A couple of months ago I posted on how the Wastegate Modulator operates when energised and de-energised.

That research was necessary because the “common forum wisdom” that the WGM operated to reduce boost seemed to be completely at odds with what the WGM control code appeared to be doing.

To quickly recap, when active the Wastegate Modulator rapidly switches between boost pressure and turbo intake pressure to regulate the pressure at the wastegate actuator. When the Modulator is inactive boost pressure flows directly to the wastegate actuator.

The effect of the switching between intake and boost pressures at 16 times per seconds is to create a blended pressure which related to current value of the two pressures and to the amount of PWM applied. Because the intake is always pressure is always lower than boost pressure (assuming the Turbo is producing boost) the WGM can only reduce pressure at the Wastegate Actuator when there is a PWM signal.

With no PWM modulation applied to the modulator boost pressure flows directly to the wastegate actuator meaning the wastegate behaves as it would if the modulator was not present.

This is absolutely fundamental to the way the WGM maps work, so
if you have any doubts I’d recommend (re)reading the post on WGM operation, and if necessary doing some testing to verify this is correct for yourself.

The WGM Controller

I’ve had a bit of difficulty trying to decide how best to write up the WGM. The software contains support for a full blown PID controller, however the WGM implementation uses only Integral component. The Proportional and Derivative components are both disabled.

For the moment I’ve decided to stick to what is actually used in the standard implementation, but keep there is a huge amount of hidden flexibility and the full PID setup should prove useful for full-blown Boost Controller applications down the track.

As a note of explanation, the parameter names I'm using below come from a list of Td5 System Constants from offical Land Rover engineering docs that were posted to a Spanish forum. While they are not natural language they have the distinct advantage of accurately defining the purpose of the value.

So let’s look at the details.

WGM Enable and Disable

The first thing to note is that the Wastegate Modulator is not “always on”. The control code is only enabled when the engine is operating above specific RPM and IQ thresholds.

The enable thresholds are set by two system constants:
- tb_fuel_mass_enbl = 24.00 mg/fire
- tb_engine_speed_enbl = 1900 rpm

Once the fuel mass (IQ) requested is greater than 24mg/fire and engine speed is greater than 1900rpm the Wastegate Modulator controller is switched on. The controller remain in the “on” state until either engine speed or IQ falls below the disable threshold.

The two system constants for the disable threshold are:
- tb_fuel_mass_disd = 20.00 mg/fire
- tb_engine_speed_disd = 1750 rpm

This state diagram illustrates what is occuring. The controller remains in either an on or off state until the conditions required to switch to the other state are met.

These on/off thresholds are confirmed by data logging. I've filtered a log with just under 1000 data points to extract the lines that have a Wastegate Modulator Duty greater than zero, then plotted RPM vs IQ. The colour bar shows the WGM Duty %.

This shows quite clearly the WGM is only active above the operational thresholds described above.

Narrowing the focus to the “off -> on” transition, I’ve selected rows where a 0% WGM Duty is followed by a non-0 WGM duty %.

You can see the engine speed values are mainly +/-100 rpm around 1900rpm, and the IQ values appear to be generally above 24mg/fire.

Looking at the “on -> off” transition, I’ve selected all non-0 WGM Duty % values preceeding a 0% WGM Duty.

While the engine speed values are still above 1850rpm, the IQ values between 20 and 24 mg/fire are apparent.

To further illustrate the off -> on transition ploting the log data in more familiar way shows the behaviour of Wastegate Modulator duty, RPM and IQ. In both cases IQ is around 33mg/fire.

The first plot shows behaviour at 1690rpm with IQ of 33mg/fire, and WGM duty at 0%.

The second data point 290ms later shows engine speed of 1970rpm, with WGM duty at 31%.

Current Boost and Target Boost

These two parameters are calculated each cycle regardless of whether they are used in WGM operation.

Current Boost is a calculated value equal to Manifold Absolute Pressure (MAP) - Ambient Absolute Pressure (AAP).

Target Boost is a value looked up using the final table in all Td5 engine fuel maps.

The X-axis value is IQ (or fuel mass) mg/fire and the Y axis is RPM and the Z-value is Target Boost in kPa.

The Target Boost table is identical across Euro, ROW, Japanese and Korean tunes, on Eu2 and Eu3 motors and Manual and Auto D2’s despite the variation in driver demand tables, and smoke and torque limiters which suggests that this table is not precisely controlling the boost to match fuelling.

I’ve highlighted the region where engine speed is above 1750 and the IQ requested is above 20 mg/fire in a stock D2 Auto driver demand map to illustrate the region where the Wastegate Modulator is operating in an unmodified map.

It should be apparent that when you start increasing the duration in the driver demand, smoke and torque limiters that the values returned from the Target Boost table will come from further to the right - in other words the target boost will tend to be higher than for stock driver demand.

Wastegate Modulator Duty Ratio Bias

This map is has Target Boost as the X-axis and RPM as the Y-axis. Z-Axis values are WGM Duty Ratio.

This is the underlying amount of WGM Duty Ratio for a given target boost level. Because the ECU map lookup “clips” axis values to maximum or minimum, anything below 100kPa uses the left column, and anything above 120kPa uses the right hand column.
Adapation to higher boost levels will require modification of the header values.

WGM Integral Gain

The Integral Gain map has RPM as the X-axis, Boost Error as the Y-axis, and Integral Gain as the Z-axis.
This is used in the ECU when calculating the Integral (summed value over time) of the error.

WGM Proportional Amnt

Not Used in factory maps.
X-axis is Boost Error, Y-axis is WGM Duty Ratio.

Boost Error

Boost Error is calculated as the difference between the current boost and the target boost:

Boost Error =  Current Boost - Target Boost  

Note that if current boost is below target boost the error is negative, and positive if current boost is higher than target boost.

Controller code

The integral map described above should give you a big clue as to what we are dealing with here - a Proportional-Integral-Derivative Controller (PID). This is a type of feedback controller that is widely used to regulate “plant” to a steady state.

Fortunately Land Rover has disabled most of the Proportional and Derivative code so what is left is far simpler. However, the fact all the PID controller is still in place does open up the possibility of some fairly advanced boost control implementations in future. At this point I’ll stick to what is actually used.

Integral Calculation

The WGM controller has a “dead band” of +/- 2kPa which is set by the tb_intgl_enbl. Boost Error values of less than this amount are ignored.

If the Boost Error is greater that +/-2kPa, it is limited to the value set by tb_max_err_intgl which is given as 10kPa, but looks more like it should be 1kPa from my reading of the code. Regardless the constant used has a raw value of 100. Given that the raw boost error figures the constant is applied to are kPa * 100, this has the effect of limiting the max error amount to +/- 1 kPa.

The limited Boost Error is then multiplied by the Integral Gain and then divided by 1000 (raw value) to give the required adjustment to WGM Duty Ratio. You’ll note the Integral Gain table values are all negative, and as previously noted current boost values below target boost result in negative boost error values.

100 * -50 / 1000 = -5   
-100 * -200 / 1000 = 20  

This plot of logged engine data shows how the Boost Error and Integral behave. As Boost Error becomes postive the Integral amount decreases, and vice versa. The other point of note is that the values are not calulated when the WGM is not currently enabled.

The adjustment amount is then added to the accumlated value (Integral) of the Wastegate Duty Ratio adjustment. The maximum adjustment to Duty Ratio is +/-0.2% each time the code is executed.

The WGM controller code runs at 10ms intervals which is set by the scalar tb_intgl_deriv_rate. At this rate the controller can adjust the Duty Ratio by up to 20% in 1 second.

The final step in calculating the Integral is limting checking against tb_clamp_intgl_hi which is set at 10%, and tb_clamp_intgl_lo which is set at -30%.

Total WGM Duty Ratio

The final step is to sum the component values of the PID controller. Because the Proportional and Deriviative amounts are set to zero in this implementaiton this is becomes effectively

 Duty Ratio % = Bias + Integral

You can see this in practise in this plot of log data.
The Duty Ratio Bias is 25.64%, and the Integral is -1.54% and the total WGM Duty Ratio is 24.1%.

This amount is then checked against tb_max_duty_ratio which is set at 82%, and tb_min_duty_ratio set at 16%.

WGM Module Disable

The WGM module can be disabled by setting the scalar which controls the PWM frequency tb_turbo_pwm_freq to 0. Defenders have a fully configured WGM which only requires setting this scalar to 16 to enable.

This is a bit of background on how the physical waste gate modulator unit operates.

It has only been in the last few weeks that it finally dawned on me that I'd been uncritically accepting the "Active to Limit" theory as correct. While it doesn't seem that important on the surface, it's difficult to make sense of code when the behaviour seems to be at odds with how you believe the hardwares should be operating.

Let's start by looking the Active to Limit Boost theory of operation:

• power off, the valve is in an open position and boost flows from turbo outlet pipe to the Wastegate Actuator;
• power on and no PWM signal from the ECU, the valve is in a closed position and boost is prevented from reaching the Wastegate Actuator;
• power on and PWM signal from the ECU, the valve opens and allows boost to reach the Wastegate Actuator, opening the waste gate.

Active to Limit Boost is the most commonly expressed explanation of how the WGM operates, even if it's not explicitly stated.

This example is from Urban Panzer's excellent www.discovery2.co.uk website...

"trying to the rev the engine over 2500 RPM the waste gate modulator info that Nanocom shows under "read fuelling" went to zero occasionally, so it basically was not controlling turbo charger boost / waste at certain times and was very "hit and miss" at best. That all seems to make sense and description in RAVE would appear to confirm that view. And the Nanocom logs seem to suggest this is what happens in practice.

You can clearly see the WGM active = controlling boost, WGM inactive = boost uncontrolled aspects of the Active to Limit Boost theory.

You'll see this theory of operation informing most forum posts relating to over boosting and WGM diagnosis. It's difficult not to fall into this way of thinking due to it's prevalence - that's my excuse anyway.

So lets look at this in detail...

Hardware first

After a bit of digging I found a couple Pierburg service information sheets for electric switch over valves. These were for parts identical in appearance to the D2's waste gate modulator, but with different part numbers. The test methodology given in the service information required a hand vacuum pump, so I decided put the site donations fund to use and purchased a Mityvac kit. Thanks again to those who have contributed!

The test procedure is fairly straight forward, but the main question I had was whether the valve on the D2 behaved in the same way as described.

Testing the modulator on my D2 suggests that the flow through the modulator does match that outlined in the test procedures.

• power off, boost flows from turbo outlet pipe into port 1 for the modulator and out of port 3 to the waste gate actuator;
• power on, engine running, and no pwm, boost flows in from turbo outlet pipe into port 1 for the modulator and out of port 3 to the waste gate actuator;

In the course of doing these tests I discovered that while the WGM seemed to be OK, it wasn't passing the Tightness test 1.3.
It was possible to pull down to 0.5 bar vacuum but this dropped to 0 within roughly 10-15 seconds. Similarly, applying pressure to port 1 with port 3 capped resulted in a loss of pressure over 10-15 seconds.

To confirm normal operation I ordered a Pierburg OEM modulator and bench tested.
Out of the box the modulator would hold 0.5bar vacuum on port 2 for 20 minutes with no visible loss of vacuum.
Testing with port 3 capped and 2.0 bar applied to port 1, there was no visible loss of pressure after 20 minutes.

The pin to pin resistance of the new WGM was 29.3ohm, which is within the 28.4ohm +/-1.5ohm specification.

Powered Benchtests

Rather than leave the testing hanging I decided to perform tests 1.1 and 1.2 from the Pierburg service information. This was done with a bench power supply and confirmed the new WGM behaved as described.

For test 1.1 Electrical Function there was distinct click as the solenoid activated, as you'd expect
For test 1.2 Internal Fouling the air flow matched the required values:

• no power: air flowed from port 1 -> port 3.
• power: air flowed from port 3 -> port 2.

The thing to note here is that port three is the common outlet. The solenoid is switching between port 1 and port 2.

Next step was to connect the WGM to my tester ECU. Power to the + pin on the WGM was taken from the "master relay" on the interface board. The - pin was connected to A21 on the ECU.

I also hooked up the Bitscope Micro with the scope probe connected to the - pin and the earth clip hooked up to the power supply earth. Add in the Mityvac and Nanocom and it all starts getting a bit messy!

The main difference between unpowered (ignition off) and powered (ignition on) without a PWM signal is the voltage level.
Unpowered and no PWM gives 0V on both +ve and -ve pins.
Powered and no PWM gives supply voltage on both +ve and -ve pins.

Because there is no voltage differential between +ve and -ve no current flows and no "work" is done.

In the case of power applied to +ve pin and PWM operation, the +ve pin is at supply voltage and -ve pin at earth reference when the PWM completes the earth path. Because there is a voltage differential of around 13.5V and the solenoid coil has resistance around 29ohms the WGM will draw 0.46 amps and dissipated about 6.28 Watts when it is activated.

I can attest that leaving the WGM hooked up to a 12V supply for 5 minutes results in the WGM becoming hot to the touch. oops!

The Bitscope capture below shows the Nanocom Wastegate Modulator Test in action.

You can see the effect of the PWM completing the earth path resulting in the voltage at the - pin of the WGM dropping by just under 12V.

The pulse width is 100ms or 10Hz, and the duty cycle is roughly 50%, which corresponds with frequency and duty cycle requested by the Nanocom test routine.

I couldn't work out how to capture on video but with the port 3 capped and port 1 pressurised to 2 bar, there was a distinct "puff" as pressure was released through port 2 each time the WGM activated.

As can been seen from the video the WGM is directing pressure to the capped port 3 when it's not receiving a PWM signal.
When PWM is active the pressure in the capped waste gate actuator outlet is released via port 2.
The small drop in pressure seen on the gauge with each activation is due to the small internal volume of the cap I've used.

Revised Theory of Operation

At this point it should be apparent the theory of operation outlined at the start of the post was a bit back-to-front.

Updating the theory to reflect real world behaviour we get:

• power/ignition off, air pressure flows from port 1 (turbo outlet) -> port 3 (waste gate actuator) - wastegate receives full boost
• power/ignition on, and no PWM signal, air pressure flows from port 1 (turbo outlet) -> port 3 (wastegate actuator) - wastegate receives full boost.
  In these cases the WGM is de-energised.

• power/ignition on and PWM signal, air pressure flows from port 3 (waste gate actuator) -> port 2 (turbo intake pipe) - wastegate isolated from full boost.

In this case the WGM is energised.

This means when the WGM is recieving a PWM signal it is operating to reduce pressure at the Wastegate Actuator. Rather than "Activate to Limit Boost" the system is "Activate to Increase Boost".

But RAVE says...

So what does RAVE actually say to support the "activate to limit boost" theory?
The following sentence is often pointed to as evidence:
When full boost is reached a control signal is sent to the wastegate modulator, and a vacuum is applied to the wastegate valve.

Interestingly this sentence incorrectly states that vacuum rather than boost operates the waste gate valve. This is often excused as a typo but there is clearly an error in the statement.

Nothing else in RAVE indicates a specific method of operating, just that the system controls boost pressure to prevent overboost.

What about the Nanocom logs....

The Nanocom samples engine data once every 1.2 seconds in the current EVO guise - Nanocom 1 was a bit quicker at 1 second.
The WGM control code samples boost pressure and engine load once every 10ms and updates the PWM amount at that speed.

This means the WGM PWM has potentially been updated 120 times for each data point the Nanocom logs show. Transitions between PWM on and off to control limit boost occur quite quickly. 500ms is not uncommon, and the actual switch off occurs faster than the 70ms sample time of the logs.

In conclusion...

Looking back at the opening quote it's pretty obvious what is occurring.

... the waste gate modulator info that Nanocom shows under "read fuelling" went to zero occasionally, so it basically was not controlling turbo charger boost / waste at certain times and was very "hit and miss" at best.

The waste gate modulator PWM drops to 0% because the ECU is trying to direct full boost pressure to the waste gate actuator. And from the diagnostic information you can surmise the failure mode is either loss of spring tension in the valve or internal fouling of the modulator.

The next post in the Wastegate Modulator series will look at the WGM control maps.

Note link to calculator spreadsheet added at bottom of post

Background

The 1.42 bar boost limitation of the stock Land Rover Td5 engine management system has been a long standing issue when increasing boost levels as a performance upgrade.

The standard solution has been to insert a boost box or some other type of "cheater circuit" in line with the MAP sensor to lower the sensor voltage to provide the ECU with false reading of the boost levels.

In the course of disassembling the ECU firmware it became apparent that the sensor parameters could altered with minor adjustments to the settings in the fuel map to accomodate alternative MAP sensors. The advantage to this approach is that the ECU receives a true reading of the boost pressure rather than a falsified reading that effects pressure readings across the range.

This modification was originally tested on a range of Td5 powered vehicles by members of the Td5Tuning.info forum in January and February 2014.

Under the hood

In order to understand why and how this modification works, it is useful to understand the signal flow from the MAP sensor through the ECU hardware and the processing the ECU firmware applies to the converted analog voltage.

Sensor Signal Flow

The starting point of the conversion process from manifold pressure to ECU representation of the pressure is MAP/IAT sensor which is mounted on the TD5 inlet manifold. The voltage the MAP sensor outputs in response to a given manifold pressure is determined by the characteristics of the sensor used. The relationship between pressure and output voltage is often described as the transfer function of the sensor.

Once the MAP sensor voltage enters the ECU housing it passes through a voltage divider formed by two resistors. The effect of the voltage divider is to reduce the incoming MAP sensor voltage to 90.7% of the original value.

The reduced MAP sensor voltage is then processed by a 10 bit Analog to Digital Convertor (ADC). The 10 bit range of the ADC allows the sensor voltage to be converted to one of 1024 ( 2^10) values. The conversion process is referenced to a 5 volt supply within the ECU, meaning the maximum ADC value of 1023 represents 5000mV and the minimum ADC value of 0 represents 0mV.

At this point the signal flow moves from hardware to pure software. The ECU code first checks that the raw value retrieved from the ADC is within the preset range. Both the minimum and maximum values are related to characteristics of the sensor being used and the range of sensor voltages that are considered within normal range. The maximum value set for the MAP in stock tunes is the equivalent of a pressure reading of 2.42 bar, which will familiar as the point the ECU limits with over boost. Any value that lies outside the initial check range causes an “out of range” fault to be logged.

In the next stage of processing the range checked ADC value is scaled so the converted value is returned to the required units. In the case of the MAP sensor this is millibar * 100. The scaling and offset values reverse the transformations applied by the ADC conversion, voltage divider and the sensor transfer curve to give the pressure value the sensor measured.

Reverse engineering from the stock MAP sensor

At this point we have a basic outline of how the MAP sensor voltage progresses to a digital representation of the pressure, and where intervention is required if we wish to replace a MAP sensor.

To illustrate the process of configuring the ECU values for a specific sensor the following section works from first principles using the stock MAP sensor.

While the explanation of the process may seem long winded and overly detailed the aim here is to explore the underlying assumptions, so that the same procedure can be applied to other sensors.

Hardware

The sensor datasheet

The starting point is to acquire the specification of the MAP sensor as this provides the key information required to accurately configure the ECU. The stock MAP sensor for the Td5 is Bosch part 0 281 002 205, and the key parameters from the data sheet are shown below.

Note that the data table pressure range refers to two points - p1 and p2 The information about these points is given in the plot of the sensor Characteristic Curve.

From the spec sheet we can determine that that this sensor has an output of 400mV at 20kPa and 4650mV at 250 kPa.

Sensor Transfer Curve

Calculating the transfer curve is fairly simple and uses only basic high school algebra.
First we calculate the slope of the sensor curve from the two points given by the datasheet.

p1(mV) = 400
p1(kPa) = 20
p2(mV) = 4650
p2(kPa) = 250

The slope of the sensor curve (m) is the change in voltage divided by the change in pressure.

$ m = \frac{p2(mV) - p1(mV)}{p2(kPa) - p1(kPa)} $

Substituting in the stock MAP sensor values

$$ m = \frac{4650- 400}{250 - 20} = \frac{4250}{230} = 18.478260869565217 mV/kPa $$

The slope m tells us that for 1 kPa change in manifold pressure the output of the sensor increases by 18.4783 mV.

The second piece of information required is the voltage the sensor outputs when the pressure is 0 kPa. This is the sensor offset.

 offset = p1(mV) - (m * p1(kPa)
= 400 - (18.4783 * 20)
= 400 - 369.566
= 30.434 mV 

The slope (m = 18.4783 mV/kPa) and offset (30.434 mV) provide us with sufficient information about the MAP to configure the ECU.

ECU Hardware

As discussed in the Sensor Signal Flow section the output of the MAP sensor passes through two fixed stages of processing - a voltage divider and the Analog Digital Convertor.

Voltage divider

The Td5 ECU uses a resistor divider arrangement on sensor inputs to provide a form of over voltage protection for the Analog to Voltage Convertor inputs. The divider consists of two resistors:

resistor1 = 121000 ohm
resistor2 = 12400 ohm

$ voltDivider = \frac{121000}{ 121000 + 12400} = 0.9070 $

The divider therefore reduces the sensor voltage to 90.7% of the original value. This means that a sensor output of 5000mV is reduced to 4535mV at the input of the ADC.

ADC Conversion

The 10 bit ADC divides the range of voltages between ground/0mV and the sensor supply voltage/5000mV into 1024 discrete steps.
Dividing the total number of steps by the voltage range gives the step size of 1 millivolt of input voltage. The output of the ADC is a value between 0 and 1023 that represents the measured voltage.

Note that there is debate as to whether n or n-1 steps is correct. Comparing both methods to the ECU curves indicates that n-1 gives the closest match when compared with stock values.

The voltage of each ADC step (or ADC code) is calculated as:

$ step/mV = 1023/5000 = 0.2046 $

Note that it requires a change of at least 4.8876 mV to cause a change of 1 ADC code.

Putting together the hardware scaling factors

The combined effect of the voltage divisor and ADC allows us to calculate the value in "adc codes" the ADC will output for a given sensor voltage at the ECU connector.

 hwScale = ADCstep/mV * voltDivider
 hwScale = 0.2046 * 0.9070 = 0.18557

Using this hardware scaling factor we can calculate the ADC codes produced by a voltage at the ECU MAP input.
For example if we have 4650mV input...

$ adc codes = 4650 * 0.18557 = 862.9 $

This can be extended to calculate the ADC output for a given pressure.

Using 242kPa as an example...

pressure = 242kPa;
adcCodes = ((pressure * sensor slope ) + sensor offset) * hwScale
adcCodes = ((242 * 18.4783) + 30.434) * 0.18557 $  
adcCodes = 4502.18  *  0.18557= 835.47 $

As another illustration lets use the sensor maximum of 250kPa...

pressure = 250kPa;
adcCodes = ((pressure * sensor slope ) + sensor offset) * hwScale
adcCodes = ((250 * 18.4783) + 30.434) * 0.18557
adcCodes = 4650  * 0.18557 = 862.9

Error handling

At this point in the process the ECU does error checking against minimum and maximum values defined for the specific sensor input. These are the values that are available as scalar editors (ai_limit_min : MAP and ai_limit_max : MAP) in my "donor-ware" Tuner Pro .XDF's.

If the sensor value in ADC codes is below the minimum the ECU logs "below minimum" and "out of range" faults, if the value is above the maximum, "above maximum" and "out of range" faults are logged.

These are the faults the Nanocom reports as "logged low" (1-2,x), "logged high"(3-4,x) and "current" (5-6,x). "Current" simply indicates that there is either a "logged low" or "logged high" fault set.

Using the stock MAP ADC limit check values as example we can work backwards to find the voltages and pressures that have been set.

min = 93
max = 836

By reversing the transforms performed by the ADC and voltage divisor we can reconstruct the input voltage.

sensorV = ( limits /  mvStepADC ) / vDivider
sensorVmin = ( 93 /  0.2046 ) / 0.9070 =  501
sensorVmax = (836 / 0.2046) / 0.9070 = 4505

So it appears the limits are set to a minimum of 500mV and a maximum of 4500mV.

To find the pressure these voltages correspond to we divide the voltage minus the offset by the mV/kPa value m

$ pressureLimits = (sensor voltage - sensor offset) / m $
$ pressureMin = (501 - 30.434) / 18.4783 = 25.46kPa $
$ pressureMax = (4505 - 30.434) / 18.4783 = 242.15 kPa $

25kPa is well below anything you'd encounter while driving on the surface of this planet but higher than the minimum sensor hardware limit. 242kPa matches the well known stock boost limit.

When recalibrating for a new MAP sensor, or using the stock sensor at higher boost it is essential that these limit values are reset to match the new boost levels and sensor curve.

Software: From ADC codes to pressure readings

Working backwards from ADC codes to pressure is a good warmup for the remaining steps of calculating new multiplier, divisor and offset parameters which makes possible substitution of alternative sensors.

In effect we are reversing the transformations done by the sensor curve m, sensor offset, the voltage divisor and the steps/mV of the ADC process in the same way the limiter pressures were checked.

The value used internally by the ECU for MAP is kPa*100. Additionally the divisor in the stock configuration is set to 1000. This is done due to the integer math used in processing - the multiplier is x1000 to give three decimal places of additional precision, and after the multiplication is completed the result is divided by 1000 to the required two places. This means the multiplier should be multipled by 100000 to bring it to correct units for use in a engine map.

$ multiplier = (1/(m * hwScale ))* 100000 $
$ multiplier = (1/(18.4783 * 0.18557 ))* 100000 $
$ multiplier = (1 / 3.429018) * 100000 = 0.2916287 * 100000 $
$ multiplier = 29163 $

The stock value for this parameter is 29163, so the calculated value is a match for the factory calculations.

Stock divisor parameter is 1000, and reverses the three places precision noted above.

The final parameter is the offset which is the sensor offset calculated in the intial steps converted to ADC codes then scaled by the multiplier and divisor.

$$ offset = \frac{sensor offset * hwScale * multiplier}{divisor} $$

$$ offset = \frac{30.434 * 0.18557 * 29163} {1000} = 164.7 $$

Rounding up to the next highest integer value gives 165.

As noted earlier the offset indicates the voltage the sensor would output at 0kPa pressure. This means that to correct so the sensor curve so the output is 0 mV at 0 kPa you need to subtract the offset if it is positive and add if it is negative.

The ECU math uses addition for this calculation, so if the offset is positive we need to swap the sign to make the number negative. And if the offset is negative the number added needs to be signed swapped to make it a positive number.

So in this case the ECU offset parameter should be -165 to remove the positive offset of 165.

In summary, the values calculated from the datasheet information match the stock parameters:

MAP ADC Maximum: 836  
MAP Multiplier: 29163  
MAP Divisior: 1000  
MAP Offset: -165  

Current XDF's have a changed naming scheme for scalars which reflects LR documentation.

ai_limit_max_map  = 836  
ai_anlg_mult_map = 29163  
ai_anlg_divisor_map = 1000  
ai_anlg_offset_map = -165  

1.5 Bar MAP Recalibration

This is a super simple mod to do!

• Change the MAP ADC Max (ai_limit_max : MAP) value from 836 to 863, which raises maximum input to 250kPa.
• Change the Boost Limit (tb_over_pres_enbl) from 14200 to 15000.
• Change the Boost Limiter Recovery (tb_over_pres_disbl) value to 14800.

This mod uses the stock MAP sensor and does not require any hardware changes.
It's a good choice if you are running stock intercooler and turbo.

In the Tuner Pro .XDF's I give as a "thank you" to donors these parameters can be edited using a simple graphical interface.

The parameters can be located by searching for the stock values using a hex editor of course, so it's your choice.

MAP Setting Calculator

There is now a MAP Parameter calculator on Google Spreadsheets.
It's read only so you'll need to download as an XLSX or ODS spreadsheet (or copy to your Google account) from the File menu.

The spreadsheet contains the values required for the VAG 3 Bar and Bosch 3.5 Bar (PN# 0 281 002 244) sensors.
- Copy the values you need across to the area highlighted in yellow or enter for the sensor you want to use.
- Set the boost limit required (pressure from Point 2 or lower *100). Recovery is calculated as 2kPa below this.
- If the calculated multiplier is greater than 32767 you'll need to lower the divisor. Try 750 as a starter.

MAP calculator on Google Spreadsheet