Electrical adaptation of the trip computer Boost gauge

[UrS4boy]

The original trip computer digital boost gauge for the ABY and AAN was based on a 2.5 Bar MAP sensor signal (AFAIK). If you move to a 3.0 Bar MAP, the readout for the trip computer boost gauge is no longer correct/accurate. Has anyone figured out a fix for this? And, yes, I *know* all about analogue boost gauges (I have one and I also have a TDS-1 from EFI Express). I am installing a trip computer in my 93 S4 and if I can, I would like to make the digital boost gauge accurate (as much as it can be)).

[prj]

Short map output to 5v, power up ecu on the bench, look at what the charge pressure signal value is going out of the ecu.
Now set map output using a pot to whatever the voltage output of your sensor is at 250 kpa.
Fit a pot instead of R298 and adjust until the charge pressure signal matches what it was at before.
Measure pot resistance and find a resistor close to it in value.

Once you are dome, post the value here. Keep in mind that the gauge can not read higher than 2.5.
Other option is to look which microcontroller the computer is running, disassemble the code and modify the scale.
Good luck.

[UrS4boy]

Short map output to 5v, power up ecu on the bench, look at what the charge pressure signal value is going out of the ecu.
Now set map output using a pot to whatever the voltage output of your sensor is at 250 kpa. Fit a pot instead of R298 and adjust until the charge pressure signal matches what it was at before. Measure pot resistance and find a resistor close to it in value.
Once you are dome, post the value here. Keep in mind that the gauge can not read higher than 2.5.Other option is to look which microcontroller the computer is running, disassemble the code and modify the scale. Good luck.

So, the answer is no.

[prj]

It's "yes", just I can't be arsed to do it just out of my free time.
If you feel like experimenting, I can provide all the info you need to get the ECU running on the bench... You need like 5-6 wires and you can use your computer's power supply for that.

It's just basic soldering and experimentation work - it's 100% possible to do it, I am just too lazy, and no one will ever pay me for doing it.

[Zoig]

Prj, you could use pot to increase voltage?

AFAIK when going from 250kpa map to 300kpa map, the 5v value changes from 2.5bar to 3bar. So when the ECU reads 2.5Bar from the map sensor (4.2v) the Computer is expecting 5v to display 2.5bar. I was thinking of using some opamps to increase the incoming voltage from ECU by 1.2 so it would read correct in the Cluster. Also Something like Zener diode to block anything above 5v from reaching the Cluster.

[UrS4boy]

AFAIK when going from 250kpa map to 300kpa map, the 5v value changes from 2.5bar to 3bar. So when the ECU reads 2.5Bar from the map sensor (4.2v) the Computer is expecting 5v to display 2.5bar. I was thinking of using some opamps to increase the incoming voltage from ECU by 1.2 so it would read correct in the Cluster.

This is the problem. However, way back in my mind, I remember Kevin D, the developer of the TDS-1 (with its built-in boost gauge) saying the relationship was not linear, i.e. not just a 1.2 factor.

Oh well. Absolute pressure is a pain anyway. Thanks for the replies.

[S2central.net]

The output should be linear - it certainly is from the raw map sensor. I've never yet benched the ecu output signal but should be very easy to use a simple passive circuit to step down the 0-5v output from ecu into correct range where it fools the AAN display into reporting actual boost values in range of 0-2.5 bar.

For raw example lets say that 4.8v signal provides 2.5 display stock.
That gives a range of 1.92V per bar absolute - same as 250kpa map sensor inside ecu

A 3bar map sensor operates at 1.6v per bar absolute to give us 4.8V output with 2 bar pressure - 3bar absolute and mismatch with display.

So 1.5 bar boost on 3bar sensor gives us 4v output.
A 1.5 display on the LCD is derived by 0.5 x 1.92 = 2.88V

So all you need is a linear pot to step signal down by a factor of 0.72 and hey presto

I can bench this to test out the theory.

The only gotcha will be that the LCD display won't give full range of vacuum - actually the lines may not intersect at 0bar boost so it might be skewed off. Just thinking off top o my head.

[S2central.net]

After a little more thought - such passive skewing would only give a rough indication in the range of 1 - 2 bar boost

To make it more accurate in display of 0 - 2 bar boost I think a simple comparator circuit would need to be factored into things. Reference voltage would be 1.6v by this method so as to scale linearly from 0bar boost - if that makes sense.

[prj]

It is just a linear multiplier... do not overthink things

[S2central.net]

It is but you need the offset at 1.6v so as to fool the display into showing boost not absolute. We need gain of 1.2 after the comparator.

[prj]

Yeah, if you want to make it show boost instead of absolute you will indeed need an offset...