4bar map problem reading above 3000mbar with AHU JB ecu (msa15.5)

[burpod]

i posted on ecuconnections, but maybe someone here might have some answers before i bite the bullet and swap to an alh ecu...

so i thought i had my 4bar map sensor working correctly on my rabbit tune, but as i have been fully testing boost above 2bar (3bar absolute) i realized something isn't right.

compared to my new vdo 3bar boost gauge, and using my air compressor and blocked off boost pipes to check for leaks and compare readings with key-on/engine-off, my vcds readings seem accurate up to around 1.9bar (2900mbar absolute) but then seems to quickly lose accuracy and vcds won't read past 2998 even when pumped up to 2.5bar boost which should read 3500mbar. a while back i thought i found a hard limiter for maximum displayable boost pressure, as i could not read past 3014mbar. i changed a number from 3019 to 4000 and then was able to see readings past 3014 (At least in logs). in logs i have taken where i had seen a maximum boost (Steady for a second or more so vcds would pick it up) of 2.5bar, vcds has read 3350 - a bit low. i know i shouldn't be boosting that much, but because my readings in the upper end seem to be much lower than reality, i had been adjusting my n75 values wrong.

i've checked all my wiring, thinking maybe i'm having a wiring issue, but i think that's all good. i can see 3.8v at the solder joint onto the ecu board using pin #1 on the map sensor as ground. based on the map linearization table, this is clearly above 3000mbar. testing voltage between pin #1 and #3 i see steady 5v - this is all with key-on/engine-off. it will not read past 2998. but when i'm doing vcds logs i AM seeing values as high as 3350mbar. however, compared to what i'm seeing on the gauge, these numbers are low - like the sensor is saturated and not reading correctly.

I have tried swapping to a different ecu (another JB ecu), and also tried another new 4bar map sensor. both are bosch, one is vw-branded, the other just plain bosch. and no change. aside from my crappy cluster that has issues (in any car), i haven't had or noticed any strange electrical problems and everything else seems to read correctly, so i don't think now it is a hardware/wiring issue.

but what gives? why the readings max out at 2998 with key-on/engine-off, but vcds logs i see values >3000, although they seem to be inaccurate? it makes no sense, i can read the voltage the ecu should be seeing at the solder joint on the board for the external map sensor and that is a sane value.

i did have a slight boost leak crop up around 2bar that i'm fixing now, but it holds the pressure long enough that i should see higher readings. but maybe somehow that's contributing to the problem. with the car actually running, perhaps it is enough to mess up the map readings.

 

[BustedBolts]

Most likely the boost diagnostic factors need adjustment unless you already did so. There is a single value boost limiter but it has nothing to do with the diagnostics.

 

[burpod]

i changed all the diagnostic factors to values like others have used for 4bar (eg, 980 -> 638, 255 -> 157). and as i mentioned, the values i see look correct up until around 2900mbar absolute, then it inches towards 2998 as i pump up the boost to 3500.

but i still don't understand why with key-on/car-off, it stops reading at 2998. but when running i see values >3000 (up to 3360 most i've seen in log). perhaps the ecu software is hard limiting this without engine running for some reason? fixing my slight boost leak and test it again today with logs. maybe it was enough to throw things way off. it would be nice to test these values for accuracy with engine off and using the air compressor, but that seem to not be possible for whatever reason...

 

[burpod]

well i'm an idiot and should have known better. that boost leak developed around 29psi that i thought was tiny enough to not throw things off too much (Based on leakage observed with air compressor) - i believe was worse than i thought, or got worse. fixed that and i think my actual MAP values now might be correct at least when actually running and taking logs. i need to fix my n75 map a bit first and re-log. still doesn't explain the behavior with key-on/engine-off readings but whatever...

tdi_my_live suggested my boost by IAT correction map might need to be scaled above 3000, so i did that, but haven't tested anything more yet

 

[burpod]

well, made some progress! i hadn't touched the boost correction by temperature map, and that map needed values for higher boost levels beyond 3000. after i did that, with air compressor, i was able to see 3014! so an improvement over 2998 lol... maybe there's another map that's limiting it with key-on/car-off. i tweaked my linearization to be a little more "correct" to my gauge. the gauge read ~100mbar higher at 3000, assuming this new gauge is accurate. so tweaking it should be in a safe direction at least. now that hopefully last boost leak is fixed, i did some logs and man it really rips. that tiny little leak (maybe had been getting worse) was really throwing things off again. max values in my log seem to correspond pretty accurately to what i saw on the gauge. 3250 is the max my log shows, but i think i hit ~3400 so perhaps it didnt quite pick it up. in any case, i think things should be pretty good now and back on track....

 

[adamss24]

I was about to suggest your diagnostic limits have not been adjusted accordingly to the new map sensor hence why vcds only seen 3 bar boost max. Seems like you sorted the issue ! Nice !

 

[burpod]

Yeah apparently I made the mistake (for the 100th time) of underestimating the effect of what I thought to be the the tiniest boost leak at higher pressures. I believe my diagnostic factors are fine (might need to tweak a tad but I'm using what most others use for 4bar map). Doing more boost logs and trying a different pump map now, it seems pretty accurate and see readings greater than 3014mbar absolute and corresponds pretty close to what I see on the gauge

There must be some map I can't find that's limiting my readings with key-on/car-off to 3014. But with no leak now, I can at least make sure my ecu and gauge match up well between 1000-3000mbar with air compressor test and will just have to assume beyond that is good too.

So good to go, now more tweaking of new pump map and n75

 

[burpod]

Well I'm still having problems with this. Need to boost leak test for the millionth time I think, I tried swapping in a 3rd map sensor. Maybe I have the tiniest leak there throwing things off again. Using the linearization everyone uses for alh, it reads about 60-70mbar low at ambient. And according to my gauge around 2.2bar it reads maybe 150-175mbar too low. I've tried various different linearization and factors. I've also hooked up my multimeter to the map sensor wires now, and when I've seen 3.6v and compared to gauge and vcds reading, vcds is too low. Using linearization table I should have seen higher at that point but a little difficult to see exactly all that stuff in real time or compare after the fact to what I saw...

Maybe I'm getting myself mixed up with too many logs and different linearizations. But I feel now maybe for this ecu somehow, I just haven't hit the right combination of linearization and factor. What a pain. Nobody has had any problems with it working on an alh.

 

[burpod]

i did one last round of testing with the air compressor hooked up. i can't for the life of me get the vcds readings to read right. i tested with multimeter - and also realized i could test the back of the pin on the ecu where the map sensor is soldered to eliminate any possible connection issues. at 2500mbar on the gauge (like 3520 absolute) i would read 4000mv. ambient at about 1020mbar, reads 1100. 2000 mbar on gauge (3020mbar absolute) read about 3500mv. so i believe the ecu and map sensor are totally fine. no leaks in my pipes up to 3500mbar absolute, unless i have a leak at the compressor housing or race pipe (but both look pristine, no evidence of leaks - i do have penty of oil in the intake where i should see seepage, as i have my ccv hooked up on a diverter valve so it dumps to atmosphere only when boost >5psi).

i also tried the tests with battery charger on, but no difference.

using linearization of 500/500 4500/4000 and factors 638/157 it still reads low according to vcds, 958mbar at ambient when baro here is usually 1015-1025. and above. ecu would say baro is 1011, just a touch low at least compared to what weather.gov was saying at the time for my area.

i tried linearization using my real-world values i measured of 1100mv/1020mbar 4000/3500. when i used factors based on 3500max mbar (729/137), it reads way low. change just the factors to usual 638/157, it seems to read correct up to close to 3000mbar absolute then rapidly drops off and doesn't read past 3014. haven't tried this one yet with car actually running, maybe it will read more correct.

also, when i compare the voltage i see at the map sensor output, to my hand calculations based on line-slope, they don't match up well at all at higher psi. so makes one think the factors are wrong. but if i increase say 157 to 160, and it is outputting higher balues at higher psi, but then my low numbers are off.

driving my NUTS!!!!

based on what i see the n75 doing at high boost 28-30psi, it seems like what the ecu is actually reading is correct despite what i'm seeing in vcds... ugh. so i believe my n75 map is very close. but near impossible to dial in better without boost logs.

 

[jwrb]

Hey!

Try changing diagnostics factors to the following, makes it more accurate than what you have used.

0.0617 in "slope for pressure conversion"
162.000 in measurement data B value
8 for factor

 

[burpod]

I can try that when my cars back together. Currently trans is out for clutch replacement and 6spd/LSD install and figuring out my shudder (almost certainly drivetrain) problem.

I'm still a little confused about those numbers. I thought it only had to do with the largest mbar reading you wanted to see. So I could use 255/3500 and 3500/255, and have slightly better granularity since I'm never going to go over 2.5bar boost.

I've fiddled with grounds thinking I have a wiring issue, but hasn't seemed to help any. Maybe this vdo boost gauge is optimistic for some reason above 2bsr boost and 3500+ rpm? My numbers shown by vcds around there still seem a bit low by like 100mbar

 

[jwrb]

I can try that when my cars back together. Currently trans is out for clutch replacement and 6spd/LSD install and figuring out my shudder (almost certainly drivetrain) problem.

I'm still a little confused about those numbers. I thought it only had to do with the largest mbar reading you wanted to see. So I could use 255/3500 and 3500/255, and have slightly better granularity since I'm never going to go over 2.5bar boost.

I've fiddled with grounds thinking I have a wiring issue, but hasn't seemed to help any. Maybe this vdo boost gauge is optimistic for some reason above 2bsr boost and 3500+ rpm? My numbers shown by vcds around there still seem a bit low by like 100mbar

100mbar isn't that bad of a discrepancy for a manual gauge.

I would suggest buying a hydraulic pressure gauge, fluid filled.
4+ bar guage is only $10 to buy, works great with a couple fittings to adapt to hose


Usually accurate to +-1mbar or less.

 

[burpod]

Hmm. I was thinking this $60 vdo 3bar mechanical boost gauge would be more accurate than that As mentioned before, it seems pretty accurate up until 1.8bar boost. Up to 2.5bar boost (above where I should be anyway) it seems like it could be maybe 150-200 mbar higher than vcds. Maybe I'll look around for some other gauges, but man. Just keep spending money I really shouldn't on this TDI habit

 

[[486]]

confirm gauge using your OEM MAP sensor
you know it's high quality and therefore reads pretty good, right?

 

[burpod]

confirm gauge using your OEM MAP sensor
you know it's high quality and therefore reads pretty good, right?

Well, I do trust the OEM map sensor to read correctly voltages, at least in a static environment . As i mentioned before, in "bench testing" with air compressor + key-on/engine-off and the standard linearization/factors, it seems to match up well to the gauge up until about 3000mbar when it won't read past that for some reason. When running, I see values beyond 3000. And then also it almost seems like it loses its linear slope, ie, I clearly hit 2.25 bar on the gauge steady for a couple seconds but vcds log only showing 3000-3100 four that brief period. Hit 2.0bar on gauge and seem to see values 2900ish.

1) possible faulty wiring, ground contributing when engine running?
2) gauge or map sensor losing accuracy due to the dynamic environment of compression/exhaust at higher rpms?
3) factor or linearization slightly off for my particular ecu somehow?
4) sampling rate for ecu/vcds simply not catching the "high" values?
5) my imagination
6) combination of the above...

 

[arazvan2002]

Post the file together with the original, here or the other forum so people can look what you did there.