Calling all MK3's

[rabbidmk1]

Well, I am happy to say that I just finished my first event in the new car. Unfortunatly though, the car had a few gremlins that I have no idea what is causing them.


Under load in 4th gear only, the car cuts out at 5600 rpms like I am hitting a rev limiter or fuel cut off. Does anyone have an idea of what might cause this? I have switched ecus, mafs, ect with no change... What is throwing me for a loop is that it only happens in 4th gear...

[OTLimit]

Good luck with that. Chris is still trying to get his to run again. Car had been running great, but started having problems during the practice day at the ARRC last year, and hasn't run right since.

[Dave Zaslow]

This seems to be an endemic problem. I had it, Kirk had it, Chris has it. The only one I know that didn't is Tim Mullen and he didn't build the car, Dick Shine did and Dick didn't have a clue.

It really, really, really sucks.

You have a problem that is sending the computer into a limp mode. The real problem is finding the problem. I found the key reading to knowing something was wrong was fuel injector on-time. If it is over 10ms then the ecu is richening the mixture to preserve the car and get you home.

The ecu must see each and every input signal as clean and pure. The first step is to take out the Bentley and go through every one of the rudimentarty component tests. If you use the breakout box fine, but you really need to test the component at both the source (itself) and at the ecu pins to see what the ecu sees.

Second, check the grounds. Every one. Make sure they are all clean and shiny.

Third, take a look at the O2 sensor readings. If all you have is a voltmeter make sure that the readings are all within spec and that you are not getting any out of range or negative numbers.

Fourth, get access to an automotive oscilloscope, learn how to use it, and make sure each of the waveforms is correct. You've shotgunned changing things out, as I did, but no new sensor made a difference.

Fifth, test the signal for the VSS at the component and the computer. This tells the ecu what speed you are going. It runs from the transmission to the relay panel to the cluster to the relay panel to the ecu. All in something that looks like a 22 gauge wire with six different connectors chopping it up. Try running the signal direct to the ecu pin.

My problem was solved when I changed the complete engine wiring harness and fuse/relay panel. I never did pinpoint the exact problem, although I have my suspicions.

If I can be of any help, send me a PM so I can get an email address. I have some files and info that may assist in getting closer to the problem.

Did I say it really, really, really sucks? Well it does.

Good luck.

Dave Z

[rabbidmk1]

I just got off of the phone with Colin @ TT and I know what my problem is. Lets just say I feel like a Dumb A$$ . My suspision is the car was about 15 to 20% down on power... After I test that part I will post up my solution!


Aaron

[Knestis]

I'm old and preoccupied with work and stuff so my memory is suspect. Cameron is young and clever so he'll remember the details but we thought it was the O2 sensor for a while. We actually ended up replacing the ECU (it had a resistor that actually LOOKED like it had smoked) and - again, if I remember correctly - repaired a VSS signal wire, maybe where it goes into the gauge cluster?

Herr Conover?

K

[shwah]

I hope you found it! I remember the first event in my car...shifting at 4200 rpm and trying not to get run over. I was about 30% down or more on power that weekend.

See you at Blackhawk.

[Dave Zaslow]

So what happened with the fix?

DZ

I just got off of the phone with Colin @ TT and I know what my problem is. Lets just say I feel like a Dumb A$$ . My suspision is the car was about 15 to 20% down on power... After I test that part I will post up my solution!
Aaron
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[rabbidmk1]

I had a faulty Vehicle speed sensor! The car still is not 100% yet though.

Aaron

[Dave Zaslow]

Trace the path if the replacement didn't fix.

Signal from VSS pin 2 ws/bl to relay panel G1/11
From G1/11 internal to relay panel U1/11
U1/11 ws to cluster T28/27
From cluster T28/7 li to relay panel U2/2
From U2/2 internal to W/1
From W1 bl/ws to T43
From T43 bl/ws to ecu T68/65

Pin 2 at the VSS should fluctuate between 0 and 4 volts. You should see the same at ecu pin 65.

[Conover]

Damn VSS strikes again, just run a new wire!

Other than that Kirk's car has been pretty much dead nuts reliable in the wiring department!

Pretty damn reliable everywhere else too, considering how much track time we throw at that car.

[Eric Parham]

Is this issue about the same for OBD2 and OBD1? Isn't Tim running OBD1 in the SRS-built car? How about the rest of you?

[Knestis]

I replaced a VSS when i first got the '96 (OBDII, threw a code) but it ended up being the harness.

Built the new car ('94 OBDI) and had the same problem - among the raft of little wiring/ECU/fusebox issues that we worked through.

Again - if I'm recalling correctly...

K

[Conover]

Yep, your right, I think part of the problem is in how they route the VSS wire up to the dash, then over to the ECU, or something like that, it's been a few months, but I seem to remeber the same wire feeding info to two seprate consumers, so I think we fixed it by T'ing at the sender and running that up to the ECU in a straight shot.

[Dave Zaslow]

Eric,

After a brief glance at the wiring diagram it looks like the OBD2 VSS also feeds another function. The routing is just as complicated, but it looks like there is a tap to feed something ???.

Tim Mullen is OBD1 as am I.

DZ

Is this issue about the same for OBD2 and OBD1? Isn't Tim running OBD1 in the SRS-built car? How about the rest of you?
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