Won't go into Open Loop mode - need troubleshooting help

[KenH]

Won\'t go into Open Loop mode - need troubleshooting help

I have been working to get all the gremlins out of my SC install. The latest issue is that the A/F keeps changing from run to run on the dyno without us changing anything. A/F is 11.5 on one run, then 10.2 on another, then 13.0 and power is down quite a bit from where it should be.

Troubleshooting with a scanner shows that the car is not entering open loop mode even at WOT, so my assumption is that the stock PCM is still trying to read the O2's and correct the A/F when it shouldn't be. We tested this theory by pulling the front O2 sensor cables which did force the car to enter open loop mode and we were able to do a couple of runs with fairly stable looking A/F readings.

TPS reads on the low side at 3.4V - 3.5V at WOT. Anyone know if a TPS reading this low will prevent the PCM from entering open loop (never sees WOT)?.

I found that the throttle plates are not opening up fully with the pedal down. I redrilled the throttle cable holding bracket to move it back some and it now opens fully. TPS now reads about 3.9V at the sensor at WOT. Before I take it back to the dyno for the 4th time, I'm hoping someone can either give me confidence that this should fix the issue or help me identify somewhere else to look. Unfortunately there are no Viper tuners in the area to turn to.

 

[GTS Dean]

Re: Won\'t go into Open Loop mode - need troubleshooting help

I could be completely wrong, but I think the keys to the ECU block registers are MAP and RPM. You should hit open loop somewhere between 2800-3200rpm. TPS should go full range from about 0-5V.

I know that on turbo cars, there is a big driveability issue with the open/closed loop handoff. Not much of a deal on the strip or the dyno, but pedaling on boost in traffic and road racing are problems.

 

[Ulysses]

Re: Won\'t go into Open Loop mode - need troubleshooting help

More than likely something to ddo with the TPS or the Throttle cable. Removing an O2 just makes the PCM think you are running real lean and dumps more fuel. TPS voltage at WOT is 3.5V. One should measure the Voltage "AT" the PCM to see what the PCM side sees.

The PCM senses WOT through the TPS.

 

[KenH]

Re: Won\'t go into Open Loop mode - need troubleshooting help

Thanks for the input. By disconnected the O2 sensors, we forced a fault mode at the PCM (Seen as bad O2 sensors) which forced the PCM to drop into open loop mode and not monitor or adjust A/F at all. At that point, it seemed to be running off the VEC2 programming only as it should be in open loop.

Unfortunately, I don't have the scan tools myself to tell if the car is now able to go into open loop mode or not since the changes I made in the throttle/TPS voltage.

 

[Mr Hemi Head]

Re: Won\'t go into Open Loop mode - need troubleshooting help

My VEC2 is wired to measure the TPS signal and I get the same values. The Viper Service manual gives a range of .5V to 3.5V @ WOT. Assuming you have the VEC2 software chart this signal, the rise time should be very short. At the begining of your next test monitor the TPS signal. Start logging, before starting the engine, floor it to verify the rise time.


Are you wired to chart the MAP signal? This may provide some clues.

If you don't have the VEC2 software consider purchasing it, with the Roe S/C it is a worthwhile street tuning tool.

 

[dansauto]

Re: Won\'t go into Open Loop mode - need troubleshooting help

yes, sounds like a bad TPS sensor.

 

[Mr Hemi Head]

Re: Won\'t go into Open Loop mode - need troubleshooting help

I'm told that TPS sensors seldom go bad, that it's more likely a bad connection cause by the cable pulling down on the connector or in my case a poor ground.

 

[KenH]

Re: Won\'t go into Open Loop mode - need troubleshooting help

By remounting the throttle cable and getting the TPS output up to 3.9 (3.8V at the PCM), the PCM is now going into open loop as it should be at WOT per the scanner at the Dodge dealer. Time to put it back on the dyno I guess.

 

[Ulysses]

Re: Won\'t go into Open Loop mode - need troubleshooting help

Ken, make sure that you get a voltage close to .5V when you are at closed throttle as well. You may have to adjust the TPS back and forth some before you hit the right combo of high and low trip points.