A Sure Diagnosis?

Is there such a thing?  With this vehicle and time there was.   This is a 1994 Buick LaSabre with a 3800 cc engine that came in with the MIL on.   The light would come on immediately every time the engine was started and remain on.  The code stored was a P1361.  This code is similar to a code 42 on the earlier year vehicles. 

My first test was to retrieve a dual trace waveform of  Bypass and EST on startup.  I wasn't sure what to expect so I chose the Bypass signal  to trigger on with a positive slope. 

As you can see, bypass went high for about 1/2 a second and dropped low with some kind of unexplained activity.  EST never went high but there appears to be some kind of activity during the period that bypass went high.  At this point I would guess at saying that EST isn't be sent to the ICM.  What are the possibilities?   PCM  not sending an EST signal, EST wire open somewhere between PCM and ICM, EST wire grounded somewhere between PCM and ICM or ICM bad grounding EST signal.  What about the EST activity?  Isn't that an indication of EST being held low by bypass?   Well, no.  Here is an image of a proper EST waveform while being held low by bypass.  Notice the difference in the amplitude when EST is low.

Now where do we go from here?  I went to the PCM connector and decided to take some resistance readings of the EST circuit.  This should narrow down the above possibilities.  What I was looking for was about 200 ohms while bypass was low and a switch to a high resistance with bypass high.  I switched bypass with a test light connected to battery positive with the ignition switch on. 

Notice the difference in the resistance readings with the different scaling.  When I used a low scale (50 Ohms/div) the readings are closer to what they should be but I couldn't get a complete view of the sequence of events.  With the high scale (5 KOhms/div) I have a complete view of the sequence of events but with inaccurate measurements. I think this will hold true for any digital meter (including a Vantage).  Why is this?  This is because the EST circuit has a very small voltage that distorts the resistance readings.

Getting back to diagnosing this failure.  By looking at the resistance measurements and switching I know that the ICM is switching and is not pulling EST to ground.  I also know that the EST wire isn't ground or open so that leaves a PCM failure.  Right?  Am I certain?  Well not quite.  I have one more procedure and I will be certain the PCM is bad.  I don't know if I should reveal this diagnostic procedure being I have used this procedure with success and as far as I know no one else knows about it.  Ok, I will give you a hint.

Seriously, use this procedure with caution.  You do not want to end up buying two PCMs.

I also discovered on these vehicles bypass will go high a lot sooner then the earlier models vehicles that used code 42 as the failure.  Instead of switching bypass high at about 400 rpm, the PCM will switch bypass high on the second or third fuel control (3x) pulse it sees.  In other words, almost instantly.

If you are interested in posting an article or case study on this site, please feel free to contact Jim Wilson.