1995 Chevy G20 Galval Sport Conversion
This is my handicap van. Loaded with goodies to help me in my wheelchair get to work, the store etc...
I work at a local Laboratory on exotic embedded applications, so when my Gladiator started acting up (read not starting) I took interest in everything used to control the engine.
Turns out I can remove power to the PCM and clear up the aberrant behavior. I want to know why!
Enjoy!
I wonder if the injector driver in the PCM is having a problem? It would interesting to know if / when you have a "no start" if the injector is spraying fuel? If the injector is spraying fuel, then the next most likely cause for "no start" is an ignition problem. Maybe the ignition module in the distributor is on its way out? The ignition module tells the PCM the engine RPM. No RPM signal to PCM, no fuel from the injectors.
dave w
Under what circumstances do you think clearing the ECM nonvolatile memory would restore functionality?
The pcm may be at fault but the failure mode you describe is very uncommon to me. Generally that would result in fairly thorough testing to confirm the fault lies in the pcm.
What diagnostics have been performed already? How does the no start present? Is there an anti-theft device integrated in this van? Is disconnecting power to the pcm also disconnecting power to another component? Is voltage steady and consistent to the pcm after shut down and during subsquent re-start attempts?
Each failure was during humid, cool weather. There were no error codes using the OBCI pin short to make the ML light flash method. All I got was the PCM OK (12) message. I pulled the ECMB fuse to erase the nonvolatile RAM (per GM shop manual). I am currently in a wheelchair and unable to reach most parts of the engine and therefore not as easy to check many of the checks in the shop manuals. The "no start" pressurized the fuel system (have not checked the actual pressure, just heard the fuel pump stop running) and the engine cranks over and over. If this sounds like an interesting/useful problem to solve, we could move the thread to a more appropriate forum.
The PCM will briefly run the fuel pump when the ignition key is turned to on / start position. To keep the fuel pump running the engine oil pressure must be above 5 psi or the PCM is getting an engine RPM signal of more than 300 RPM's from the distributor ignition module.
The PCM triggers the injectors on by sending "ground pulses" to the injectors. The injectors get 12 vdc when the ignition switch is in the on / start position.
Fuel / Air / Spark are the basics. When the engine does not start, it is missing fuel or spark ... assuming the exhaust and air intake are free breathing.
Connecting a timing light to a spark plug wire might be something you can do when the engine will not start? If the timing light strobes while cranking, we can logically think the engine is getting spark.
Removing the air cleaner can help determine if the injectors are actually spraying fuel when the engine will not start.
I realize you will likely need assistance with some of these troubleshooting steps.
I find it odd that the Van will start after removing power from the PCM. My experience with PCM's is they work or they don't.
dave w
Given that the failure does not occur with with certain frequency, I'm looking at the systems level for a failure mode not defined by an OBCI code. Else I would expect to see the code when shorting the the OBCI plug. It is common for embedded systems to store temporal parameters in NVRAM used to alter functional behavior. If the application is not rigorous, I have no reason to believe it is not, odd behavior can be seen. By zeroing out the NVRAM, the micro-controller program initializes the parametric store to "known" values that can be counted upon to produce exact behavior. If I find a way to reliably reproduce the error, I will perform a more in depth analysis.
I thank you for your feedback, I will keep the forum informed.
For an intermittent problem with unknown but low occurrence frequency I've found it best to design tests and equip the vehicle with the tools necessary in case the problem occurs. I have developed a fairly standard test sequence for "cranks but won't start" on older GM vehicles. I always find it interesting when someone has inadvertently come up with a unique test that appears to point to a strange or uncommon failure. Offhand I can't think of any variables stored in NVRAM which would prevent a restart but I'll be watching to see how this develops.
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