Intermittent no-start , no ECM connection... need ideas
10-07-2018, 12:05 PM
#1
Registered User
Thread Starter
Join Date: Mar 2000
Location: Eastsiiiiide , NH, USA
Posts: 112
Intermittent no-start , no ECM connection... need ideas
this only happened once, but I don't believe in good luck so it will happen again 
1996 formula btw... I've had multiple 4th gens so I'm not new to the platform.
I drove the car to work, check gauges light came on (for no real reason?), I shut it off in the parking lot and it would not restart. It would crank fine, but not fire (no sputtering either). I had the car towed home and 4 days later when I went to work on it it fired right up and has been starting fine ever since... only clue I can give you is that when it was not starting I could not get a connection through OBDII port, I would just get "no link" as if it's not even there, now I can read the ECM fine. The only tool I had in the car was the OBD reader so I can't confirm whether there was spark or fuel etc... but with computer being offline I would imagine it had no spark.
Any idea what would cause that? I think I can rule out things like crank sensor or maybe even opti, I've had more than few bad optis and usually there are codes of some kind and computer can be read... I also think the ICM or coil could be ruled out right?
what gives power to the EMC/PCM? some relay I guess? could that be flaking out? if anyone has a wiring schematic for the PCM I'd love to have it.
1996 formula btw... I've had multiple 4th gens so I'm not new to the platform.
I drove the car to work, check gauges light came on (for no real reason?), I shut it off in the parking lot and it would not restart. It would crank fine, but not fire (no sputtering either). I had the car towed home and 4 days later when I went to work on it it fired right up and has been starting fine ever since... only clue I can give you is that when it was not starting I could not get a connection through OBDII port, I would just get "no link" as if it's not even there, now I can read the ECM fine. The only tool I had in the car was the OBD reader so I can't confirm whether there was spark or fuel etc... but with computer being offline I would imagine it had no spark.
Any idea what would cause that? I think I can rule out things like crank sensor or maybe even opti, I've had more than few bad optis and usually there are codes of some kind and computer can be read... I also think the ICM or coil could be ruled out right?
what gives power to the EMC/PCM? some relay I guess? could that be flaking out? if anyone has a wiring schematic for the PCM I'd love to have it.
Last edited by Zrout; 10-07-2018 at 12:56 PM.
10-07-2018, 03:39 PM
#2
Administrator
Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 70,648
Re: Intermittent no-start , no ECM connection... need ideas
We’re there any codes when it started again?
Sounds like the PCM briefly lost power or ground - no fuel, no spark, no nothing, codes cleared. Check Gages light might have been indicating sudden low voltage. Get the 96 factory service manual (free download link below) and check the fuses (loose?), ignition switch, and connectors that supply power to the PCM. There's a direct link to the battery (red + box near battery) that has a fusible link at the box, and the PCM BAT fuse in the box under the hood. That provides power to the volatile storage in the PCM. Then there's switched ignition power through a fuse in the panel at the end of the dash. The ignition switch itself could be developing problems. Burnt contacts and melted plastic housing parts seem to become more common on these 20+ year old cars.
Couldn’t possibly be the crank sensor. That was only added in 1996 for OBD-2 misfire detection, has nothing to do with how the engine runs.
https://www.mediafire.com/?40mfgeoe4ctti
Sounds like the PCM briefly lost power or ground - no fuel, no spark, no nothing, codes cleared. Check Gages light might have been indicating sudden low voltage. Get the 96 factory service manual (free download link below) and check the fuses (loose?), ignition switch, and connectors that supply power to the PCM. There's a direct link to the battery (red + box near battery) that has a fusible link at the box, and the PCM BAT fuse in the box under the hood. That provides power to the volatile storage in the PCM. Then there's switched ignition power through a fuse in the panel at the end of the dash. The ignition switch itself could be developing problems. Burnt contacts and melted plastic housing parts seem to become more common on these 20+ year old cars.
Couldn’t possibly be the crank sensor. That was only added in 1996 for OBD-2 misfire detection, has nothing to do with how the engine runs.
https://www.mediafire.com/?40mfgeoe4ctti
10-08-2018, 03:27 PM
#3
Registered User
Thread Starter
Join Date: Mar 2000
Location: Eastsiiiiide , NH, USA
Posts: 112
Re: Intermittent no-start , no ECM connection... need ideas
nope no codes (I wish) , my reader is sort of dumb so I can't tell if the EMC got reset although that would be nice to know.
IDK if I can suspect the ignition switch/cylinder, it was cranking fine, I would think at that point the ECM would have power?
thank you for the link, should I replace all relays just to be safe? but again, would a relay allow crank and no start? I really wish I had a chance to troubleshoot it further to see if it was no spark or no fuel and if ECM had power, but by the time I looked at it it fixed itself :-/ The fuses I pulled while I was waiting for a tow truck, I will replace the related ones just in case as one looked little shady, but I did test it at home and it is still fine, can't hurt to replace them I guess.
IDK if I can suspect the ignition switch/cylinder, it was cranking fine, I would think at that point the ECM would have power?
thank you for the link, should I replace all relays just to be safe? but again, would a relay allow crank and no start? I really wish I had a chance to troubleshoot it further to see if it was no spark or no fuel and if ECM had power, but by the time I looked at it it fixed itself :-/ The fuses I pulled while I was waiting for a tow truck, I will replace the related ones just in case as one looked little shady, but I did test it at home and it is still fine, can't hurt to replace them I guess.
Thread
Thread Starter
Forum
Replies
Last Post
ForcedInductionPonti
LS1 Based Engine Tech
9
12-22-2008 10:22 AM