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91 Chevy Silverado 5.7 standard cab
trouble started with a little stumble here and there, Then stalled at light. Then stalled while driving. No spark, no start, 1/2 hour later started and limped home. From that point forward no spark, well once in awhile, just randomly it will start, but after turning off no spark.
I’ve changed the distributor, coil, checked all the grounds, no vacuum leaks.fuse checked.I have 12v at the coil, but there is no continuity on the white. I’ve read it could be Ignition relay, or i ignition I switch. I don’t know how to Test em.
Which white wire are you talking about - the one in the 2 wire harness between the ignition module and coil? Or the white wire in the 4 wire harness that plugs into the ignition module?
Also did the new distributor come with an ignition module or did you swap the old module?
In the primary harness along the firewall, you’ll see a tan/black wire with a connector (you may need to remove some tape or harness wrap to find it) - this is the spark bypass circuit. Unplugging it removes ECM control from the spark circuit. If you have spark with that wire unplugged, first double check that the base timing of the distributor is set correctly - manually set the #1 piston to TDC on the compression stroke and check that the ignition rotor is pointing at the #1 tower on the distributor cap.
Hi Gumby, the white wire I was refers to was the connector from the firewall/fusebox (I assume) that connects to the coil. I checked the two wires from the coil to the icm, they are good, after that I don’t know what I’m looking for.
I learned my lesson last time, I bought the whole distributor. I still keep a spare pick up coil and icm for testing.
I did a top dead center, with the little advance connector undone, but I wanted to do it again, with my sons help, just to make sure I did it right.
Thanks for the help Gumby22, eliminated something’s at least.
Originally Posted by Barak Reis
91 Chevy Silverado 5.7 standard cab
trouble started with a little stumble here and there, Then stalled at light. Then stalled while driving. No spark, no start, 1/2 hour later started and limped home. From that point forward no spark, well once in awhile, just randomly it will start, but after turning off no spark.
I’ve changed the distributor, coil, checked all the grounds, no vacuum leaks.fuse checked.I have 12v at the coil, but there is no continuity on the white. I’ve read it could be Ignition relay, or i ignition I switch. I don’t know how to Test em.
That’s the signal wire to the tach. If that wire is shorted to ground, it’s grounding the coil and will prevent spark.
Check for continuity to ground with the connector unplugged - I can’t remember if it should be completely open, but I’d guess if it’s got less than 5 ohms resistance, it’s shorted. Cut the wire a few inches from the coil and check spark again - if it’s present, find and repair the short in the tach circuit or run a new wire for it.
The 4 wires that connect to the other side of the ICM are all to/from the ECM. I’ll have to do some more digging on what voltages you should see on them.
[weird day Gumby, can’t just not work on it lol, traced that white wire back to ecm, and your right the other white is the tach. I looked at the back of the fuse box, nothing, made sure connector was in ecm, really just grasping at straws.
Then I had my sone turn it over while I wiggled the connector, boom it starts, we had done the tdc, so we timed it. Sounded beefy, I grab my wallet to go for a test drive, i put it in park cuz it was still in neutral from doing the tdc, rev it a couple times and it dies! You guessed it no spark. I can only think of 3 things, ecm, ignition switch, or electronic spark relay. I have no idea how to test any, lol. Just reading other poor unfortunate souls problems.lol. Thanks for the help. I know a bit more, or a least u confirmed what I thought.
QUOTE=Gumby22;460739]That’s the signal wire to
the tach. If that wire is shorted to ground, it’s grounding the coil and will prevent spark.
Check for continuity to ground with the connector unplugged - I can’t remember if it should be completely open, but I’d guess if it’s got less than 5 ohms resistance, it’s shorted. Cut the wire a few inches from the coil and check spark again - if it’s present, find and repair the short in the tach circuit or run a new wire for it.
The 4 wires that connect to the other side of the ICM are all to/from the ECM. I’ll have to do some more digging on what voltages you should see on them.[/QUOTE]
If wiggling a connector got it to start, I’d recommend cleaning and tightening the terminals before any further troubleshooting. If they’re not heavily corroded, a thin layer of dielectric grease should help as well.
Was it the connector to the ECM or ignition module you wiggled?
Poor terminal contact can be very frustrating to diagnose sometimes because it can be triggered by seemingly nothing.
Just a couple weeks ago I was trying to diagnose a totally random problem on a Grand Prix GT. It would suddenly revert to reduced engine power with no warning and regardless of driving conditions. I did live and dead checks (including wiggle tests) on every related circuit and found nothing. A service bulletin pointed to a connector as the likely cause, but even focusing on the connector didn’t reveal any clear problem. I finally decided to temporarily splice the wires and bypass the connector as a way to test if it stopped - so far no problems, but I’ll have to wait and see if the customer brings it back to know for sure.
Also if any pests/rodents have nested in the truck, thoroughly inspect the nearby wiring for damage. A harness can look OK on one side and have chewed insulation on the other.
That’s a really good idea Gumby, temp the circuit, I was also thinking maybe the connector between the icm and coil needs to be looked at closer. It is pulled really really tight, not sure of design there, but it could be showing connectivity when testing on bench, but when pulled tight it shorts out. Well it’s Working on my truck on a Sunday again, what more can a man want.
When I was younger I had a Ford Focus svt, fun car, 6 speed, anyway I could never find this little glitch, it was sporadic but would reset my computer and pull a code, only time I ever took a car to a dealer to diagnose, $275 later and it was a wire and connector in the alternator connector that had come undone, but the wire was still in it, so when you tested it the wire would push back into connector. They put one of their big machines on it. I do have to say I would have never found it.
QUOTE=Barak Reis;460742][weird day Gumby, can’t just not work on it lol, traced that white wire back to ecm, and your right the other white is the tach. I looked at the back of the fuse box, nothing, made sure connector was in ecm, really just grasping at straws.
Then I had my sone turn it over while I wiggled the connector, boom it starts, we had done the tdc, so we timed it. Sounded beefy, I grab my wallet to go for a test drive, i put it in park cuz it was still in neutral from doing the tdc, rev it a couple times and it dies! You guessed it no spark. I can only think of 3 things, ecm, ignition switch, or electronic spark relay. I have no idea how to test any, lol. Just reading other poor unfortunate souls problems.lol. Thanks for the help. I know a bit more, or a least u confirmed what I thought.
QUOTE=Gumby22;460739]That’s the signal wire to
the tach. If that wire is shorted to ground, it’s grounding the coil and will prevent spark.
Check for continuity to ground with the connector unplugged - I can’t remember if it should be completely open, but I’d guess if it’s got less than 5 ohms resistance, it’s shorted. Cut the wire a few inches from the coil and check spark again - if it’s present, find and repair the short in the tach circuit or run a new wire for it.
The 4 wires that connect to the other side of the ICM are all to/from the ECM. I’ll have to do some more digging on what voltages you should see on them.[/QUOTE][/QUOTE]
The tension on the wires is due to the wiring shrinking over time because of several thousand thermal cycles. A complete replacement harness is available for the 2 wire lead - or you can just splice in a few extra inches to the wires. Sites like rockauto.com or picowiring.com have pigtail assemblies ready to be spliced in.
I’ve never considered the factory mounting location of the coil as a good one because it has no protection from the heat of the engine, and few things will kill a coil faster than heat. I’d recommend relocating it to the firewall or fender - there are remote mount kits available but it’s really nothing more than lengthening the control wires and getting a longer coil-to-cap wire - easy enough to do if you know how to use wire cutters and crimpers. The mounting bracket can just be transferred to the new location.
Well Gumby she continues to confound. Lol. Did what ya recommend, cleaned everything all up, did a double check on the tack wire and coil. My son and I wiggled that thing on and off just like yesterday and it would not get any spark. Just confused.
We came in watched the race, and worked on some projects.
My buddy, an ol hot rodder has a guy he uses when he can’t figure something out. That’s rare, but I’m dumbfounded. Perhaps the computer is just done, but that’s not in my wheelhouse.
Ima keep trying until I get ahold a him though. Thanks for all the help. Learned some stuff.
Are you checking for spark at the coil or from one of the plug wires off the cap? If the new distributor cap or rotor is bad for some reason, maybe spark is present but not making it past the cap. Just a thought.
I’m still trying to find more specific info on the signals between the ICM and ECM, but have you tried checking for diagnostic codes? Even if the codes stored don’t seem to have anything to do with spark control, sometimes they can help point in the right direction.
The OBD pinout is below - all you have to do is jumper pin A to pin B (or pin B to ground if the jumper doesn’t seem to work) and then turn the key on, engine off, and count the blinks of the SES light. Each code will repeat 3 times before the next code is displayed, and code 12 (system pass) will be the first code regardless of any other stored codes.
This sequence reads out code 12 once the key is on. After the 3rd display, any other stored codes will blink out. They will all be 2 digits - even if you don’t know the definitions, post them and I can look them up.
February 18th, 2021, 11:30 AM
No spark
