Chris Luongo

Hi everyone. Typing from my phone in my dead car in a restaurant parking lot! I have limited tools with me but I've tried a few things.

1991 Trans Am convertible. 305 TPI 5-speed. All stock.

Drove fine on the way here, now cranks and doesn't start.

--Has fuel pressure at fuel rail.
--Does NOT have spark on plug wires.
--DOES have spark coming from coil.

I had some hope of maybe a cracked rotor or distributor cap, so I took an Uber to buy a few hand tools and a new cap and rotor.....old cap and rotor look OK, and even after replacing those, still no change.

I dont have any electrical testinf sting equipment with me, but all fuses good including the 30-amp by the battery.

Tryin to get it towed home now, but what's next? I guess the ignition module at the base of the distributor fails a lot in these cars? Could that be it? What's the best way to diagnose it?

OrangeBird

iTrader: (1)

If your 100% certain this is true , then your spark is most likely either weak or badly out of time , since you say you tried a new cap & rotor with no change in the malfunction . Yes indeed the ICM is a prime suspect but the coil itself could be at fault , if it does end up being the ICM buy some good thermal paste at a computer supply store for the new ICM since the stuff that comes with a new one is garbage compared to the stuff the computer stores sell .

NoEmissions84TA

iTrader: (1)

Does this type of ignition use a HV coil wire?
You said you have spark at the coil, but not at the plug wires.
I would have suspected the rotor, but since you changed the cap & rotor, what's left?

GeneralDisorder

iTrader: (1)

The spark must be going somewhere. It's either arcing somewhere, or it's out of time. I've been told by old timers that if the timing chain is going to jump it will do it on shutdown and then not start. Probably has something to do with the chain going slack just at the right moment of rotation when the engine is slowing.

GD

zed-028

ICM or the pickup coil in the dist is shot.

ICM is easier to change out, pickup coil is a PITA - just replace the dist

blacksunshine'91

iTrader: (10)

I had the exact same situation happen to me. Was a bad coil.

zed-028

you could be onto something there, as with another post further up, you might have a hairline crack on the casing of the coilpack, spark might be arcing down to the mountings under any real world load.

Tootie Pang

iTrader: (2)

ICM

Chris Luongo

I want to thank everyone for all the responses.
Managed to get the car towed home. I read some of these posts before bed but I was pretty tired.

I should note that after I replaced the cap and rotor,
when I cranked the car it would almost fire up but not quite.
A couple of times it would even seem to run extremely rough for about one second and backfire from the tailpipe.

Hopefully now at home, with better tools, I can get this thing figured out.

I'm going to start from scratch, since maybe I wasn't thinking clearly yesterday-----I'll test fuel pressure with a gauge this time, as well as testing for spark at the coil and at the plug wires, and report back.

Thanks for the tip on the thermal paste.

Yes, the coil is separate from the distributor, with the typical short "plug wire" style wire between the coil and the center of the distributor.

I'm with you on the 'what's left' thought. One thing I read on another forum is that if the timing is way way off, the coil could be firing at the moment the rotor is between two of the spark plug wire connectors, meaning no spark would get out to any of the spark plug wires.

Hmm. Makes sense. I'll have to look up how to check for a bad timing chain. Car has 56k miles but that doesn't mean it couldn't have failed. I agree with the "spark must be going somewhere" too.


Do you know how to diagnose one versus the other?

It's daylight now, but if I can't figure this out by nightfall, I'll crank the starter and look for stray sparks.

Chris Luongo

Two basic questions, guys, if any of you know.

Is it possible for the ICM to be bad, and the coil still produces spark?

Is it possible for the pickup coil to be bad, and the coil still produces spark?

NoEmissions84TA

iTrader: (1)

If the ICM is bad, being an electronic part, it is either good or bad, usually NO in between. This is the easier of the 2 parts to change. If the pickup coil fails, that could drive you crazy trying to diagnose it, because it could randomly work - or not. Not easy to change - buy a new distributor.
My 95 GMC truck has the same ignition system as yours. When the truck was nearly new, the coil wire at the coil end was corroded. Fell apart when I pulled the wire. Pickup coil in the distributor failed a maybe 8 years ago. Replaced the distributor. Maybe 2 years ago, the engine started skipping, but would recover. One night on the way to work in a rain/snow storm, it died. Very hard to diagnose. Turned out to be the rotor. Cheaper rotors arc out internally to the center post of the distributor. Almost impossible to see. You have spark at the coil wire, so spark goes into the cap, but never comes out. I carry a spare rotor now.
Test your ignition coil with an ohmmeter.
A shot of ether can quickly test for lack of fuel.

brianlibby791

Definitely the ICM, my coil had spark but got no spark after the distributor, replaced the ICM and the car cranked a bit and started right up.

NoEmissions84TA

iTrader: (1)

Great!

Chris Luongo

I wanted to come back and report results, and also thank everyone for their help.

Turns out it was the ICM. After installing a new one from AutoZone, the car started and ran perfectly on the first try! (When I can, I try for OEM parts and/or discount sites like Rock Auto. But the car was taking up two parking spaces on the street, so AutoZone it was this time around. $48 and lifetime warranty.)

The ICM that was in there had the GM logo, and the car only has 56,000 miles, so it was probably original. The heads of the bolts were rusty, but they were still good otherwise, so I cleaned them up a little and put a little anti-seize when I put them back in.

In case anyone finds this in the future while doing a search, I found a really well-written article about diagnosing a bad ICM. Well......it doesn't actually show you how to diagnose the ICM itself, but it shows you how, after making sure the wires/cap/rotor/coil are good, then you test the pickup. If the coil and pickup are good, the ICM then must be bad.

http://easyautodiagnostics.com/gm/4....ed-icm-tests-1

If I had to do this again, I might have just replaced the entire distributor---it's not much more money. The cap and rotor were $37, and the ICM was $48......and a whole new distributor, which includes all these parts, was $118.

Also, take note that the ICM requires a 5.5mm socket. I happened to have one, but not all tool kits have this. You can use the same 5.5mm to replace the distributor cap, if you'd like. And don't forget you'll need a Torx bit to remove the decorative plastic cover along the TPI runner, before you can get to any of it.

So, after getting the car running, I was able to give it a wash, and get it back in the garage and on the battery tender, ready for the next nice day.

Thanks again for all the help, everyone!