[lonallen]

I have been struggling with a dead ZJ since October and after everything it still won't turn over. It cranks like crazy and gets fuel, but the spark coming out of the coil is weak. When I put a noid light on the coil primary the light flashes as expected when cranking. The light is strong when it is on, which tells me that the power to it and the ground signal from the PCM is good, but the spark off of the coil secondary is hardly noticable. This is my second Mopar coil so I tried an aftermarket one today and it is weaker than the Mopar one.

I can't figure out what the issue is. I cleaned all the grounds last weekend. I cleaned the metal mount that the coil attaches to (at the point where it is bolted to the engine and where the coil connects to it). The ground on the fender is good an clean. Where the ground cable connects at the coil mount is clean. Since the starter gets its ground from the engine I have to assume this ground point is good since the starter cranks the engine nice and strong.

My wife is ready to leave the ZJ on the side of the road to get rid of it except we would have to push it there.

I am baffled and completey confused. New CKS, cap, rotor, wires, coil, MAP sensor, fuel pump, fuel filter, new battery terminals, cleaned grounds...

No codes presented when checked with ON-OFF-3x... ignition method.

Help wanted...

Lon Allen -

94 ZJ 5.2L 327K miles...

 

[Bob Lincoln]

How are you determining that the spark is weak from the coil secondary? What is your exact method? If you are grounding the coil wire to the block or head, you can easily damage the power control module. You should use an inline tester with adjustable spark gap to evaluate spark strength.

 

[lonallen]

I don't have an inline tester with an adjustable spark gap. I am just using a screw driver touching the coil secondary terminal and putting the tip of the screw driver close to the ground terminal at the head (without touching). I would expect that if the output were normal the spark across the gap would be rather large (for several Kvolts). Instead the spark that is being produced is very small and hardly noticeable. I'll have to look into getting a tester, but I do not think I have caused any damage to the PCM. It has been doing this since I started troubleshooting this issue. I originally just pulled a spark plug and grounded it to the block to look for a spark. The spark produced here is non-existent. This is why I started testing at the coil output.

 

[Bob Lincoln]

Not safe to ground the coil wire or center dist wire to the engine - it can damage the MOSFETs in the PCM. Grounding one spark plug wire is safer.

A weak spark with two different coils and known good wire connections suggests that the PCM may be damaged. You are not getting enough current flow.

An inline tester is under $10 and has an adjustable gap so that you can get a rough guess of the secondary voltage by how far the spark jumps vs the markings on the tester.

 

[lonallen]

Understood. I will get one of these testers. As for the PCM, I think it was the problem to begin with. The vehicle went from running perfectly, to starting to stumblem, to just startable and then nothing. Like something deteriorated little by little (I am betting the caps in the PCM failed). I started replacing parts out of despiration. Sadly I spent about as much as a new PCM in replacement parts. The PCM is supposed to be under warranty so I am checking on a replacement.

Thanks,

 

[lonallen]

I pulled the PCM last night to ship to my vendor. I removed the plastic cover to have a look at the silicone covered PC board and was surprised to find that the silicone was split open everywhere and falling apart. Looked like a drived up river bed with all the gaping cracks. Some of the components are exposed to the elements. I feel more confident that the PCM might be the problem.