Bad 89 Ignition Module


New member
I'm pretty sure I know the answer to this, but I want to confirm. The patient is an 89 MJ 4.0l that cranks but doesn't start.

100% sure its not the CPS. It measures good at 197 ohms, and I'm getting about 0.5 volts a/c cranking. I have voltage pulses at the injectors (also confirms good cps).

I have a tach signal showing up at pin D1 on the small diag connector when cranking. On a dc voltmeter, its bouncing around mostly 10 volts. This pin comes from the ign module, so the ignition module it getting signals from the ECU. Cap and rotor are 5-months old ans still look good. Still no spark

Sound like a bad ign module to you? I can't tell, but can you replace just the coil, or do you replace the whole thing?

88 Wagonman

New member
Pull the wire off of the Dist cap to the coil and check spark there, if there is no spark there then check the connections to the module to see if they are clean, if still no spark I would say module.


New member
Use a hair dryer to heat up the module specifically - ICM failures are usually heat-related.

You'd need an O-scope to properly measure ICU output voltage - it's "pulsed DC," and a DCV meter won't know what to do with it (it's 12VDC in a rough square wave. The drops between pulses are what makes it look like 10VDC - you can tell something is there with a DCV meter, but you won't know for sure what.)

The ignition coil is retained with two screws into the module. I'd suggest removing those screws, heating up the coil, then dropping it in place and holding it down while a helper tries to start. If nothing changes (you can still read some voltage, or you get spark,) cool the coil off and heat up the module. Repeat.

I've isolated a couple of failed RENIX ignition problems this way - one was a coil, and one was an ICM.


New member
I walked in from discovering you can separate the coil from the module. Low side of the coil is shorted. High side has 6 ohms. Looks like I'll be replacing the coil and hoping it didn't kill the ign module. I'm guessing the low side should have been in the neighborhood of 3 kilohms.

I was just using the dvom to look for "something" that indicated tach pulses. My next step was to drag the o-scope out into the driveway. :}

Hmmm. I think I still have a few motorcycle coils out in the parts bin that might work temporarily..... Or maybe I'll graft that extra EEC-IV coil pack on there instead. Or maybe just stop by the pull-n-save with my torx screwdriver and grab one for $3.