icm has been replaced and still nothing. cleared the codes and everything is installed properly. wouldnt start and only threw the h41 code no 62. i still only have .2vAC on my signal wire which the fsm says is a bad pcm or pcm connector since my continuity is good. i have never touched the pcm or any of its connectors when i switch optis so im utterly confused why replacing it has fixed my problem. coil has been replaced with known working coil.

 

Well, I'll be dipped in ****. H41 is almost always the ICM. This is the first occurance where it wasn't.

I went out to my 92 and tried these tests:
> Disconnected ICM, rigged my VOM to cavity B and ground and cranked it. I saw 2.0 VAC while cranking. It also set H41 in Module 4, no codes in Module 9. I cleared the codes.
> I rigged Cavity B to Ground. I cranked it for about 5 seconds. This set H42 in Module 4, no other codes. I cleared it.

I wanted to know A: What the AC voltage is during cranking, and B: I wanted to be sure that the ECM responds to open or grounded circuit with the appropriate codes, which it did.

H41 is open circuit, H42 is shorted circuit.
> Try grounding the white wire at Cavity B and crank it. See if you get Code H42. If = Yes, the PCM is seeing the difference, responding with the appropriate code, and therefor the connections and wire to the ICM connector is intact, and the processor is running the diagnostics correctly.
> Is there any possibility that your ".2 VAC" is actually 2.0 VAC and your meter is not scaled correctly? (I had exactly 2.0 VAC.)

Neither of the above will suggest why it doesn't start, but may provide input to solve the mystery.

I am really surprised that the ICM wasn't "it". Sorry.

 

Dont worry about it! im glad we eliminated that from the list of causes. the list is narrowing down by the day. ive got my battery on a tender but tomorrow ill be back and ill run all those tests n respond with the results. the voltmeter im using now is a crappy autozone special i couldnt find my good one...well i found it tonight so ill run the vAC test again and see if my .2 is really a 2. the autozone special doesnt have options for ac it just has one mode unsure of its scale. my good one i can select the scale. if my .2 is actually 2 ill follow the fsm flow chart to the remaining steps and report back the results.

 

did a bunch of tests this morning these are the results. my .2vAC was in fact 2.1 vAC scale was off on the cheap meter. so i moved onto the next test. test light on battery positive and terminal c to test for ground and the light came on. next test was test light to ground and terminal a, light came on. then ground and terminal d, light came on. so now we know a has 12v, b is getting a proper signal from the pcm, c is a good ground, and d has 12v everything should be good. the fsm says its either a bad icm (just replaced it so highly unlikely) or bad icm connection. not sure if that means i should repin the 4 wires as maybe theyre not making a good connection with the icm pins?

I tried your test and could not get h42 to appear only h41 every time. i also couldnt clear the h41 code like i normally would. i pressed eng/met on 4.7 until three lines appeared, shut car down, waited 10 seconds, then turned the key to on. h41 still appeared. i had to disconnect the battery for a minute then connect it back to clear the h41 code unsure why a history code would remain after clearing

Let me know how to proceed. im going to try your ground connector b test again and see if i can trigger that h42 code. a replacement 4 pin connector is only a few bucks and some time repinning but that seems like the logical next step

 

You're making progress. You're eliminating possibilities. Verifying that you indeed do have the pulse from the PCM is huge.

It looks to me that you should concentrate on the connections at the ICM connector. Particularly Pin B, the PCM control circuit. I know you should be able to force Code H42 by grounding Pin B, because I did it yesterday. That you can't, and still get H41 (open circuit) suggests that that terminal, wire, circuit etc is open-circuit. I'm aware that you tested continuity to the PCM and "it was there", but still, your recent testing suggest that something is still amiss. That you can read the 2.0VAC, but can't get the PCM to recognize the same circuit as grounded doesn't make sense. But that you only get H41 suggests that the circuit is open.

I'm pretty sure the ICM connector is a "pull-to-seat" Metripack 150.2. These terminals aren't serviceable. You probably have to replace the ICM connector pigtail.

 

I've had two instances of a terminal fracturing inside the connector block. These were tough to find.

> Remove the connector. While observing the terminal side, gently push/pull the wire in and out. Does the terminal 'follow' the wire? If you can move the wire, but the terminal doesn't move, it has broken internally.

I agree, a terminal connection problem at the ICM is likely. Replacing the opti also disturbs the ICM and connections. So when you replaced the opti, you re-established connection at the ICM and it ran again. Until the connection again shook loose. << This is my wild *** guess for now as to why "replacing the opti" made it run again.

 

Wow what a day. checked the pins on the icm connector and 3/4 were stretched out to hell. sat there with a pick and bent everything back to shape. plugged it in first start no problems. no codes fired up just fine. now of course something else had to break. harmonic balancer seems to have backed off the hub so im buying new bolts and gonna loctite em today hopefully that fixes that problem. but all in all so glad this ignition nightmare of 2+ years is finally over! Thank you all for your help!

P.S. if annyone knows the gm or arp part number for those 3 harmonic balancer bolts thatd be greatly appreciated. cant seem to find them online it keeps trying to show me the center bolt which i do not need

 

Hallelujah!

Sometimes it is the simple stuff. BUT. You won't soon forget all that you learned from this experience. Glad it's done.

 

yea exactly thats why i bought this car. i was 18 and didnt knew absolutely nothing. now i know slightly more than absolute nothing LOL. its been a joy over the years fixing this thing n building her up. again thanks for all your help!

 

Really glad you tracked it down, finally! IHBD's statement reminds me of when I was 20 or 21 and bought a 1989 Mustang LX notchback 5.0 from the Texas Highway Patrol. It was the nicest one and lowest mileage of any at their auction (they had 15-20 to sell), by a lot. It had a new battery and alternator, too! Should have been a clue, right? Bought for $4800, only a couple hundred higher than the others went for. Drove it home that evening from Austin to College Station (I are an Aggie). A mile or two from home, every electrical thing on the car shut down: it was obviously running on battery alone and not charging the battery at all. Got it home, got a friend with access to the FSM to help me trace wires. Turns out that the wire from the battery to the field magnets of the alternator (they have to get 12v or the alternator won't charge) had a break of its conductor inside the insulation, but the insulation was intact and hiding the break. As soon as we jumped across that break, it started charging again. Spliced in a 2" piece of wire and it was fixed for the price of free. Highway Patrol sold off a Mustang way early and in too good a condition because of a "simple stuff" problem that wasn't simple to troubleshoot...but two college boys solved it in an apartment parking lot. I drove and raced that car for 5.5 years and put another 110k miles on top of the 75k it had when I bought it. After all that, I sold it for $5000, which was $200 than I paid for it. Best deal I'll ever make...

 

I read this whole thread. Tons of good tech info in here. The whole time as I read I kept thinking it has to be the harness. But it wasn't. Its great you got it fixed!

It reminds me of when I built my 96 LT4 and installed the motor and it wouldn't start and its giving me codes that didnt make sense. I start testing wires and turns out the plug for the MAF and the Crank Position sensor are the same and I had them switched. I had the car apart for a year so it was likely something would be off. Switched them and off it goes! Been a wild ride ever since. These cars are fun and they will make you learn how to be a tech real fast!