zupmanZ28

Ok, so heres the chain of events so this might a little easier to diagnose. For the past couple days everytime i accelerate anything other that very very lightly, my aftermkt tach jumps a couple hundred rpm, like a glitch. My factory tach, which doesn't even have a signal going to it, also jumps up a few hundred rpm. My Fasten seatbelt light comes on as well, which also shouldn't have a signal (i have harnesses, factory belts are gone).

When im on it fairly hard, the engine begins to miss really bad when this happens. Well today i get on it, the belt light comes on, both tachs jump around, the engine misses really hard, and smoke starts pouring out from under the passenger side dash. I get home take the ECM apart and find a blown up component (pictured below, labed C15).

What boggles my mind is that the ecm doesn't even do anything anymore... The car has a 355 with old style vac. HEI, and a Speed demon. It has no control over the engine at all, or so i thought.

How the hell do i fix this? can i just leave the ecm unplugged? I'm assuming the battery lead to the distributer is still connected to the ecm, and thats what is causing the miss. Can i rewire that, unplug the ecm, and be ok?

Please help me out with the quickness, i'm going to the track tomorrow!!!

Eric

zupmanZ28

by the way, this is an 86 z28, originally an lg4 t-5 car....

the top casing on the ecm has a label that says:

SERV. NO. 1227169 ANW
860631 M523517594


theres another label with a barcode that reads:

16060631

and under the barcode it reads:
860631M523517594

Vader

Zup,

I've seen a few of those before. The component with the hole in it is an MOV (metal oxide varistor). That usually happens when the system goes overvoltage. The clamping voltage of the MOV is 17.8VDC, but they can only pass about 260 Jou;es (very little transient energy). I;m guessing that the system is generating high transients and the ECM could no longer handle it, thus the blown MOV. I've got a small supply of them from repairing other "blown" ECMs. The other problem is that the 5VDC regulator usually takes a dump along with the MOV, as well as a few other non-critical components.

If your car is not using the mixture control solenoid, EST, or electric fans, you really don't need the ECM.

The reason the tach "jumps" is probably also the voltage transient, as well as the indicator lamps and warning buzzers, gauges, etc. One of the first things I'd check is the alternator output. You may have a shorted diode in the rectifier bridge, and such damage can show up as AC voltage at the alternator. You may also have poor grounding.

Another possibility is the ignition coil and system. Are you using a CD box like a FireBall or MSD? Is the coil well-grounded?

RB83L69

You know, everything electronic works on smoke. As long as the smoke stays in, it works; let the smoke out, it stops.

All you have to do is catch all the smoke and put it back in, then it'll be OK.


All right, I'm not ready to quit my day job.

Like Vader said, it's a victim of another problem, probably the alternator. I'd check the wiring to the voltage regulator, which is the short red wire that goees from the big terminal over to the plug. Make sure it's all plugged in good and everything is tight, and the wire isn't broken or anything. If you still see the transients, replace the alt.

You can clip the MOV's leads and the ECM will probably start working again, but the next time a transient comes along, it will toast the ECM instead of the MOV. If you have any soldering skills you can buy one of those from Newark or wherever and change it out.

zippy

that's not litteraly blowing up as an ecm isn't explosive. by the way it sounds, the ecm wasn't the root of the problem either.

zupmanZ28

its interesting that the alternator might be suspect, i did notice tonight the headlights were dim, and my power windows had trouble going up. The car has been starting a little harder lately too. i'll have to stick another alternator on it tomorrow and see if that does anything.

I talked to my friends dad tonight, hes an engine builder, and he's pretty well known to know his sh*t, and he said it could be the alternator itself is going bad, or i have a plug wire grounding out. I checked to see that all of the plug wires were in good shape, but i did not check to see that i had spark on all cylinders.

Can a distributer just up and go bad?

Eric

Edit: i am not using any kind of CD ignition. Only thing different from stock is a pertronix coil and module. (already checked both)

Vader

True. This one let out much of its smoke all at once - the stains were still on the driver's seat:



See how the MOV is always the first to know? Funny, how the common failure mode is supposed to be open circuit, but it never seems to work out that way in an ECM.

zupmanZ28

So are we all in relative agreeance that the alternator is most likely the culprit, and this is causing the miss???

Sounds good to me, just wanted to make sure.

Eric

c4boom

yup that would be it really it is the rectifier in the alt but ya they are one peice these days

Vader

Zup,

Before you go wasting time replacing the alternator, I'd suggest testing the output on the car first. Check the voltage directly at the output stud. Set your meter on a 20V AC scale. If you get more than 0.2-0.5VAC you may have a fornicated rectifier. If not, you've saved some time and money, and can continue to diagnose the problem.

zupmanZ28

i just brought the alternator to Advance auto to have it tested, it only put out 10 volts, and he said the regulator was completely fubared. I just put another on it, and it would appear that the problem is gone. I can't drive it though cause it's raining, so who knows whats gonna happen when it sees a load. Thanks for everyones help, hopefully i won't be needing any more advice on this topic.

Eric