weird electrical problem

[mouchyn]

ok, i just put the engine back in my 88 lxi sedan. as soon as i connected the battery, i heard a loud pop. damn. I look around and see that the charge wire for the alternator was bent back and touching the housing. It was shorted. I bent it back and everything works fine except the ECU.

I swapped out the ECU thinking i fried it with a known-good unit. ECU still won't come on. I grab this pinout and start testing.

http://pages.infinit.net/omus/1989A20A3pinout.html

A17= 12V
A2, A4, A16, A18 = no continuity to ground. with the key turned on, these wires have 12V. wtf?

This leads me to believe that something is still shorting somewhere. 12V battery current is still ground out somewhere. The weird thing is no fuses are popping, no wires are hot or burning, and everything else works fine. The starter even works. the interior lights work, the clock works, the radio works, the cluster lights work, everything works except the ECU.

I tested the ECU and it's fine. I used a known-good one and it still didn't work in the sedan.

Any ideas what could be causing this problem?

[BlueBead]

Did you check the ECU fuses in the dash and under the hood?

[mouchyn]

yeah, all the fuses are fine. checked them with a digital fluke multimeter for continuity. the ECU is GETTING power from the harness. I checked that with the meter, too. for some reason, those ground wires aren't ground wires anymore. They're switched 12V, now.

i thought i posted this in the technical section. damn.

[mouchyn]

ok, so as it turns out, the 4 ground wires on the Plug "A" at the ECU were the problem. There were getting voltage when i started the troubleshooting, but replacing the fuel pump main relay fixed that. after that, the wires no longer had voltage, but they still didn't have continuity to ground. As a temporary fix, i just used a wire tap, tapped on of the wires, and grounded it right next to the ECU.

The ECU and the car fired right up.

Question: Where do those 4 wires ground? All 4 have continuity to each other, but none of them had continuity to ground before I grounded them myself. I followed the wires from the ECU to the under side of the dash. That's where they disappeared. I didn't want to cut the looms apart to keep tracing. Does anyone know where those wires go?

[SteveDX89]

I think they might be the ground for the injectors. Don't quote me on that, I'm only 50% or less sure. I'll have to check some wiring diagrams when I get home.

[shepherd79]

according to Chilton's and Honda shop manual, those wires are just ground wires. there not going anywhere. they go all the way back to engine bay and they are grounded somewhere there. I don't know why.

[mouchyn]

yeah, that's what's weird. I don't know why they run all the way to the engine bay and ground in there. For some reason, all 4 of those wires still have continuity to each other, but not to ground. Wherever they are grounded...they aren't grounded anymore. I'd rather not use my wire-tap ground solution much longer than i have to. I'm kinda of anal about keeping things the way they were engineered.

Honda had a reason for running those ground wires all the way into the engine bay. The chassis for the ECU is ground, the bracket it's bolted to is ground, the holes the bolts go into are ground, and, hell, there's even a small ground wire that sits under one of the ECU bolts. There's a very specific reason why they didn't just ground it there. I wanna know what it is... hmmmm

[SteveDX89]

Sounds like those 4 wires run together into one since they all have continuity to each other.

[mouchyn]

Sounds like those 4 wires run together into one since they all have continuity to each other.

yeah, that's what i figured. when the charging wire grounded out to the alternator housing, if fried my main relay. I was wondering how that happened until i saw that in the dask fuse panel the alternator and fuse pump share a fuse. bingo.

after i replaced the main relay, the ground wires no longer had voltage, which is good. Maybe the come together and ground at or near the main relay. Maybe the alternator? who knows...

[HostileJava]

Sounds like those 4 wires run together into one since they all have continuity to each other.

Not really, they may all end up going to the same metal device which in turn is grounded to the battery or chassis. So they would still have continuity between each other but not to ground. If you remove one of those wire from the device then they all would not have continuity. For instance your injector idea. They would all go to the injectors, which would have continuity to the fuel rail and from there to ground. However pull one of those injectors out and you loose the continuity with each other. (Just an example, I'm not sure if that's how it's setup).

[mouchyn]

i tried all the injectors. they don't ground there.

[HostileJava]

I'm not saying that it is the injectors. I was using that as an example :D

[88LXi/87CRXSi]

Maybe the come together and ground at or near the main relay. Maybe the alternator? who knows...
I looked in my electrical troubleshooting manual, and as best as i can tell, it's closer to the alternator, actually attached to the intake manifold, right side of engine behind valve cover. I took a peek at mine ('88 lxi sedan) and the wires you are looking for come out the back side of the plastic fuel injecter wiring trough (sp?), and bolt to the manifold. It's bolted sorta behind and below the #1 fuel injector. Hope this helps.

[mouchyn]

i know exactly what bundle of wires you're talking about. i'll check it out.

[mouchyn]

steve, you nailed it. you're the man. Mine was folded under and hidden. I never even saw it. Thanks!

[88LXi/87CRXSi]

I'm glad I could be of some help, and that you solved the problem!