Short to ground-'87

[jeffcoslacker]

Working on a friend's '87, has a wicked short to ground, overheats the pos cable end, and runs the battery dead in hours. Finally isolated the 2 wires affected, but don't know what they go to. I do know that it is not in any fused sytem, and not related to the alternator or starter.

I pulled out the underhood fuse/relay center, and unplugged the two wiring connectors going to the front of the box, that's when the short stopped. There are two connectors, one with eight terminals (inboard) and the other with six. The path to ground is in the top left terminal of the outboard connector, and the bottom right of the inboard.

Wondering if anyone has enough familiarity with these to know what they go to?

One hint: the problem started when he was changing the thermostat. He accidently crossed a wrench to one of the temp sensor terminals and the pos battery cable. He said it threw a good arc, then the fan came on, despite the ignition being off. It did go back off, but would sporatically stay on after car was shut down, running the battery dead.

I'm wondering if he fried the ECM at the fan control terminals?

Don't know Hondas. Where is the ECM on this one?

Thanks!

[mpumas]

Every circuit is fused in the Honda except for the starter and alternator and the power window relay and then its output is fused for each of the windows. You need to pull all the fuses and see if the high current draw drops. Otherwise you have a wiring problem. ECM?? Do you mean ECU?? The fans do not go through the ECU. The coolant sensors are really just temperature switches and in the case of the radiator fan operates a fan relay. The condenser fan assembly is controlled by the cooling fan timer unit and you might have fried it. Try pulling fuses and see if you can determine the circuit by fuse.

[jeffcoslacker]

Originally Posted by mpumas

Every circuit is fused in the Honda except for the starter and alternator and the power window relay and then its output is fused for each of the windows. You need to pull all the fuses and see if the high current draw drops. Otherwise you have a wiring problem. ECM?? Do you mean ECU?? The fans do not go through the ECU. The coolant sensors are really just temperature switches and in the case of the radiator fan operates a fan relay. The condenser fan assembly is controlled by the cooling fan timer unit and you might have fried it. Try pulling fuses and see if you can determine the circuit by fuse.

Thanks for the reply. I'm ahead of you on the fuses. I disconnected the battery an jumped with a check light while we pulled every fuse and relay in the car, under the hood and in the interior. The excessive load remained. I disconnected the wiring to the alternator and starter, the load remained. Only when I took that underhood box out, and disconnected the plugs from the front side did the load stop. Then, probing those terminals, I found that the two I described are shorted directly to ground, which is not acceptable, as this is a positive power feed, so there has to be a place in this circuit when the wiring is pinched, burned or whatever, and it is laying on something grounded. With the ignition off, and nothing active except the clock and the ECU (thank you, I'm used to GM) there should be no other loaded path to ground, much less a direct ground pulling over 20 amps. So the problem, I assume, would have to be ahead of the fuse box inside the car, as it is definately after the box under the hood. And the load comes off the harness fed by the MAIN FUSE, the big one in the power center under the hood, if that helps.

Thanks again.

[jeffcoslacker]

Next time I go over there I'll take a couple of pictures of exactly what I'm talking about, maybe it'll make more sense. So you are saying that the fan control circuit operates solely on sensor reading to control relays, the ECU has no control over that circuit? If that's true, it would seem the problem has to be in that circuit, if the harness I described was part of that, it would seem to confirm that.

[mpumas]

I suppose the wires that you found shorted are the white ones. But two wires, that has me baffled. After the main fuse, is the ignition switch which is fused with 2 40 amp fuses, radiator fan fused at 30 amp, voltage regulator fused at 10 amp, defogger fused at 30 amp and the main relay fused at 10 amp. Have you pulled the radiator fan relay? If it was in the radiator fan circuit, pulling the fuse would have removed the current draw. Baffling

[jeffcoslacker]

Originally Posted by mpumas

. Have you pulled the radiator fan relay? If it was in the radiator fan circuit, pulling the fuse would have removed the current draw. Baffling

You got that right

Yes, in fact he is in the habit of pulling that relay after driving it, because like I said, the fan would run for no apparent reason and run the battery down before this other problem started.

The more I think about it, since it began with a temp sensor zorch, and then the fan acted up, and now we have a dead short, I'm guessing some wiring just off the power block melted and is contacting or fused with a ground. It's gotta be in that fan circuit. I'll just have to keep digging.

Thanks for the input!

[mpumas]

The fan may be running because the switch is fried or the relay contacts are fused together.

[AccordCodger]

Well, the fan would run all the time if the relay contacts were fused closed., and it's only "for no apparent reason"

The fans are controlled by the "Radiator fan control module" and I'm not sure where that it. Also, there are TWO engine coolant temperature switches - either could be causing the fan problem.

Now - back to the short. I don't think we established what color the wires are that you identified. If I know that, maybe I can identify them on the wiring diagram.

[BIAR]

This may sound crazy,but did he hook the ground back up at the t-stat houseing.I am almost postive the old carb cars had a ground wire there also.Maybe it is seaching for a ground.It is easy to check,Just a thought.Carl

[AccordCodger]

Maybe it is seaching for a ground

No - that's against the laws of physics. Regular old direct (or even alternating) current isn't like lightning.

The problem is a nasty short to ground - plain and simple (but hard to find).

[BIAR]

accordcodger,I understand.I was thinking if the ground was off it got hot enough to melt and then shorted to ground.It is the only think that I could think of that changing a t-stat would have done.
jeffcoslacker,What fuse number is the main fuse.I have shopkey and does not label them that way. Carl

[BIAR]

I will check in with you guys later.Bud shootout is starting.Later.Carl

[AccordCodger]

jeffcoslacker - just a follow up:

then the fan came on, despite the ignition being off.

The cooling fan (on the right side of the car) is not controlled through the ignition. It can, and does, go on (and off again) for some time after the car is turned off, especially after a summer high-speed run.

My feeling is that if either the temp sensor or the fan control module were fried (a good possibility by the sound of it) one of them might be the culprit.

[jeffcoslacker]

Originally Posted by AccordCodger

jeffcoslacker - just a follow up:

The cooling fan (on the right side of the car) is not controlled through the ignition. It can, and does, go on (and off again) for some time after the car is turned off, especially after a summer high-speed run.

My feeling is that if either the temp sensor or the fan control module were fried (a good possibility by the sound of it) one of them might be the culprit.

Right, but it would come on after driving, maybe many minutes later, then run until the battery was dead. I took it home one day, parked it. Many hours later I opened the hood cuz I heard a faint noise, and the fan was running slowly, having drained the battery. So it may be signalling to run on after boil, but never getting signal to shut off, or the relay sticks.

I understand what he was saying about the ground. Have seen high amperage items try to gain a ground path through other sytems they are in conjunction with if the ground path was not good enough, causing unwanted operation of another circuit. Kinda like on old Fords (a classic example) when you loose a ground to a taillight, the indicator on the dash will glow when you have the lights on. Ground loop feedback, it's called.

I didn't know about the fan control module, that sounds promising. It just amazes me that this short can pull so much amperage without frying a fuse, cuz like the one guy said, EVERYTHING seems to be fused on this model.

As for which wires, I'll have to get back over there and let you know. I made note of their position in the connector (see original post), but not color. I tried to start the car with those connectors pulled apart, but it won't crank and ignition is dead with either one disconnected, so they are apparantly real high-priority primary wiring.

Thanks to all of you for your input. I love a good mystery

[BIAR]

jeffcoslacker,That is what I was trying to get say.I guess I should have said, feeding back trying to find a ground through a different circuit that is was tide into.The reason I mentioned the ground is it seemed when I worked at the dealership we would have about two fuel injected cars a year towed in.That someone had put an engine in and it wouldn,t start.It is the first place we learned to look.I think the carb cars had it to .It is really easy to miss.I believe it has three wires and one eyelet.And attaches to t-stat housing.I just thought it might be worth a quick look.Carl