1985 911 3.2 Horn Electrical Short Troubleshooting

[SpiderCider]

I'd appreciate a spare set of eyes on an electrical issue I've been troubleshooting. A few weeks ago I was woken in the early morning by the sound of my car horn blaring non stop in the garage. I pulled the steering wheel horn pad off and disconnected the horn wire from the pad but the horn kept going until I disconnected the battery ground. I initially suspected a bad horn relay, but replacing the relay did not solve the issue. I then tested each of the relay socket pins for continuity and got the results below.



Based on these findings, I believe I have a short somewhere between the 87 pin leading to the horn and a 12V source.



I'm curious if anyone has had a similar horn issue, if you agree with my findings, and if there is anything else I should check. I've pulled the fuse panel off the body and don't see any issues or obvious frayed wires or shorts on the back side of the panel. And there is no obvious wire damage between the relay socket and the horns. Curious if there are any other suggestions before I cut off the electrical tape and trace the individual wire from the 87 socket to the horn looking for a short along the way.

[wazzz]

I don't know what you are using to test continuity, but make sure you do so with the battery ground disconnected. Also be sure to have both doors closed and the frunk light switch pressed in with a clamp so that the frunk light is not included in the circuit you are testing. Otherwise you can be tricked by seeing what you think is continuity while in fact the meter is seeing the frunk light filament resistance (or interior light resistance if a door is open).

Do this and see if you still get the same continuities.

[SpiderCider]

Thanks Gilles. I am using a multimeter set to the diode / audible continuity setting and I'm claiming continuity based on the audible beep the multimeter will make and an ohms reading close to 0. The doors are closed and the battery ground was disconnected, but I did not clamp the frunk switch in my original test. I rechecked with the frunk switch depressed and now I am getting intermittent continuity between 87 and 30/86. It seems random and almost like a relay or diode is intermittently working to prevent reverse current. I will have continuity for 2 seconds between 30 and 87 and then it will suddenly stop. And then if I reverse my test leads I will get continuity again between 87 and 30. I'm seeing the same random issue when testing between 86 and 87. There is always continuity between 30, 86, and the battery positive terminal. Any thoughts?

[blucille]

Did you remove the wires from the horns? The horns can short internally.

[brianlay]

Testing by measuring continuity is the wrong approach.
When you pull the horn relay, does the horn stop?
If so, I would focus on the the wire going up through the steering column to the horn pad. Likely shorting to ground somewhere.

[wazzz]

Originally Posted by SpiderCider

Thanks Gilles. I am using a multimeter set to the diode / audible continuity setting and I'm claiming continuity based on the audible beep the multimeter will make and an ohms reading close to 0. The doors are closed and the battery ground was disconnected, but I did not clamp the frunk switch in my original test. I rechecked with the frunk switch depressed and now I am getting intermittent continuity between 87 and 30/86. It seems random and almost like a relay or diode is intermittently working to prevent reverse current. I will have continuity for 2 seconds between 30 and 87 and then it will suddenly stop. And then if I reverse my test leads I will get continuity again between 87 and 30. I'm seeing the same random issue when testing between 86 and 87. There is always continuity between 30, 86, and the battery positive terminal. Any thoughts?

OK so now what you may be seeing (or your multimeter continuity tester for that matter) is the charging of capacitors by the continuity tester voltage. Could be capacitors in the clock, which is about the only device that remains in the circuit once you have excluded the frunk and cabin lighting. That would explain why you hear the beep for a couple of seconds, then no more once the caps are charged.

That's tricky and also the reason why I never trust continuity testing in the car unless I'm absolutely sure no devices remain connected in the circuit under test.

Think of it, in your illustrated pic you reported continuity between pin 30 (which is permanent battery positive) and ground. How would that be possible? Once the battery is plugged back something would burn instantly, since it is a short between +12V and ground (0V). So what you are seeing in fact is not continuity, but the resistance of some load that remains connected in the circuit under test.

[76FJ55]

Cross post from my RL response:

Does the horn continue to blare if you connect the battery with the horn relay removed like in the picture?

If it does, then you are correct there is a short on the wire from 87 to the horn, where it is shorted to a 12V source somewhere.

If it doesn't make noise with the relay out, then the short is elsewhere.

86 showing continuity to ground seems odd. if it truly has a significant ground path that would be a direct short between Battery 12V+ (terminal 30) and ground. either the battery would have a significant drain, or the wire to 86 would go up in smoke.

Edit: Does your car have an alarm? could it be the alarm horn, not the normal horn?

[wazzz]

Originally Posted by 76FJ55

Cross post from my RL response:

Does the horn continue to blare if you connect the battery with the horn relay removed like in the picture?

If it does, then you are correct there is a short on the wire from 87 to the horn, where it is shorted to a 12V source somewhere.

If it doesn't make noise with the relay out, then the short is elsewhere.

86 showing continuity to ground seems odd. if it truly has a significant ground path that would be a direct short between Battery 12V+ (terminal 30) and ground. either the battery would have a significant drain, or the wire to 86 would go up in smoke.

Edit: Does your car have an alarm? could it be the alarm horn, not the normal horn?

Read my post right above yours. There is no continuity between 86 and ground, it appears there is continuity because of remaining loads in the circuit that make the test deceiving. This is a possible explanation and the reason why this continuity test cannot be trusted.

[76FJ55]

Originally Posted by wazzz

Read my post right above yours. There is no continuity between 86 and ground, it appears there is continuity because of remaining loads in the circuit that make the test deceiving. This is a possible explanation and the reason why this continuity test cannot be trusted.

Yes, I'd responded to his post on Rennlist (as noted at top of my post), which seems to be a dead thread as everyone is posting here. I realize that some of it was redundant, but didn't think it worth my time to retype the post and just copy/paste it here.

A couple points from my post that haven't been addressed is whether or not the horn sounds with the relay removed or not? I think that is critical info for narrowing down the failure.

also there is the possibility that it is the alarm horn and he may be completely misdirected, if he has an alarm on his car, and was just chasing a horn sound, but didn't identify the actual horn source.

[wazzz]

Yes, lack of details as usual. Didn't know what RL means. Now I know.
Actually I didn't focus at all on the horn fault and still have no clue about it, but was more concerned about the method used to test continuity in general, because I recently helped another fellow member on the French forum who was using the same technique on his car and doing the same kind of continuity tests and was facing the same weird findings, i.e. a so-called continuity between battery pos (30) and ground, which of course was due to the continuity tester finding a path through the frunk light filament.

[ant7]

Had exactly the same thing happen to me, i.e.; got woken up early one morning to the sound of the horn blasting, quickly disconnected the battery to stop it, after doing some checks, I looked at the relay points and they seemed to be exceptionally close, almost touching, so possibly a difference in temperature was enough to bring them together just enough to set the horn off, who knows ? anyway, I re-adjusted the relay points to a more acceptable gap, and all has been well with it since.
Ant.

[SpiderCider]

Thanks everyone for the replies and insight. It was a busy end to the week and I didn't have time to troubleshoot further until tonight.

All your input made me realize there was likely not a short between the 87 wire and a 12 volt source because the horn stopped once I pulled the relay out. If there was a short then the horn should have continued sounding even with the relay removed.

I did previously take the steering wheel apart looking for a short on the 85 brown wire to the steering wheel but everything in the steering column looked good. I did retest the 85 pin to ground continuity and found that the horn pad was functioning as intended: no continuity when the horn pad was not pressed, continuity when the horn pad was pressed.

Gilles, thanks for the tip on a capacitor throwing me off with the 87 to 30 and 87 to 86 continuity. With the doors closed and hood light button taped down, I found that I would get continuity if I put the positive lead from my multimeter on the 30 or 86 pin and the negative lead from the multimeter on the 87 pin which kept throwing me off thinking there was a short. But when I then put my negative lead on the 30 or 86 pin and positive pin on the 87 pin, I would get a second or two of continuity and then it would stop. I assume the voltage coming from the multimeter was enough to charge a capacitor somewhere when I had the first configuration and reversing the multimeter leads allowed the capacitor to drain to ground and the continuity to stop?

In any case, after these retests I felt more confident that there was not a short along the 87 wire to the horn, and that the 85 wire to the horn pad was functioning properly. I swapped the relay from the cruise control plug into the horn relay plug and decided to retest the horn. And guess what, THE HORN WORKS JUST FINE NOW!

Which still has me scratching my head. When this issue first popped up, I swapped relays to see if a bad relay was the cause and I was still getting a blaring horn. A bench test of the horn relay indicated it was working properly and not sticking closed, but who knows maybe it was a bad relay to begin with. Or maybe like ant7 I wiggled the relay socket wires just right while unscrewing the fuse panel to inspect for any obvious shorts. I also did find some moisture in the area which I believe came from my windshield washer hoses, so perhaps that fluid caused a short along the 85 wire which has since dried out and resolved itself.

I wish I had a sure answer, but for now I'm happy to have a functioning horn again. I'm ordering a few new relays and will be replacing my windshield washer hoses and joints over the next few weeks. Hopefully the issue doesn't reappear but I'll post here if it does.

Thanks again!