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Join Date: May 2010
Location: Eagle, ID
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1986 928 rear wiper problem

ok boys I am at my wits end. Here is the issue...

When you start the car and tap on the brakes the rear wiper arm turns on. I have literally disassembled the interior trying to find a shorted wire, bad ground, dirty plug, etc. I have also pulled the rear bumper cover and pulled the tail lights apart to see if there was a short in them since some of the wires and grounds seem to run to both.

I have changed out to a new relay, swapped wiper motors, checked the brake light switch, changed the actual switch(which is new), completely pulled out and cleaned all the contacts and plugs on the CE panel. While i was in there I cleaned all the wires and ground spots behind it. I thought i had found the issue once or twice. First there was a little bit of corrosion the end of they relay. I cleaned that and the corresponding wires(also changed out that relay), but to no avail. I have traced every wire I could find that goes from the CE panel back to the license plate lights. All the wires are in great shape... they are all very pliable and almost like new... Yes even the ones running under the headliner that go to the back hatch and third brake light.

What am I missing or has anyone ever had this issue. I don't have the option to just unplug the rear wiper or get rid of it since my friend is trying to get this back to 100% working order.

Thanks for your time and any suggestions you may have.

Ramon
Old 02-05-2018, 06:58 AM
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ramon - you probably should include what year car you're working on - i assume the relay you replaced was the rear wiper relay - some of the systems on these cars run through other systems relays for power and if those "supposedly unconnected" relays are bad they will affect the system you're working on - when i first got the 84 i could not turn the front wipers off - turns out they run through the fog light relay - i replaced that and the wipers worked fine - i would never have thought to test the fog light relay until Alan told me about this known issue - hopefully someone with more insight than me will chime in and steer you in the right direction - good luck
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Old 02-05-2018, 07:32 AM
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I do have the year in the thread title but i did not add it to the actual post. I have replaced a few of the relays in the CE panel, but since I do have my 928 next to it I may switch them out one by one and see what happens. Anything is worth a try at this point. Yes I did replace the actual wiper relay in the rear though with a new one.

Thanks for the input I will give that a shot.
Old 02-05-2018, 08:28 AM
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yep i missed the year in the title - i said fog light relay but now i'm thinking it could have been the rear defroster relay - your plan to swap the relays individually sounds like a good idea - let us know how it works out
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Old 02-05-2018, 12:10 PM
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Yes, it's the rear defroster relay that could be the problem. I've pulled the rear defroster relay in all my cars. It is a known potential trouble issue for battery drain as well.
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Old 02-06-2018, 07:59 AM
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Ok so I have gone through and replaced each relay by swapping one by one with the relays out of my 86. No change... After reading Stepson's post i took out the rear defrost relay and it did cut power to the rear wiper completely. I also went as far as disconnecting the two wires from the rear defroster itself to see if it was somehow looping power back into the system through a short or something(grasping at straws I know), but that had no affect. I also pulled out the switch for the defroster to see if there was any corrosion but found nothing.

Ramon
Old 02-14-2018, 08:25 AM
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Lamp Control Unit -

Hey Ramon - Did this problem just suddenly develop ? Have you noticed any wiring mods by the previous owner of your car. Sometimes a PO will do some silly splicing repairs or jumpering around when installing a stereo or something.

You might try pulling the brake switch fuse to verify that the B+ leaving the brake light switch is directly triggering your rear wiper. Have you made sure your brake lights are both coming on (brightly) when the brake is applied. The 86 should also have the 3rd brake light wired in. We've all had to make a few laps around our cars cleaning a poor ground or two.

You also could disconnect the rear wiper ON/OFF switch on the console just to isolate that circuit.

Then disconnect the lamp control unit behind the kick panel down by your drivers left foot - and hit the brake. I've seen some weird feedback electrical problems being generated by a corroded in/out pin connector on the lamp control unit.

Keep us posted - Michael
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Last edited by JK McDonald; 02-14-2018 at 04:20 PM..
Old 02-14-2018, 03:53 PM
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Ramon,
If I read you correctly, when you removed the rear defroster relay, the problem was corrected and the wiper did not function? It's a ground problem, nothing more. the defroster relay is powering that ground issue. Until you find your ground fault, leave the relay out. You just won't have your rear wiper. Big deal. The rear wiper should have absolutely nothing to do with the defroster relay......unless there is a ground fault. I would look to the brake light circuit for something plugged in wrong.

Sorry if I have misunderstood what you posted and you still have the issue.
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Black Metallic 1984 Euro S 5 speed (The Schwartz)

Last edited by stepson; 02-14-2018 at 07:22 PM..
Old 02-14-2018, 07:18 PM
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please post pictures of the CE panel
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Old 02-15-2018, 04:51 AM
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