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Old 06-13-2022, 02:44 PM   #1
blkaplan
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Default Brake light / rear fogs cross talk

My 92 240 wagon has something funky going on with the taillight wiring.

If i hit the brake pedal the bulb in the rear fog switch illuminates, the rear left brake light does not work.

When the brake pedal is depressed, the rear right brake light comes on along with the right side fog lamp.

If i turn the rear fog light switch on, the high beams illuminate when i hit the brake pedal. Theres also a trace illumination of the rear left brake light that normally wont turn on.

Anyone have any suggestions on what to check?
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Old 06-13-2022, 02:59 PM   #2
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I'd start by swapping the bulb failure relay.

Cheap shot in the dark I know, but with this being TB and the operative word being ¢heap, what's to lose?
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Old 06-13-2022, 03:01 PM   #3
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If I remember correctly, there is a wire that goes to the headlamp dip relay from the rear fog lamp harness. Is used as the power wire for the rear fogs, run through fuse 14. It's possible that you may have more than one issue, in that the relay harness plug might be miswired, and there's an issue somewhere between the brake pedal switch and the taillamps.
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Old 06-13-2022, 03:25 PM   #4
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Check the ground wires off of each taillight assembly - there can be strange failures if the grounds aren't good. If you have a helper, or a stick, push down the brake pedal and measure the voltage from the ground wire lug on the taillight to a chassis ground. If you see voltage, that ground wire isn't working.
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Old 06-13-2022, 03:34 PM   #5
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Originally Posted by bobxyz View Post
Check the ground wires off of each taillight assembly - there can be strange failures if the grounds aren't good. If you have a helper, or a stick, push down the brake pedal and measure the voltage from the ground wire lug on the taillight to a chassis ground. If you see voltage, that ground wire isn't working.
I'm seeing .39 V between the tail light ground strip on the chassis on both sides.

Cleaned them up, now seeing .08V still have fog lights illuminating with brake lights.

Last edited by blkaplan; 06-13-2022 at 03:55 PM..
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Old 06-13-2022, 04:54 PM   #6
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There is a ground wire inside each hatch hinge. When that fatigues and breaks you start getting strange feedback loops. My car did it with the rear defogger dimly lighting when hitting the brakes.

If the ground wire in the hinge is broken. I drill a small hole inside the hinge and use a teflon wire to replace the ground. The teflon is tough, flexible, and smaller so it fits in the flexing part of the hinge easier. I just use a small red ring terminal and a sheet metal screw to screw it down to the hole you drilled. Drill the small hole inside the body and the hatch side of the hinge.
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Old 06-13-2022, 05:11 PM   #7
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We solved the rear left brake light issue by wiggling the bulbs in the holder a bit.

As it stands, the current issue.

Rear fog lights illuminate with brake pedal,
(Bulb in the fog light switch also illuminates with brake pedal)

if rear fog lights are switched on the high beams come on with the brake pedal.
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Old 06-13-2022, 07:24 PM   #8
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That's certainly a strange set of behaviors.

I looked at the '93 wiring diagrams (my crossword puzzle for the day), and I think you have a short between the brake light wire and the fog light wire. The foglight wire comes direct from the switch to the taillights, the brake lights come from the switch, through the failure relay, to the taillights.

If the wires are shorted, it can back-power the main headlight relay through the fog light switch. When brakes and fogs are on, can you flip between high and low beams?

I'd inspect the wiring harness in the rear spare tire well for damage. Did the wagon ever have an aftermarket alarm/immobilizer installed? If so, someone may have been clever and used the fog light switch for some sort of starter disable.
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Old 06-28-2022, 03:46 PM   #9
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Originally Posted by bobxyz View Post

I'd inspect the wiring harness in the rear spare tire well for damage.
bing bing bing

It wasn't damaged but someone did run a jumper between the brake lights and the fog lights that way there would be 5 brake lights... It was back feeding and causing havoc with the headlights.

After removing the jumper everything went back to normal... with exception of my EZK box going into diagnostic mode and throwing a CEL. could have been random... or not. Replaced the EZK and diagnostics starting working and CEL went away.
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Old 06-28-2022, 06:26 PM   #10
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Glad you got it sorted out.

When you wire that up like that. You need add a diode in the jumper so when the brakes work it gives you all the lights but when the fogs are on it will only be fog lights. cathode toward the fog light.

Last edited by dl242gt; 06-28-2022 at 06:27 PM.. Reason: add comment
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Old 06-29-2022, 12:31 PM   #11
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Glad you got it sorted out.

When you wire that up like that. You need add a diode in the jumper so when the brakes work it gives you all the lights but when the fogs are on it will only be fog lights. cathode toward the fog light.
yea, that was one detail the PO overlooked.
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