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Old 02-07-2019, 07:46 PM   #1
Wesdunns70t5m
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Default Brake light help

So I just discovered while backing into my parking spot at home that none of the three brake lights are working in my 1990 765. Iíve checked just one bulb and it was good, and Iím assuming (yeah I know) that the others are good as well. Weird part is, my bulb failure light isnít illuminating in the dash when the brakes are applied. All other lights appear to be working fine. Any suggestions?
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Old 02-07-2019, 08:28 PM   #2
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Get out your test light. Check for current at the brake light switch. One side should be hot. If not fuse bad? Other should show current when the pedal is pushed. If not bad switch bad?
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Old 02-08-2019, 07:48 AM   #3
Wesdunns70t5m
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Switch works as I’ve got the launch control from my wasted spark hooked up to it and a switch and it works as should. I checked the fuse before work this morning and it was blown. Changed it and that fuse immediately blew as well. Once I got to work I changed it again and the brake lights worked for the three second test I performed. I’ll grab a pack of 15a fuses after work and hope for the best. Sounds like a wire is bare somewhere
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Old 02-08-2019, 07:16 PM   #4
Wesdunns70t5m
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So I replaced the fuse with a 20a one since the parts store was out of 15a and the brake lights are working again. However now, the launch control function isn’t working. I’m getting 5v from the wasted spark board going to the switch, the switch itself has continuity, and I’m getting 13v coming from the wire to the brake switch from the toggle switch. This is turning into a real head scratcher now

To make matters worse, I just disconnected the launch control wire from the brake switch and attached it directly to ground, and it works as it should. Put it back on the brake switch and still nothing. Anyone got any ideas on this one? Electrical isn’t my strong suit

Last edited by Wesdunns70t5m; 02-08-2019 at 07:30 PM.. Reason: Added info
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Old 02-08-2019, 09:58 PM   #5
dl242gt
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Well the brake light switch doesn't ground anything. It is always hot with 12v then passes that through when you press the pedal to the bulb out relay then on to the rear brake lights from there. If you need a ground signal for launch control you won't get it there.

You could add your own second switch to the brake pedal that connects that wire from the launch control to a ground wired to the other side of the switch.
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Old 02-09-2019, 06:20 AM   #6
Wesdunns70t5m
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It’s been working for a month or two wired up this way. There’s a video on YouTube that has someone using the brake switch as well, and that’s where I got the idea
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