Hit some electrical snags

AndrewB · #1 (**permalink**) 06-20-2019, 02:18 PM

Hi all,

I am wrapping up the wiring on my ERA 289 but I have run into a issue where the brake lights activate with the key in the accessory position, and after a few minutes, the trailer relay gets quite hot. I have gone as far as unplugging the harness from the brake light switch yet the lights remain on. I've jiggled the connectors and verified the pins are pushed in and there are no blown fuses. Any ideas? Bad trailer relay?

Thanks,
-Andrew

DanEC · #2 (**permalink**) 06-20-2019, 03:49 PM

I think the trailer relays do heat up as I remember Bob mentioning recently that he might look at Velcro for mounting the relay to the backing plate but he would need to check if the Velcro self-adhesive was heat resistant. I've never touched mine while driving the car. Suspect it heats up when turn signals are activated or brake pedal is held down. It's possible it could be a bad relay - I remember Patrickt saying it is difficult to trouble shoot their circuitry.

Since you have unplugged the brake light switch that would seem to take it out of the equation. What about the turn signals - have you been able to test them with this issue? If the front ones work but the back ones are acting funky that would seem to point in the direction of the trailer relay. I suspect Patrick will be along to help after bit.

AndrewB · #3 (**permalink**) 06-20-2019, 03:55 PM

Quote:

Originally Posted by DanEC

I think the trailer relays do heat up as I remember Bob mentioning recently that he might look at Velcro for mounting the relay to the backing plate but he would need to check if the Velcro self-adhesive was heat resistant. I've never touched mine while driving the car. Suspect it heats up when turn signals are activated or brake pedal is held down. It's possible it could be a bad relay - I remember Patrickt saying it is difficult to trouble shoot their circuitry.

Since you have unplugged the brake light switch that would seem to take it out of the equation. What about the turn signals - have you been able to test them with this issue? If the front ones work but the back ones are acting funky that would seem to point in the direction of the trailer relay. I suspect Patrick will be along to help after bit.

Thanks Dan. The turn signals and parking lights are working as expected front and rear.

DanEC · #4 (**permalink**) 06-20-2019, 04:19 PM

About the only other thing I can immediately think of is to look for a pinched wire running to the brake light switch that is shorting it.

patrickt · #5 (**permalink**) 06-20-2019, 04:39 PM

Well let's try an easy test. Remove the cover to the brake master cylinders box that is underneath the driver's side of the car. That's easy to do, it just involves about four or five sheet metal screws and four or five nuts. Then turn the key to the ACC position and observe that the brake lights are on without you having to touch the brakes. Now pull the red wire off the brake light switch that is on the back of the master cylinder that is closest to the frame. See if the brake lights go out.

AndrewB · #6 (**permalink**) 06-20-2019, 04:47 PM

Quote:

Originally Posted by patrickt

Well let's try an easy test. Remove the cover to the brake master cylinders box that is underneath the driver's side of the car. That's easy to do, it just involves about four or five sheet metal screws and four or five nuts. Then turn the key to the ACC position and observe that the brake lights are on without you having to touch the brakes. Now pull the red wire off the brake light switch that is on the back of the master cylinder that is closest to the frame. See if the brake lights go out.

With the key in the ACC position, the brake lights do not go out when the red wire is pulled.

patrickt · #7 (**permalink**) 06-20-2019, 05:14 PM

Quote:

Originally Posted by AndrewB

With the key in the ACC position, the brake lights do not go out when the red wire is pulled.

Now disconnect the red wire at the trailer relay and see if the brake lights go out.

AndrewB · #8 (**permalink**) 06-20-2019, 05:23 PM

As the red wire originates under the cover of the trailer relay, I removed the red wire from the #74 connector block. With that removed the brake lights are not light.

patrickt · #9 (**permalink**) 06-20-2019, 05:28 PM

Alright. You now have the red wire disconnected at both the brake light switch and the trailer relay, and the lights are out. Both ends of that red wire should not show 12v+. Put a VOM on it and see if that wire is hot with 12v+.

AndrewB · #10 (**permalink**) 06-20-2019, 05:38 PM

Quote:

Originally Posted by patrickt

Alright. You now have the red wire disconnected at both the brake light switch and the trailer relay, and the lights are out. Both ends of that red wire should not show 12v+. Put a VOM on it and see if that wire is hot with 12v+.

There is no voltage on the red wire. However there is 12v on the pink wire in block #70 that the red wire would connect to if it was not pulled.

patrickt · #11 (**permalink**) 06-20-2019, 05:48 PM

That's the problem. The RED wire and PINK are the same wire. When operating normally, the 12v+ comes through the brake switch from the green wire and out through the PINK/RED wire to the trailer relay. With that wire disconnected from the switch there shouldn't be 12v+ on it, but there is. Somewhere that PINK wire is shorted to a hot wire. Now, if you run a separate jumper wire from the brake switch to the RED wire that goes in to the trailer relay, everything should work normally. You can then figure out why that pink wire is shorted to hot at your leisure.

AndrewB · #12 (**permalink**) 06-20-2019, 06:13 PM

Quote:

Originally Posted by patrickt

That's the problem. The RED wire and PINK are the same wire. When operating normally, the 12v+ comes through the brake switch from the green wire and out through the PINK/RED wire to the trailer relay. With that wire disconnected from the switch there shouldn't be 12v+ on it, but there is. Somewhere that PINK wire is shorted to a hot wire. Now, if you run a separate jumper wire from the brake switch to the RED wire that goes in to the trailer relay, everything should work normally. You can then figure out why that pink wire is shorted to hot at your leisure.

Thanks Patrick - very much appreciated. I'll trace out the pink wire tomorrow and post my results.

66AC · #13 (**permalink**) 06-21-2019, 02:53 AM

Patrickt, I am always amazed at your thorough understanding of ERA issues and your concise instructions to help out those of us in need! Bravo Amigo. You are a true asset to the ERA and Cobra Community here on Club Cobra. A virtual beer is on the way!

I am sure I am not alone in this assessment.

DanEC · #14 (**permalink**) 06-21-2019, 04:15 AM

In association with Patrick's guidance, it's a good idea to go through all the major harness plugs under the dash and compare the wiring layout on both sides with the electrical manual. I found one plug where two wires were in the wrong position - it's hand built, it happens.

AndrewB · #15 (**permalink**) 06-22-2019, 12:32 PM

OK, found the issue. It was on the brake light switch wires that branch off the main harness and run vertically down the front left wheel inner panel. All fixed and everything works great. Thanks again Patrick!

DanEC · #16 (**permalink**) 06-23-2019, 04:19 AM

Was it a pinched/shorted wire?

AndrewB · #17 (**permalink**) 07-01-2019, 01:20 PM

Apologies for the delay - I've been having a hell of a time with login errors here lately.

Anyway, it appears that the brake light switch harness was a bit too close to the headers when I fired up the engine. I've rebuilt it and moved it a safe distance away. Thanks again Patrick and Dan!