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1994 Chevy C1500 Truck Brake Light Mystery


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antonia 
Member - Posts: 16
Member spacespace
Joined: April 16, 2009
Location: Illinois, United States
Posted: April 16, 2009 at 9:41 AM / IP Logged  
This has me pretty stumped. A friend suggested your forum.
A couple of days ago I heard a high pitched vibration in the engine compartment. I stopped the engine and it continued. I opened the hood and it was coming from a relay just above the brake master cylinder.
I unplugged the relay, started the truck, and proceeded to test everything, finally discovering that my brake lights weren't working.
When I got home, I plugged the relay back in, and brakes worked fine. No vibration or noise coming from the relay. An hour later, I looked out at the driveway and the brake lights were lit. I went out to the truck and had to unplug the relay in order not to drain the battery. I plugged the relay right back in to test just after unplugging and the brake lights stayed on again. I left it unplugged for the night.
Next morning I plug in the relay and the brakes lights function normally with the pedal. I drive the truck and occasionally back up to any large window to check the brake lights. About twenty minutes into my drive, the relay starts to vibrate (my friend said the contacts are "bouncing") for about five minutes and then suddenly locks into the on position, keeping the brake lights on. The only way to turn the lights off is to unplug the relay.
So I have about twenty minutes of use from the relay whenever I plug it in, engine on or not, until somehow, somewhere, a short builds up strength, or something bizzare, and the relay coil gets progressively energized to the point where it locks.
When it's locked on, I've gone as far as I'm able to, under the truck, back bumper, behind the light assemblies, etc, to wiggle wires to see if I can isolate a short. Nothing.
What could be going on?
ckeeler 
Gold - Posts: 1,461
Gold spacespace
Joined: June 20, 2008
Location: New Mexico, United States
Posted: April 16, 2009 at 9:47 AM / IP Logged  
purchase a new relay.
antonia 
Member - Posts: 16
Member spacespace
Joined: April 16, 2009
Location: Illinois, United States
Posted: April 16, 2009 at 9:52 AM / IP Logged  
It's a 1994 C1500 Chevy Work Truck.   4.3 liter v6. Manual transmission.   There is a topper on it. Topper installed a third brake light tap on the left tail light assembly harness, but the job looks pretty professional and straightforward. I had doubted that it was part of a "high mounted brake light" circuit since it tapped into the main brake light wires, however (the mystery deepens) there was a blown fuse on the tap that I discovered when investigating behind the light assembly. I replaced the fuse and the high mounted light worked for the first time since owning the truck. However, when next the relay stuck in the ON position, and I had to unplug it to extinguish the brake lights, the high mounted light STAYED LIT even after unplugging the relay! It must be on another circuit. I had to unplug the wires leading to the high mount light to turn it off. Next day I plugged the high mount light back together, and plugged in the relay. The regular brake lights worked with the pedal just fine, but the high mount now refused to light!   It's fuse was fine.   Go figure.   Twenty minutes later, the relay started buzzing/bouncing, and then the brake lights stuck on.
Exasperating.
antonia 
Member - Posts: 16
Member spacespace
Joined: April 16, 2009
Location: Illinois, United States
Posted: April 16, 2009 at 9:56 AM / IP Logged  
Isn't a relay kind of a go/no-go unit?   How can it be half bad? I removed it from its casing and it looks brand new. I watched the contacts vibrating and watched them close.   Later, I plugged it back in and the contacts were resting open. I even cleaned the contacts with 400 sandpaper just in case they might be dirty or arcing or something.
The coil is evidently getting power that it's not supposed to. A new relay will do the very same thing. No?
ckeeler 
Gold - Posts: 1,461
Gold spacespace
Joined: June 20, 2008
Location: New Mexico, United States
Posted: April 16, 2009 at 10:03 AM / IP Logged  

i dont think so because you say unplugging the relay resets it. if it was a power issue to the relay coil it would do it as soon as you plugged it back in probably. to be sure you could measure the voltage at those terminals while trying to duplicate the problem to see what exactly is going on.

antonia 
Member - Posts: 16
Member spacespace
Joined: April 16, 2009
Location: Illinois, United States
Posted: April 16, 2009 at 10:17 AM / IP Logged  
I realize my post was a bit long-winded. I don't know enough about this stuff to be concise. But the only mention I made (i think) to plugging it back in immediately was
I plugged the relay right back in to test just after unplugging and the brake lights stayed on again. I left it unplugged for the night.
...where I indicate that it did not reset the circuit. Unless I don't understand 'reset' correctly. It did not reestablish 'normalcy'. It kept the brake lights burning. Only after the truck sits still, relay unplugged, does it reset (or return to a normal state) and allow the relay to stay open.
antonia 
Member - Posts: 16
Member spacespace
Joined: April 16, 2009
Location: Illinois, United States
Posted: April 16, 2009 at 10:20 AM / IP Logged  
What on earth could be causing a slow, fairly predictable, build-up of energy to the coil in this relay?   I've never heard of such a thing before in an automotive system.
ckeeler 
Gold - Posts: 1,461
Gold spacespace
Joined: June 20, 2008
Location: New Mexico, United States
Posted: April 16, 2009 at 10:24 AM / IP Logged  
ckeeler wrote:

measure the voltage at those terminals while trying to duplicate the problem to see what exactly is going on.

antonia 
Member - Posts: 16
Member spacespace
Joined: April 16, 2009
Location: Illinois, United States
Posted: April 16, 2009 at 12:35 PM / IP Logged  
voltage is 12.4 volts across the terminals when buzzing.   12.4 volts when 'locked' on. I have not had the system 'settle' long enough now to measure voltage when the relay is not keeping the brake lights powered on. I called three chevy parts/service departments and they don't have record going back more than 10 years and were unable to help. Noone had any idea what the realy above the brake master is. I cross referenced a couple of more numbers and came up with a blower motor relay and backup lights relay.   The second is interesting.
I removed the harness from the brake light switch on the pedal arm and it made no difference. Lights still locked on. While I had it out I disassembled the switch and cleaned the contacts. No melt or arc or problems.   Put everything back together and right back where I started.
I frankly cannot afford to have a couple of hundred dollar service charge for this.   I cannot drive the vehicle as it is. I think I'm going to cry.
ckeeler 
Gold - Posts: 1,461
Gold spacespace
Joined: June 20, 2008
Location: New Mexico, United States
Posted: April 16, 2009 at 12:39 PM / IP Logged  
myself and others on here will help you get it figured out, please post back with the results of the final portion of your testing. and can you post the part number off of the relay so i can check it out and see exactly what relay it is and the specs on it? a pic of it would also be helpful.
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