1994 Chevy C1500 Truck Brake Light Mystery
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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=113195
Printed Date: April 29, 2024 at 8:48 AM
Topic: 1994 Chevy C1500 Truck Brake Light Mystery
Posted By: antonia
Subject: 1994 Chevy C1500 Truck Brake Light Mystery
Date Posted: April 16, 2009 at 9:41 AM
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?
Replies:
Posted By: ckeeler
Date Posted: April 16, 2009 at 9:47 AM
purchase a new relay.
Posted By: antonia
Date Posted: April 16, 2009 at 9:52 AM
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.
Posted By: antonia
Date Posted: April 16, 2009 at 9:56 AM
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?
Posted By: ckeeler
Date Posted: April 16, 2009 at 10:03 AM
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.
Posted By: antonia
Date Posted: April 16, 2009 at 10:17 AM
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.
Posted By: antonia
Date Posted: April 16, 2009 at 10:20 AM
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.
Posted By: ckeeler
Date Posted: April 16, 2009 at 10:24 AM
ckeeler wrote:
measure the voltage at those terminals while trying to duplicate the problem to see what exactly is going on.
Posted By: antonia
Date Posted: April 16, 2009 at 12:35 PM
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.
Posted By: ckeeler
Date Posted: April 16, 2009 at 12:39 PM
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.
Posted By: antonia
Date Posted: April 16, 2009 at 1:09 PM
Omron 12077867 Z7365
Parts counter at dealer has no record for a 'brake light relay'. Can't tell me if the brakes even use on. Neither could the service dept.
He said that number crosses to a blower relay. Online I discovered a cross ref to a 'backup light relay' listed as an AR173 Echlin. No pic or pinout so I couldn't confirm. I crawled under the transmission and and there are only two switches on the housing. I unplugged them both to see if the backup switch (this is manual transmission) was at fault. It did nothing. Brake lights stayed on. I exposed the bulbset on the assembly and discovered that the lit filament is the lower of two on the middle bulb (of three bulbs). I can't seem to confirm in any manuals or datasheets what this filament is dedicated to. I put the bulb assembly in view of the rear view mirror, started the truck, and put trans in reverse. No other filaments lit. I'll have to wait an hour until I can plug in the relay again, and try to observe which filaments light when I use brakes and backup lights.
Posted By: antonia
Date Posted: April 16, 2009 at 1:14 PM
Just to recap the main points.. If the truck has been sitting still for an hour or so, I can plug in the relay and all seems fine. About twenty minutes later, engine on or not, the relay will start to buzz/chatter, and then about five to ten minutes after that, it will fully close and keep the taillights burning. The lit filament is the middle bulb, lower filament (closer to the base). If I unplug the relay, the filament goes out. If I plug the relay back in immediately, the filament goes on. All of this is completely, so far, independant of the brake switch and backup light switch. If I wait about an hour, I can plug the relay in again and the brake switch controls the filament/s for about twenty minutes. Then the relay starts to buzz/chatter and then locks on.
Posted By: ckeeler
Date Posted: April 16, 2009 at 2:35 PM
does the relay look like this? and what were the final results you found with your meter? 
Posted By: howie ll
Date Posted: April 16, 2009 at 4:54 PM
Relay for the brake lights? Doesn't sound right, please check ALL your fuses. Had the brake lights come on with the ignition on a new Astra recently, it was a missing fuse.
Posted By: howie ll
Date Posted: April 16, 2009 at 4:57 PM
Sorry I was rambling a bit before but surely that relay has to be wired as ground to 85 brake light switch (at pedal) to 86, power in to 30 aqnd 87 out to lights, surely it only activates when brakes are pressed, therefore we've either got a faulty switch at the pedal did you not disconect the pedal switch at any time to eliminate the pedal switch or the relay is faulty so it's worth trying another.
Posted By: antonia
Date Posted: April 16, 2009 at 6:01 PM
I did disconnect the brake switch at the pedal. It made no difference. No person, at Chevy parts counter, or at service dept, nor at two aftermarket autoparts stores, was able to tell me what this relay is dedicated to. I pulled fuses one at a time while the tail lights were stuck on. All the likely candidates did nothing. So I went at them one at a time. I got to the CTSY light and bingo. I'll assume that CTSY means courtesy, or dome, light. Now the dome light never did work on this truck. In fact, ironically, it was the only light that would not work when I bought it. I stopped at autovalue on the way home from buying the truck and got a bulb, but to no avial. But ALL the other lights worked properly. Today, when I discovered that the CTSY fuse opened the troubled circuit, I began to really wonder. The door switch for this light is missing on the driver's side. There are a couple of small plugs near this point (in the vicinity of the parking brake) that are dangling unconnected to anything. I'm next going to get a switch to replace the missing one and connect it, although logically I cannot imagine why a missing switch should gum up the works. A switch is open normally. If it's missing/disconnected the system just thinks it's open... no? Why the heck to engineers do this crap? Someone told me that on some of these Chevys there is a separate fuse for 'courtesy' and for 'dome'. If one or the other is circuit is not complete, it keeps the other one from working. I have a Puch moped at home. I was online getting info on it one evening and someone on a forum was having a hard time starting the bike. Another member told her to make sure the bulb was good in the taillight. They explained that on some s the ignition would not function without a complete circuit through the taillight. No, this is not a safety feature. It's a PITA gremlin that the engineers just let pass. And this from a MOPED! This isn't computer or microcomputer stuff folks. Why does it have to have been made so complicated? The guy at Autovalue tonight, after poking around with a meter for 15 minutes (extremely kind of him) advised me that if all else fails, I ought to work around this relay entirely and hard wire the brake lights myself. I'm afraid that this is just asking for more trouble. I'd next find my headlights screwing up, and my turn signals, etc. Can't I just enjoy a basic chevy with manual transmission and roll up windows? I was so happy last week.
Posted By: antonia
Date Posted: April 16, 2009 at 6:03 PM
ckeeler, that's not the relay. Mine has four spades in square arrangement.\ I'll post a pic if neccessary.
Posted By: ckeeler
Date Posted: April 16, 2009 at 8:12 PM
sounds non factory added in to me if thats the case. if it was mine id rip it out and go straight from the brakelight switch to the lights, after all thats how its supposed to be wired from the factory.
Posted By: howie ll
Date Posted: April 17, 2009 at 7:43 AM
As I said before, even with hi-level brakes, your only pulling 5-7amps, why would you need a relay? It's far too early for silly late model electrics. Incidentally re your Moped, the Germans made wonderful, advanced tanks in WWll lucky for us they had design faults just like your Moped!
Posted By: antonia
Date Posted: April 17, 2009 at 3:50 PM
howie, indeed why would you need a relay? Why is this relay interfering with my brake lights? Does this truck use a relay for the brakes? The dealer is unable to say yes or no! I'm just gobsmacked and beside myself that information about this truck is so hard to come by. It's not a Maserati. It's one of the best selling vehicles of all time!
Posted By: antonia
Date Posted: April 17, 2009 at 3:55 PM
I just went to the junk yard and got a new door switch for the courtesy lights. I plugged it in and immediately the "key in ignition" bell started sounding off. This is rich.
I also grabbed a spring from the turn signal innards that, broken, was keeping the signal from defeating after turning the wheel. I cleaned and relubed all the contacts. After both these fixes, I held my breath and reinserted the CTSY fuse, and nothing was remedied. The CTSY circuit still has a short somewhere that is energizing the HMSL relay which in turn is activating my brake lights! And fixing the CTSY door switch has now remedied one of my least favorite alarms... the "hey your keys are in your ignition" bell.
Posted By: ckeeler
Date Posted: April 17, 2009 at 4:06 PM
that truck does not use a relay for the brakes. at least it's not setup that way from the factory.
Posted By: antonia
Date Posted: April 17, 2009 at 6:08 PM
If it does not use a relay for the brakes, can you theorize why it would use a relay for the HMSL? The 'third' brake light would seem, to me, to draw even less amperage. I've been told by a few people now that the relay as I've described it is an HSML relay. Yet it is affecting the funtion of the brakes. 
This is an image of the relay. Two yellow wires, a black, and a white. It is mounted above the brake master cyclinder on the firewall.
It is stamped Omron 12077867 Z7365
Posted By: howie ll
Date Posted: April 17, 2009 at 6:09 PM
Sorry I'm missing something, what's an HMSL relay?
Posted By: howie ll
Date Posted: April 17, 2009 at 6:16 PM
OK, I've seen the photo, next stage would be to trace this relay's wiring! Can you not get hold of a Chilton manual for that year from your local library? I would think about rewiring the brake lights from the brake pedal switch back avoiding that relay. What's the link to the dome (courtesy) lights?
Posted By: antonia
Date Posted: April 18, 2009 at 9:50 AM
I wish I knew what the link was to the courtesy lights. The Chilton and Clymer manuals are no help. There is no mention of HMSL in either of them. There are five wires leading back to the rear bumper on my truck and only four of them are leading to the taillight assembly harness. A yellow one is loose and terminated. It used to be connected to the HMSL (high mount stop lamp) on the topper. The topper installer had connected the HMSL to this yellow and tapped into a black wire behind the left taillight assembly. I have since removed this HMSL wiring that the topper installer used. The clymers diagam I have shows yellow going to the brake lights. But the plug has not been altered in any way. It is a four pin plug on the harness going to the taillights, and there are five wires leading out of the split loom plastic conduit leading to the back of the truck. I'm really starting to suspect that this yellow got shorted somehow in the messy wiring that the topper installer had going up to the HMSL on the topper.
Posted By: howie ll
Date Posted: April 18, 2009 at 2:20 PM
Not only would I reroute the brake pedal switch back to the back lights, I 'm suspicious of the yellow being taped to the black. Now I don't know the vehicle and I'm 4,500 miles away! ..But black is normally a ground on US, old UK colours and some Japanese. If the yellow came from the top light, check what the black did. 5 wires seems too few because of rear fog lamp(s) and separate indicators to brake lights (amber, look at any Euro or Japanese, they all have separate amber flashers, our equivalent would be at least 8 wires also and I think this also applies to N. America, sidelights are separated (ie fused, either left right or diagonally) as a safety precaution. Next disconnect that darn relay and then test drive the vehicle, see if anything isn't working apart from what we already know about. I don't know how relevant this is because big trucks are a rarity here but I've worked a few Ford Litenings and they have a courtesy light on the outside rear of the cab to illuminate the load area. Could there have been one on yours or something similar that's been disconnected and has a lose cable that's shorting out?
Posted By: Mad Scientists
Date Posted: April 20, 2009 at 8:04 PM
Alldata doesn't show a relay for the brake lights.. I don't see a wiring diagram for the third brake light, but without an adapter module, I'd expect the feed for the third brake light to come from the harness, between the stop light switch on the pedal assembly and the turn signal switch. Based on the additional information you've given about the door switch, I'm inclined to think you've got a wiring problem. The 'key in ignition' wire from the door switch is the tan wire.. there's should be three wires on the switch; tan, black and white. I don't see any connection between the courtesy wiring and the brake wiring. I don' t know what the relay is.. while there a relay listed for snow plow lighting package, I doubt a v6 2wd has that. More troubleshooting is going to be required.. I'd consider pulling the relay and testing the wiring to try to determine what that relay is wired to. Jim
Posted By: antonia
Date Posted: April 21, 2009 at 6:15 PM
Ok, I'm pretty much home. Should I quit while I'm ahead?
Good idea to eliminate the relay despite my doubts. Autovalue had the relay for 24.00 dollars. I'm really not inclined to swap in another relay/harness/plug since it seems to be any deviation from the norm is usually the culprit in these headaches some way down the road.
I went to the local junk perveyor and got one off of a '90 c1500. Got home, plugged it in, and it commenced buzzing. Oh well. It was a good idea. If anything, it was buzzing more consistantly than my original. Probably sign of a healthy unit!
I removed the CTSY fuse again and got a good strong flashlight up under the dash. (one of a pair of anodized aluminum 9 led flashlights I just picked up at Harbor Frieght in Grand Rapids, MI. 2.99 for the pair with a clipped coupon. Holy crap these are bright!)
So here's what I discovered. There is a terminal block on the firewall straight back of the parking brake. It has about 25 sockets with recessed spade plugs (never know whether to call these male or female. Even electrical systems are getting confused now'days) All were empty, except one. Probably an indication of the simplicity of this 'work truck'. An orange wire was plugged into the top socket of the third row from the left. With a good light on it I could see that the socket had the letters GRN next to it. The sockets are idiot proofed (poka-yoked?) so that you cannot stuff the wrong plug into them. Interestingly, There was a slot on the first row marked ORG that accepted the plug as well. I switch these slots, and plugged the CTSY fuse back in, and presto! Everything works as it did before. Brakes, turn signals, running lights (forgot to check the backup lights). All, unfortunately, except for the courtesy and cargo lamps themselves.
Now the hell did all the outside lamps work for three months with this wire wrong? My courtesy and cargo lamps didn't work before, and they still don't. I was so sure that the CTSY lamp would work now that I fixed someones previous mistake, but they didn't.
Oh well. I can certainly live without the courtesy and cargo lamps.
Thanks everyone here for your help and suggestions. And for putting up with my ill temper.
Posted By: antonia
Date Posted: April 21, 2009 at 10:36 PM
I wonder if anyone out there also has the HAYNEs Repair Manual for 88 thru 98 Chevrolet and GMC Pickups?
On page 12-3 there are four photographs. The upper left photo, 6.4, shows the 'convenience center'.
Upper right of this photo is a four pin inline plug.
In my truck the left part of the plug is dangling and the right side is nowhere to be seen.
Does anyone know what this plug does? Right now my CTSY and cargo lamps are not working. Nor are my backup lights.
On my 'convenience center' there is currently only one orange wire plugged in. No other receptacles are being used. Does this seem right?
If noone has this manual, I'll try to post a digital image tomorrow.
Posted By: i am an idiot
Date Posted: April 21, 2009 at 10:39 PM
What all does the "Convenience Center" contain? Does the Cigarette lighter work? The convenience center should have more than one wire. Orange is probably constant 12 volts.
Posted By: howie ll
Date Posted: April 22, 2009 at 2:34 AM
Black ground? Whatever colour GM uses for illumination, possibly an ignition or ACC. That's a nice name for a ciggie lighter and an ashtray. Antonia, are you a female person or did you use the name to get a faster responce? It worked! Make sure the orange has 12volts perm, feed a ground to one of the other pins, I'll bet the third pin should be coming from the door contacts.
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