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Silverado & Fullsize Pick-ups The Silverado has been one of the best selling trucks in the US for decades, and is truly proven to be "like a rock".

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  #1  
Old 04-16-2009, 02:27 PM
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Default Tail light mystery. Stuck on!

I recently purchased a 1993 red Chevy pickup. It's a 4.3L six with manual transmission, 2WD, rollup windows, no air, AM radio, excellent condition. I fell in love with it immediately. I've been seeking simplicity in my life. Damn!

Couple of days ago I heard a buzzing/vibrating sound in front. I stopped the engine and it continued. I opened the hood and discovered that it was a relay mounted to the firewall above the brake master cylinder. I unplugged it and went through everything electrical to identify the relay. Everything worked just fine. Then I noticed that my brake lights were not working. I had tested everything when I bought it so I knew this was new.

I drove the truck home and parked it. An hour later I went out with some tools to see what I could do. I plugged the relay in and it worked just fine. The brake pedal worked the lights. Hmm. I went for a drive and about twenty minutes into the drive the relay started to buzz again and after about five minutes it stopped buzzing and my brake lights were permanently on. I had to unplug the relay to shut them off. As soon as I plugged the relay back in, the lights went permanently on. I drove home again. I put the relay back in and went through all the wires looking for a short, wiggling here and there, and the light stayed on. I unplugged the relay.

Next day, I put the relay back and the brake lights worked fine. Corresponded to the pedal. I left the truck like this and went in to go online and try to find some info. When I went back out to the truck half an hour later, the brake lights were stuck on again. I had to pull the relay.

So basically this... after an hour or so, if the relay is unplugged, no power is measured at the harness connection to the coil. When plugged in, no power to the coil unless i hit the brakes. When left plugged in for more than twenty minutes, the system seems to slowly build up to sufficient voltage to energize the coil and lock the brake lights on.

I removed the harness from the brake light switch that rides on the pedal arm. It made no difference. I even took the switch apart, inspected it, polished the contacts, and it fixed nothing. I removed the harness to the backup lights on the manual transmission. I changed nothing.

The dealer's service database goes back no further than 10 years and the parts dept cannot confirm that this 93 even uses a relay for the brakes. Someone told me that this relay might be a HMSL (high mount stop lamp) relay. If that is so, then why does it affect the regular brake lights?
How is it physically possible for a relay to behave this way? I can only imagine that a new 35 dollar relay is going to do the same thing. It's clearly getting energy/voltage that it is not supposed to.

Relay is an Omron 12077867 Z7365 bolted to the firewall about five inches above the brake master cylinder. It has four spade connections. It is a very basic spst 12vdc relay. There does not appear to be anything physically wrong with it at all.

Any ideas? I'm at my wit's end.
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Last edited by Antonia; 04-16-2009 at 02:30 PM.
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Old 04-16-2009, 03:37 PM
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Could be the relay coil is going bad inside the relay or bad contact points in the relay. Can you see inside the relay? or does it have a cover that you can take off to check the contact points? The low power input to the relay ??? got me?? maybe someone else can chime in on that issue. The onle thing I can think of is a corroded or loose wire someware.
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Old 04-16-2009, 04:42 PM
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The contacts in the relay are sticking. If you get a new relay, it'll fix it.

Being bolted to the firewall like it is, it sounds like someone rigged it to get the brake lights to work. Its supposed to be wired to a relay on the fuse/relay block, but apparently there was a short in the wiring and they didn't feel like fixing the wiring, so they just wired it strait up to a boughten relay and bypassed the fuse block.
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Old 04-16-2009, 06:13 PM
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Another forum suggested that it was a HMSL relay. They even provided a photograph that showed the same relay mounted above the master cylinder just like mine. Mine also bolts onto the firewall in what appears to be a factory designed location, with threads, as well as being routed into a main bundle of wires in a fashion that is really unlikely to be an aftermarket tap job. But the HMSL on the cab does not work, and I removed the harness that led to the topper HMSL along with the trailer harness. Everything is right back plugged together just like it came out of the factory.

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 made an appointment with the dealer that is a week away. I'm not optomistic. The parts guys were no help at all, and the service techs that I talked to said their computer data only goes back ten years and they didn't seem any more knowledgeable about this wiring than I am. When a couple of them actually stood there in the lot and watched as I plugged in the relay, waited five minutes, and the brake lights lit up on their own, noone on the brake pedal and no keys in the ignition, they started laughing nervously and scratching their heads (prettty scary).

Last edited by Antonia; 04-16-2009 at 06:20 PM.
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Old 04-16-2009, 07:10 PM
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If I were you, I wouldn't have made an apointment with a dealership. Thats costly...

And why the rant about your moped?

Anyway, yeah it seems like there was a HMSL relay on the firewall.... I can't be sure. I'm not as familiar with the pre-1995 trucks as I am with the 1996-1999 models.

Just hang around a little while and I'm shure shawnvw or another tech we have.
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Old 04-17-2009, 09:29 AM
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I did have the relay apart. It looks brand new with no arcing or other damage. I polished the contacts. I did the same with the brake pedal switch.
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Old 04-17-2009, 04:10 PM
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As you know its hard to "tele-diagnose" things, but it may be that the relay is indeed bad. I know in some cases the low-amperage control wiring has a tendency to let a relay buzz without damaging the contacts.
Its really hard to tell by looking at it if the magnetic coil is bad or the spring has lost its tension. Replace the relay with a brand new one, or if its the same, you can use the horn relay to see if its bad.

If that doesn't work, I'm not sure what to do. Get a new relay. I can't imagine why a relay would be 35 bucks.... Usually they run less than $5.
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Last edited by EvansBlue; 04-17-2009 at 04:21 PM.
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Old 04-17-2009, 05:16 PM
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buzzing relays usually indicate poor power\ground contact

Sounds like the wiring for the relay has chaffed against something that has constant power.
I searched mitchell on demand for this relay and couldn't find it. I also couldn't find the wiring for the CHMSL.....but the wiring diagrams are not very nice and are somewhat hard to read.

I would trace the wiring while the problem is occuring.....probe the wires, find out which wire has voltage when it shouldn't, and trace it back to see where the problem is
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Old 04-17-2009, 06:01 PM
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But why on earth would the CTSY fuse cut power to the brake lamps? I checked the cargo lamp on the back of the cab since my Clymers manual shows the cargo lamp in the CTSY circuit. The screws were too close to the topper to get the lens off so I HAD TO REMOVE THE TOPPER TO GET THE CARGO LAMP LENS OFF! The bulbs were fine and there were no bad connections or shorts. It doesn't work. Never did. It doesn't work even with the fuse in. All the harnesses are connected. With the fuse in and the Cargo switch on and the door switch opened, there is no power at the pins of the plug that leads to the cargo lamp or to the dome light. The harness just disappears within a bundle of wires protected with split loom... into a riddle, wrapped in a mystery, inside an enigma. I know.... most forums take a dim view of using caps. I can't help it. I'm physically and mentally demoralized by this whole affair. I have NEVER encountered such an inexplicable nightmare in a vehicle. I'm going to have to take it in for repair and just grit my teeth. You and I know that the service dept is not any more likely to find the source of this problem that we have, and will simply default to the "we need to replace 1200.00 dollars worth of wiring and harnessess to fix your wiring" excuse. I can jerry-rig a wire to the brake lights from my pedal switch, but odds are good that this is just going to initiate more whack-a-mole fun as that hack messes with the headlamps, horn, radio, and god knows what else. If you read a police blotter about a red chevy pickup erupting in flames on the front lawn of a small town in Michigan, you'll probably be able to guess the culprit.
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Old 04-17-2009, 10:48 PM
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Wow, I agree , just get a 5 dollar relay at MegaLowMart or wherever and hook it up, if it does not fix the problem just take it back. Have you checked to see if your pedal switch is sticking or bas as well?
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Last edited by Mike Sigmond; 04-17-2009 at 10:57 PM.
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Old 04-17-2009, 10:48 PM
 
 
 
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08, 1996, 2500, brake, brakelights, chevy, express, fix, light, lights, omron, pickup, relay, reverse, stuck, tahoe, z7365


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