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1981 GMC k25 electrical problems


Jason_l

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OK trucks running now. However it is not drivable do to lighting issues. No lights to rear were working, got under dash and they started working, short somewere I guess. Well the taillights stopped working and brake lights continued to function until today. Now nothing in rear and when u put the signals on it blinks in cab like the hazards are on. There was a nasty mess in harnass coming from switch, melted insulation but wires still had continuity. Cut out bad wires. Still same symptoms. My Silverado needs a couple days of rest for some repairs and I really need this truck. How would I go about running a fused toggle to get rears? I know its not the right way but need it badly. I plan to rewire truck over winter but just need something now. Thanks guys

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You have a bad ground. DC wiring is actually very simple. One wire to provide power, one wire to ground (or return) the power. What comes out of the battery, has to go back in. Don't get hung up on the technicalities, just think of it in simple terms.

 

A circuit consists of three parts, power source, load, power return. There may be a switch before the load to turn it off.

 

A "short" in the wiring means the power is being sent to ground BEFORE it reaches the load(the thing you are trying to power, like a light bulb). A "short" usually blows fuses.

 

An "open" circuit is one where the power is not being returned to the battery. Open circuits do not generally blow fuses.

 

If the ground (or power return wire) is not connected properly it can create some funny results. These results are generally caused by the power trying to get back to the battery, and since the ground is not good, it will find another way to get there. That may mean it leaks through another circuit (sometimes called "feedback"). A common source of feedback issues is a light bulb socket. Sometimes the brake(or signal) light may end up using the parking light wire to try to return to battery, causing all the parking lights to illuminate (usually dimmer than normal though) when brake pedal is depressed.

 

Treat it simply, and by the basics first. In your case you have something wrong with a major ground, or, one of those wires that are bare are touching another wire from a different circuit. Repair all bare wires before doing anything else. It MUST be done if you want to see this working this year.

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Well everything was working last night, now today I've lost all lights in rear again and when I do put the lights on both my indicators light up steady in the cab. I've looked at all simple things and just have no clue what's goin on with it

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AC is just like 12v, only it switches positive & negative 60x per second, and, instead of trying to get back to the battery, it's trying to get back to the actual ground - as in the earth. It's really simple, actually. Black is hot, white is neutral and green is ground. Code says ground and neutral are supposed to be separate, but for years they were tied together - circuits will work with them together, but code doesn't allow that anymore.

 

Electricians unions have managed to scare the general public out of attempting to mess with it (at least in this part of the country), and government / state "codes" has made things WAY more complex than it should be. I'm talking household 120v single phase - industrial 3-phase is a whole other animal. Yep - it can kill you ... but if you understand that and how electricity behaves, and have a reasonable amount of mechanical aptitude, you'll find it's very easy to work with.

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Sounds like you've got a loose connection in there somewhere. Power or ground ... or both. Those are a pain in the ass to troubleshoot, especially when they're intermittent like that.

 

Best thing to do first is do a visual, one section at a time, and look for anything obvious - loose/corroded grounds, frayed wires, green copper, etc ... & if that doesn't turn up anything obvious, get out the meter and start testing circuits one section at a time. Eventually you'll find the problem.

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I would start by taking all the lenses off the lights, so just the bulbs and harness are hanging out. Do what you do to make it fail, then pull the bulbs out one at a time. If it still fails after pulling a bulb, put is back in before moving on to the next one. The most important thing to remember is to only make one change at a time. Also look inside the socket with the bulb out and make sure the contacts at the bottom of the socket are not corroded or damaged. Also check the number of the bulb while it is out. Generally a 1157 bulb needs two connectors in the socket, and the 1156 needs just the one contact. The 1157 bulb will also have the two little tits on the base of the bulb on two different levels, one close to the base of the bulb, the other is on the opposite side, and it about half an inch out of alignment with the other. This is supposed to stop people from putting the 1156 where the 1157 is supposed to go. Problem is, you can make 1156 fit in the 1157 socket but just twisting harder.

 

If pulling a bulb corrects the issue, swap the bulb with another socket to see if it changes anything.

 

After checking bulbs, find the harness connector for the tail lights, and unplug it. If it is still broken, plug it back in, and move forward up the harness to the next connector, and repeat. Along the way, verify that there are no extra wires just hanging around freely. Also look for any connectors attached to the frame, but no wire is attached.

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On a trailer, I've seen a burnt 1157 bust one filament and arc across, joining the brake/turn and running lights together. That was a strange one! If I remember right, left turn made every light blink off and on, alternating from side to side. Removing the bad bulb fixed the problem. Pulled my hair out for a bit that day. Gotta remember to always check the basics first, and closely.

 

In a pinch, you can wire up just about anything to work - if you need it to work, nothing wrong with cutting corners, so long as you have a solid understanding of 12v circuits.

 

I've rigged up push-button start circuits in the past, instead of attempting to rip apart a tilt steering column with no tools. Got the vehicle moving again - no tow truck needed. As a matter of fact, a good friend of mine just had to do this exact thing on a 5 hour road trip across Florida in his '85 T/A. Where there's a will there's a way ... so long as it was built prior to '96 ... :lol:

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OK. Tails are working, brake lights are also. Signals work, oopposite side brake light works when signal is onn,both when off. Both indicators are always lit inside cab, they don't flash unless turn signal is on, there just constantly lit. Also no dash lights or wipers.

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Could be a bad flasher relay - try swapping that out and see if the dash indicators go out.

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On a trailer, I've seen a burnt 1157 bust one filament and arc across, joining the brake/turn and running lights together. That was a strange one! If I remember right, left turn made every light blink off and on, alternating from side to side. Removing the bad bulb fixed the problem. Pulled my hair out for a bit that day. Gotta remember to always check the basics first, and closely.

 

In a pinch, you can wire up just about anything to work - if you need it to work, nothing wrong with cutting corners, so long as you have a solid understanding of 12v circuits.

 

I've rigged up push-button start circuits in the past, instead of attempting to rip apart a tilt steering column with no tools. Got the vehicle moving again - no tow truck needed. As a matter of fact, a good friend of mine just had to do this exact thing on a 5 hour road trip across Florida in his '85 T/A. Where there's a will there's a way ... so long as it was built prior to '96 ... :lol:

 

I have had the 1157 issue as well. Also had the 1156 in the 1157 socket. The tabs on the 1157 are offset, at least they are offset to a mechanic. 1156 tabs are on same level as each other. Some customers figure the 1156 bulb is just being a bit of a bitch by not allowing the bulb to turn, and they just twist harder.

 

I completely agree with Jsdirt regarding checking the simple stuff first. That same advice also applies to issues with the way the engine is running, or the brakes working correctly.

 

I have no issue diagnosing DC systems, but A/C systems just confuse the hell out of me. I have zero formal training or exposure to A/C circuits, and due to that, I just cannot grasp how the current actually flows in alternating current circuits. I mean if the power is constantly reversing direction (60 times a second in North America), does that not mean the current never actually gets anywhere? I think my issue is simply that I am trying to apply DC circuit logic to A/C circuits. Doesn't help when the terms do not say positive and negative either. Since I have never had the need to repair or do anything more than installing a ceiling fan, or pony panel, I have never bothered to teach myself.

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