Electrical Issues....please help!

[nomad350z]

I've got an 83 Blazer, and its been giving me some electrical issues over the past few months. Where to begin....

When I first started on it, everything seemed to work well. It needed a transmission, so I swapped the worn out TH350 for a 700R4 for the obvious overdrive. Had to buy a new tranny output shaft, and swapped in a Caprice 700R4 with the 4x4 output shaft.

Shortly after the swap, I was romping the truck and smashed the windshield, and had some minor firewall damage (tree limb through the grill, bent the firewall a little near the wiper motor). Since then, the radio doesn't work and my windshield wipers won't work (probably because the linkage is bent). The radio is shorted somewhere, I know because the second I change the fuse it blows a new one, you can actually watch the spark the instant the fuse makes contact, so Its currently without a fuse. Around this time, I noticed the voltage getting low, so I figured the voltage regulator was bad, and I swapped in a new one.

On a trip, I would notice the voltage would swap, between ~11V and ~16V. No matter the load (lights on, AC on, etc), it would go back and forth.

Now, during the day under no load (no lights, no AC, nothing) it will stay constant at 13 V on the dash gauge. At night, with lights on, the voltage will drop dramatically, and if driven enough the truck won't start again when turned off. I've been using a 2 amp charger and night to keep it up and functioning.

I haven't tested the voltage regulator yet, seeing as I don't have a multimeter and haven't had time to check the magnetism on the alternator itself, but is it likely I've gone through two alternators in less than a year? Granted it was a rebuilt Autozone POS, but still.....

My biggest concern is that there is a short somewhere near the firewall, and its draining my battery. I don't want to burn the truck to the ground either, so I'm in need of some advice.

How can I check for a short with the radio? Should I change the alternator? The truck runs fine under no load, so I'm curious about the voltage regulator being bad....

Please Help!

 

[496truck]

You need to just start doing some physical checking of the wires in the dash. A hit like that could've knocked some wiring loose causing the wires to rub on the backside of the dash or metal supports or many open screw threads. Could also have made a bad connection worse, whether it was power or ground. There is no "direction" to point you in, you just need to dig in and look for something obvious first.

There is a chance the radio failed internally, something fell apart inside and is grouding out. Unplug the radio and then try putting the fuse back in. See where it goes from there.

 

[nomad350z]

I removed the alternator this afternoon, and took it to two different places to be tested....it passed at both. I checked all wires, all grounds, and it still is having the same issues. It still runs low on voltage under load (lights on) and will run normal or even high under no load. How is this possible if the voltage regulator works? (which I'm told it does).

On another note, I checked the radio with a multimeter. With the key off, and the radio unplugged, I checked resistance on positive and negative sides of the radio fuse, and it read infinite and 0, respectively. Does this mean that there is a short in the radio? thats the only thing I can figure.....What if there is a short in a speaker?

I checked all wires near the point of impact on the firewall. No visible damage to any wires or grounds.

I believe this is separate to the firewall damage, but still worth mentioning since it might give some insight as to why I'm having the alternator/voltage issues. The rear driver side turn signal is out, as is the brake light. I'm guessing its the same bulb. I replaced the bulb, no change. Checked the voltage, its only reading 5V as compared to 12 volts on the other side. Is it just a bad ground?

It seems this truck is falling apart on me. As the rust gets worse, all the electrical grounds seem to waste away.

 

[496truck]

one side of the fuse terminal should have 12v, the other should be zero. The 0 should be the side that goes to the radio. Take your ohm meter and put one terminal on the 0 side of the fuse and the other terminal to ground. Is this where you got 0 ohms? If so, this is a short to ground. This will blow a fuse immediately.

Unplug the radio and perform the same test. If still 0 ohms, short is in the harness. If infinity, short is in the radio.

Is this an aftermarket radio? How are the connections? Are they just wires twisted together and taped up or are there proper connectors?

You need to start looking at the tail light wire harness. Does the truck have a hitch and wiring for a trailer? If so, some connection there could be bad or corroded causing a voltage drop. If not, check the harness connectors under the tailgate area. Keep checking terminals for your 12v, eventually you'll find the problem.

 

[nomad350z]

I checked the radio fuse, like you said, negative side reads 0 ohms, so its shorted out. I'll start digging around in there.

As far as the taillights, it does have trailer wiring and its pretty rusty under there. I did notice that on the bad side, two bulbs light up (or try to, very dim) when the turn signal is on, whereas on the good side there is only one bulb that lights up. I also noticed that on the same side, same bulb as the blinker, when the headlights are on, the other filament lights up nice and bright. I guess this rules out a bad ground, but means that my 12V source is bad, and splitting to the other bulb. the worse part is that the wiring harness runs inside of the framerail.

Could this be causing my drop in overall voltage when the lights are on?

 

[496truck]

It is a possibility.

Start by removing the trailer wiring harness items. Return the harness to stock as best you can. It sounds like this is the problem. Maybe the old connectors are crimped and not sealed from the elements which caused corrosion to build in the connections.

The stock harness runs on the inside of the frame rail but is easy to see by sliding under the truck, it's only a C-channel frame most of the way down.

 

[nomad350z]

I checked the trailer harness, sure enough there was a corroded wire connection. Replaced it and it all works fine.

I also removed, wire brushed and replaced all the headlight/body/battery ground wires, and my voltage is much more consistent now. Still drops when the blower motor is on, so I'll check that ground next.

As far as the radio goes, I pulled the harness and followed it back to the fuse block, I can't see where the break is. I checked the resistances for the three power supply wires (yellow, black, grey) and all read 0 when referenced to ground. Since I can't find the break in the wire (its all still wrapped up like it was from the factory), I'm thinking I'll just replace the yellow hot lead with a wire off an open ignition space on the fuse block. Should work yeah?

To check all the wiring, I pulled the instrument cluster out for a better view. Now after putting it all together, my fuel gauge and my instrument backlights no longer work. Do these two share a common ground?

*Edit: I figured out the instrument lights, just switched up two wires in the fuse block while putting it back together. The instrument cluster still doesn't work though.

 

[nomad350z]

Anybody know if I can wire up the radio as mentioned above?

And do you have an idea why the fuel gauge doesn't work? *Edit: I wired up the radio as mentioned, it works fine. I did however cut the wire for the radio backlight, since I thought it was shorted out as well. All this did was knock out my instrument cluster lights. I guess they were all in one loop.....

 

[496truck]

I usually don't touch a computer on the weekends, but it sounds like you made some good progress.