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Alternator wiring?

85 Jimmy

85 Jimmy

Sheepdog
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Ok, I'm having an issue with my alternator. When the truck is running and the alternator is all connected, the positive wire from the battery to the stud on the back of the alternator gets hot and will melt the insulation off the wire. If I disconnect the positive wire it stops, but it doesn't charge, and if I disconnect the 2 wire plug on the alternator it stops, but doesn't charge.

This is on a 91. I checked the wire's at the plug and the black does not have continuity to ground and goes to the ecm (which I'm not using b/c I'm running a stand alone harness for TPI) and the pink with a black tracer runs to the fuse block, and is grounded. Is that wire supposed to be grounded? the schematic I have doesn't have a pink with black tracer wire on it, it shows a black, rust, and red...

If this is not readable or understandable it's b/c I'm sick with a fever of 103...
 
Ok, that output post in the back is the charging lug. When its running it should be putting out like 13 or something volts, if its melting the wire, either its cranking out mega volts, or that wire is got to much resistance or a short in it. Im assuming the battery is under the hood, so unhook that big red cable, and run a new one of equal or greater size from the charging lug straight to the battery positive terminal.

I cant remember the plug, but I do know one side of it goes to the ALT light and battery gauge, and it needs that light and uses it as an "exciter" to get the alt to charge below a certain RPM.

Is this a factory regular ALT or a high output one? I have seen where guys use higher output ones and cook the battery charging wire when they are supposed to upgrade it with a bigger cable.
 
It's a standard alternator for a 91 blazer. I just replaced it under warranty, but still have the same issue. I'm not running standard guages either, I'm running autometer, so none of the wires for the guage or light are hooked up. The autometer guage just uses the positive wire to show charge.
 
That could maybe be your problem. One of them 2 wires on that plug in "im not sure so dont use me as gospel but someone here for sure will" needs resistance against it for the ALT to work correctly. On a side note, is it possible the wire was maybe rubbed raw and grinding against the side of the ALT and causing a short in it? Im kinda reaching I know, but another thing, is the battery terminals all clean and blah blah.
 
Two wire plug...is it rectangular or kidney shaped? As I recall the '91 should have a CS-style alternator, just want to clarify.

The CS requires resistance in the form of a light or resistor to start charging, is it possible the harness you have isn't quite setup right? (in other words, for the earlier SI alternators?)

It's probably something far simpler than that, but you just raised a couple of question marks. :)
 
I actually just pulled the harness and checked every wire yesterday and all the wires look good. I've been running this setup for a few years with no issue, then swapped bodies from an 85 to 91 and still no issue, then just recently changed the fuel pump and after I swapped the fuel pump this issue came up...
 
I have a CS alternator, and the plug is for a CS alternator. the plug is a 2 wire plug, one wire is black and runs to the old ECM and the other wire is pink with a black tracer and runs to the fuse block. Then on the stud on the back there is a positive that runs to the battery and another that runs to the junction block on the firewall.
 
Since it's what you've been running without issue, I don't see the wiring being the problem then, that makes it a BIT easier. :)

Have you been able to measure the output on the charge lug with a voltmeter? I'd think that the next logical step. If it's over about 14.5V, alternator is probably the culprit.
 
With everything plugged in it charges between 13.5 and 14.5, but the positive wire gets hot. It's weird because the other wire on the lug that goes to the junction on the firewall doesn't get hot, just the one going to the battery.
 
Your battery might have a plate that's broken and causing a short. That would cause high current in the wire which is what would make it hot, not high voltage.
 
I'm far from an electrical pro, but if the wire is getting hot, and the voltage is correct, the only thing I can think of is that the draw on that wire is too much, or the wire itself is bad.

Does the wire get hot as soon as you start the truck, or does it take awhile?
 
I'm not real sharp with wiring but I've got a 91 factory manual if you need to use one.
 
dyeager535 said:
I'm far from an electrical pro, but if the wire is getting hot, and the voltage is correct, the only thing I can think of is that the draw on that wire is too much, or the wire itself is bad.

Does the wire get hot as soon as you start the truck, or does it take awhile?
Click to expand...

It starts to get hot as soon as the truck is started. If the ALT is unplugged when I start it, then plug it in while it's running you can hear a difference like a load is being put on the ALT and the wire starts to get hot.
 
That's 'cuz it's completing the circuit. Current won't flow through a wire that's only connected at one end.
 
I guess my main question is, is the pink and black wire that goes from the alternator to the fuse block supposed to be grounded? That wire on mine is grounded.
 
If that wire is stock, and you did not upgrade the alternator, then you have a bad short somewhere.

Its not uncommon for someone to buy a heavier alternator and not upgrade the wire. Check the voltage at the stud on the alternator with it running, and if its not over 14.5 volts, then the smaller wires are OK.

Turn your radio on, but turn the volume all the way down. Crank the truck and listen closely to the speakers.
If you hear a loud whine that changes tone with engine speed, then you may have a blown diode in the alternator.
A slight whine is OK.

Check the wire carefully from the alternator to the battery for a short of some kind.

A shorted plate in the battery would cause this problem, but the truck should fail to crank after sitting for a short while.
It will self discharge just sitting with a shorted plate.
 
Meltdown = too much load and/or short. A bad connection can cause a hot spot also.
 
I've experienced this before.

It was actually a corroded connection on the charge wire to my "hot wire split block". The block was connected with a heavy gauge wire to the starter, and starter of course to the battery (rear mounted battery).

I had forgotten to tighten the lug in the block, so it was loose, allowing for tons of corrosion build up. It was charging and all, but noticed things were getting hot.

Resistance or loose contact point (corrosion) can cause excessive heat in any 12v lead wire.
 
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