Alternator/voltage regulator issue

[bwilhite1]

First time poster here....

I bought an 86 K10 a couple of weeks ago and have an issue with the electrical system. A couple of hours into driving it home, I had to turn on the headlights, which began to go dim after about 30 minutes. I've got a new battery and a new alternator less than a month old, but the wiring seems to be screwed up (picture below). There are three wires coming out of the back of the alternator. One to the positive terminal of the battery, one to ground, and one that loops from the side of the alternator to the rear of the alternator. The rest of the original wires have been cut. The guy that sold me the truck said the voltage regulator is internal to the alternator, but I suck at electrical stuff and don't know that for sure.

Also, the truck blew a 30 amp fuse near the firewall (pictured below) when we reached highway speed. I swapped the fuse to a 40 amp because we were on the side of the interstate and got it home. The fuse has been added, and when it blew, the truck immediately quit running at 60 mph. The ignition switch was dead, but it immediately fired back up after I replaced it.

The truck runs fine with the ground post of the battery disconnected, and the battery seems to have charge enough to turn the starter normally. I've started it probably 8 times since then with no evident battery drain. The gauges don't work in the cab so I can't tell what's going on there either.

Any help would be much appreciated. Thanks.

Can't add pictures on first post. Will add as soon as it will let me.

 

[bwilhite1]

pictures

Here are the pictures.

 

[chengny]

Dammit - I just lost everything I wrote. Short version:

The jumper is okay - don't worry about it for now.

The PO burned up the fusible link and replaced it with an inline fuse holder. He probably blew a more few fuses. He guessed that the short was in the big red lead that normally runs from the link to the stud connection on the alternator. So he cut that lead at both ends. Here is the cut at the alternator end:

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and supplied the power straight over from the battery positive. Nothing really wrong - just looks like crap.

Problem was/is - the short wasn't in that lead. This is evidenced by the fact that you are still blowing that inline fuse.

So now you have that hack job and you still have an unresolved short to ground in the primary lead that runs from the fusible link (now a fuse holder) into the cab. Once through the firewall, it splits and supplies power to the ignition switch and also the part of the fuse block that feeds the wipers, radio and brake lights.

You have to locate the short to ground in that circuit.

 

[bwilhite1]

Thanks. I'll digest that and take a look at it. The wiring in the cab is a mess as well. Most fuses are there and the wipers and brake lights work, but not the radio.

 

[chengny]

The radio not working could be due any number of reasons.

The guy that sold me the truck said the "voltage regulator is internal to the alternator"

That is true, but I fail to see what that statement has to do with anything - nowadays, virtually all alternators have internal regulators. I have to wonder why this even came up in conversation - unless he knows more than he is telling you.

Regardless, what is most important and still remains to be determined is whether the alternator is actually putting out any voltage.

This statement would seem to indicate that - no, the alternator isn't supplying the system voltage ( it is all coming from the battery - that's why the H/L's got dim):

A couple of hours into driving it home, I had to turn on the headlights, which began to go dim after about 30 minutes.

A brand new- fully charged - battery can run a truck for quite a while before it's is so drained that it can't supply enough voltage to power the ignition system. How long is mostly dependent on how much of a load you place on it. If you just run the engine alone (without using the wipers, headlights, HVAC blower, radio, etc) and the engine starts easily - with just a click of the key, you can probably get a week or so of running time before the voltage drops to the point where the engine can't fire.

When the engine is running and driving a functional alternator (one that also is connected to the system correctly), the battery - for the most part - does nothing. Thee alternator does two things. First, it restores the battery to a fully re-charged condition and at the same time it usually supplies all the truck's power requirements. Only in those rare situations where the alternator is incapable of meeting the total system demand, does the battery get off it's ass and actually help out. Even then, all it has to do is supply the small amount of power that the alternator can't provide. Then, when demand drops back to normal, the battery gets re-charged by the alternator and then it goes back to sleep. If everything is working correctly, and under normal circumstances, the only thing the battery is ever required to do is provide power when the engine is off and the starter motor is cranked.

Couple of questions:

1. Does the fuse in the in-line holder ever blow when the key is off?

2. Is the brown wire to the alternator cut ( next to the red wire at the plug)

3. Is this dwg accurate - as far as the cut wires and how they appear to have been connected (when not cut)?

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[chengny]

Wiring diagrams - as I think the system looks now - and the primary side of the circuit that has the short to ground:

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[bwilhite1]

Thanks again for the in-depth response. I called the 1-800 number on the new alternator and the support rep had me reconnect the brown wire on the alternator pigtail (it was cut). Now things seem to be back to normal. 14.2 volts at the battery with the motor running per the tester at AutoZone. I cycled the headlights and the alternator responded normally.

Couple of questions:

1. Does the fuse in the in-line holder ever blow when the key is off?

The fuse doesn't blow with the key off, but blew once at highway speeds with a 30A fuse installed. I got it up to highway speeds again this weekend after replacing the carburetor with no issue. There's a 40A in there now. When it blew, the motor quit and the ignition switch didn't work.

2. Is the brown wire to the alternator cut ( next to the red wire at the plug)

Not anymore, but the red wires seem to be cut as you had drawn in the earlier picture.

Nothing works in the cab, but there are some fuses in place. Some of them are there for things that don't work. No gauges, no AC blower, radio, etc.

 

[chengny]

1. Does the fuse in the in-line holder ever blow when the key is off?

The fuse doesn't blow with the key off, but blew once at highway speeds with a 30A fuse installed. I got it up to highway speeds again this weekend after replacing the carburetor with no issue. There's a 40A in there now. When it blew, the motor quit and the ignition switch didn't work.


The size of the fuse doesn't matter. You have a intermittent dead short to ground. That condition would blow a 100 amp fuse - if they made them.

Also, I can't imagine any scenario where road speed would have any effect on the amp draw in that lead. You can probably assume that the fuse blowing when you reached highway speed was just a coincidence. It could just as well happen backing out of your driveway.

What is important is that the fuse hasn't blown with the key in the OFF position. That tells you that the problem exists "downstream" of the ignition switch - rather than between the in-line fuse and the ignition switch.

Unfortunately, that makes locating the short way more difficult.

If the fuse had blown with the key OFF, it would have kind of narrowed things down to one wire. But since it is happening after the ignition switch, it opens a good number of branch circuits as possible shorts.

Here's what I would do:

Get a big handful of fuses (20 amps is plenty)

Put a good one in the fuse holder

Get under the dash and start shaking wires.

With any luck, the fuse will blow.

To tell when the fuse blows:

1. You can have an assistant watch the fuse (not the best option)

2. Wire a test lamp into the big red lead and bring it in the cab with you (if the light goes out when you shake in a certain area you know you are close)

3. Fix your radio and turn it on (the radio will quit when you shake the sensitive spot)

It may take a few fuse changes, but eventually you should start to get a feel for where the short to ground is occurring.

When/if you find it, come back here and someone will help you straighten out the rest of the wiring mess.

 

[bwilhite1]

Chengny,

I just got the truck back from having some steering components replaced and have started to poke around with the electrics a bit. I removed the gauge cluster, which was unplugged when I bought it, and found a burnt wire and a short in the flexible circuit board. The burnt wire was for the oil pressure gauge and the short was at the main connector where it hooks to the wiring clip. The fuse was also blown at the fuse box. I replaced the fuse and got 12 volts to the clip that plugs into the circuit board. Small victory....

I stuck fuses in the AC and radio slots, but neither one of those work. Do you have a wiring diagram that shows which wire should be hot to each with the key on?

Regarding the big fuse under the hood..... it (thankfully) hasn't blown since the 40A fuse was put in. When it blew, the motor quit. Any chance the items that don't work are related?

 

[bwilhite1]

Here are the pictures. The radio looks like it's been added in via the two blue clips and the yellow wire that on the left side of the fuse panel.

 

[bwilhite1]

Also, since the truck quit when the fuse blew... if I'm shaking wires with the motor running, it should just quit when it shorts again right?