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Discussion Starter #1
If any of you have read any of my topics, you may have noticed that there is a charging issue that keeps popping up. And then going away :banghead:

This is a little long-winded but intermittent issues seem to have a long story behind them.

Well, it's back. Today I was driving and noticed my fuel pressure was dropping to 0 when I accelerated on the freeway. Since I installed the P4070 pump, I haven't seen it drop below 3 PSI. At this point, I noticed that my voltmeter was showing just under 13 and not going up when I revved the motor. I stopped and put a real volt meter on it and it showed 12.10. When I started it, it dropped down to 12.08. While it idled, it slowly dropped down to about 12.06 at which point I got on the freeway. On the freeway, it went up to about 12.14 but very slowly and the sat there until I got home.

This didn't happen on Friday when I drove to work and back.

A while back, I made a 70 mile roundtrip to a worksite and didn't notice until I was on the return trip that the battery voltage was dropping. By the time I got home, it was below 12 volts and the alternator wasn't charging.

The alternator was custom wound by a local shop that I have always had good luck with. However, it wasn't charging so I replaced it with a cheap Autozone alternator figuring I'd take the old one back to the shop that wound it.

With the cheap alt in everything was charging fine and everything worked for a while. I took the old alt back to the shop and it tested fine :wtf:

I didn't put the old (good alt) back n and ran the cheap one for a while. Then suddenly it wasn't charging. This was after less than a month. When I pulled that alt out, the main post was loose so I figured that was my problem.

I also figured that the alternator wiring was causing the issue at this point: two alts just suddenly stop working and when tested seem fine, I assumed that the alternator wasn't getting current. I replaced the wires and installed the old (recently tested) alt. Everything seemed fine until today.

So, I have the same symptom with two alternators (not charging). Replaced the main power wire to the starter, #1 wire to switched power and #2 to the starter and battery positive. After replacing the wiring, I am now getting a weak charge out of a tested alternator.

It is possible that the alt died between Friday and today but it seems unlikely. Could it just be a bad ground? I have new 1/0 ground wires coming because I wanted to do that anyway but I’d like to track down this problem before the new wires arrive.
 

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Eagle Eyes
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Bad situation you have there because its infrequent so hard to diagnose and its going to leave you stranded someday.

You mentioned grounds so I'll start there. Always a likely suspect, and easy to dismiss. I imagine you have the standard setup that each battery is wired to the engine with same gauge wire. This should be of itself enough ground to not cause issues. Just has to be tight and clean. This includes no paint underneath. I'm sure you know the drill.

Otherwise it simply sounds like your alternators are dead or dying, however, that should not be the case because of how quickly it is happening and how new they are. However, I have run into that case myself when I first bought my truck. Mine was the belt slipping issue, and having the alternator constantly charging depleted batteries with a slipping belt killed it fairly quick apparently.

Usually with weird electrical problems its best to get the obvious out of the way (like the grounds, connectors, etc) then try to isolate the problem. If it was my truck with a mechanical pump I would have everything turned off, the alternator drive belt disconnected, and with the truck running, watch the voltage. Perhaps you can do the same with your pump wired to a spare battery. Can do the same with the alternator wiring disconnected. The of course having the alternator fully connected and the rest of the power harness not.

Because your problem is intermittent its difficult to do those tests.

Its interesting that while you are driving you are suddenly that low on voltage. Normally if you had no power demands and the alternator suddenly quit the battery would just taper down to standby voltage, like 12.5v on a strong battery. I'm guessing you would know it, but a heavy load with a not up to par alternator would do that.

The only thing you didn't mention in your post was batteries, how old they where and quality. They can be either dead, or act weird depending on problem. Have seen them good one minute bad the next, or slowly not want to hold a charge, and even take a solid charge but give it up quickly when a load is supplied. Again with my above example, when my belt slipping was causing alternator issues it quickly because battery health issues as well...

Hope something in my reply sparks an idea. Besides, its a good time to be thankful its not a new truck..good luck figuring out electrical problems then !
 

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Discussion Starter #3 (Edited)
Thanks. Some more info from a few minutes of testing today.

When I put a voltmeter to the batteries, they show 12.32 which is higher than I was getting on the freeway. So, I thought maybe it was just the wiring in the cab and the alternator is fine. I ran a test wire from the main terminal block (direct between the starter and the alt) to inside the cab. Before I started the motor, I got 12.32v. Once I started it showed 12.06 no matter how much I revved the motor.

Next step is to replace the grounds to the batteries. The stock ones seem okay but I don't know what they look like under the insulation.

I could pull the alternator out and have it tested but I want to rule out the grounds first. It seems that every time I pull out the alternator and swap it with another everything is fine at first and then goes wrong. Maybe disconnecting the ground cable for other stuff (like swapping out the switched accessory power solenoid) is fixing/unfixing the issue.


Oh, and the batteries are Diehard Golds that are about 2 years old. If I was still living in Phoenix, I would have started with the batteries but they should be good still.
 

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Eagle Eyes
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When I put a voltmeter to the batteries, they show 12.32 which is higher than I was getting on the freeway.
Just to clarify, what is the situation when you did this test? Truck sat a while off, or on, or at idle, higher RPM? 12.32V from just sitting is low voltage if the battery is good with no drains.

If you have a battery charger and some time, I would charge both batteries disconnected to 14.5 and then watch how fast they drop. If they slowly go to 12.5 they are fine, if they shoot down they are not good. Leave them like that overnight. If you see an imbalance of good proportion it will show you if you have one bad bringing down the other.

Before I started the motor, I got 12.32v. Once I started it showed 12.06 no matter how much I revved the motor.
A typical drop I suppose on a non charging system. Either the alternator is not producing power or the batteries are not receiving it, or taking it. May not be a true test as everything is connected, but use your meter to test voltage at the alternator terminal.

Would take a bit of work but a real test would be to see how much amperage the alternator is putting out with the tests. Need an expensive clamp on or wiring in a gauge to your charge wire. It is the alternator's amperage that does the work.

Keep in mind severely depleted or bad batteries will refuse a charge or take forever doing it especially with an alternator.

As far as the ground, when I bought my truck the battery cables where shot and a hodgepodge. One had been spliced together and everything. Wound up blowing a connector right off a battery side terminal. The NAPA replacements have the ground on the PS going to the alternator bracket, but I relocated to the intake stud. The DS battery ground goes to intake stud. Each have a smaller gauge ground to the radiator support. Hope that helps.
 

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Discussion Starter #5 (Edited)
Based on that, I think the next steps are (in order of effort):

1. Disconnect the alternator charging wire and test that. This is easy since it goes straight to a terminal on the firewall before going to the battery and the truck will run on the batteries while I test.

2. Pull the batteries, charge them and check for draining below 12.5. I'll also take them to the parts store for testing.

3. Replace the ground wire - this should arrive in a day or so.

4. Pull the alternator and have it tested.

If that doesn't do it, I have some weird wiring problem and it is time to just light the whole truck on fire and leave it down the block.
 

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Eagle Eyes
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The alternator was custom wound by a local shop that I have always had good luck with.
It would be interesting to know the history and specs on it. Why did you have one custom wound? What was its amperage? Was it a 12SI body or different?

The autozone one you had in lately, a 12SI as well? What amps?

What belt are you using? Pulley size is stock?
 

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Eagle Eyes
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1. Disconnect the alternator charging wire and test that. This is easy since it goes straight to a terminal on the firewall before going to the battery and the truck will run on the batteries while I test.
Am I understanding you right that your heavy charge wire does not go to the PS battery? It goes to the firewall terminal instead?

The alternator charge wire is supposed to go from alternator stud to PS battery positive terminal.
 

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"ol` smokie"
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You have a voltmeter, no charge light, correct. Possible your resistor wire to terminal 1 is going bad. Just make a jumper with a small dash light( preferably 2) wired in line for resistance. Most 80`s gm diesel cars that had charge lights also had what they called "hidden" charge lights tucked behind the dash. I tried my truck with the single hidden light, (because there is no resistance wire) and had to put 2 lamps in line. It`s possible your not exciting your alternator
 

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"ol` smokie"
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I`m not familiar with your trucks wiring, but the "BAT" on the alternator can go anywhere that is capable of carrying the load, ie. main + lug on starter, power distribution block on firewall, + whip from battery cable. I`ve seen them all over the place. Terminal 2 on alternator is a monitor wire and should be upstream away from the alternator to monitor usage/demand. Making jumpers for #1 and #2 are simple enough, #2 could be losing connection and showing no demand, #1 (resistance wire)could be going bad not exciting.
 

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Discussion Starter #10
Am I understanding you right that your heavy charge wire does not go to the PS battery? It goes to the firewall terminal instead?

The alternator charge wire is supposed to go from alternator stud to PS battery positive terminal.
Stock it actually goes to the starter and then the battery. I just replaced it with a 4 gauge wire to a terminal block right behind the passenger side valve cover then the starter (with a fusible link at the starter). From the starter, it has a 1/0 wire to the battery.
 

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Discussion Starter #11 (Edited)
It would be interesting to know the history and specs on it. Why did you have one custom wound? What was its amperage? Was it a 12SI body or different?

The autozone one you had in lately, a 12SI as well? What amps?

What belt are you using? Pulley size is stock?
They are both 12SI the custom wound one had a 140 amp output and a stock pulley size. The Autozone one has the same pulley size and is like 50 amps (from memory)

You have a voltmeter, no charge light, correct. Possible your resistor wire to terminal 1 is going bad. Just make a jumper with a small dash light( preferably 2) wired in line for resistance. Most 80`s gm diesel cars that had charge lights also had what they called "hidden" charge lights tucked behind the dash. I tried my truck with the single hidden light, (because there is no resistance wire) and had to put 2 lamps in line. It`s possible your not exciting your alternator

I think you may have just solved my problem. I don't have an ALT light so I skipped it altogether. No resistance in the line. I need to understand circuit better.
 

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Eagle Eyes
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Discussion Starter #13
You know what? I actually came across that Pirate 4x4 table but I didn't actually read the note at the bottom. Now to figure out what ohm resistor to use.
 

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"ol` smokie"
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If you really need to put a resistor in line, instead of a dash bulb like GM did to 100`s of thousands of vehicles, use a 10 ohm resistor rated at 6 watts or more.
A bulb IS A RESISTOR, and when was the last time anyone here changed a burnt out charge light bulb?

If you experience a feedback situation after installing a resistor instead of a bulb, you`ll know where to look
 

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"ol` smokie"
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I made up a test harness years ago for the delco`s, I`ll dig it out and post a pic. Simple check.
 

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Discussion Starter #16
If you really need to put a resistor in line, instead of a dash bulb like GM did to 100`s of thousands of vehicles, use a 10 ohm resistor rated at 6 watts or more.
A bulb IS A RESISTOR, and when was the last time anyone here changed a burnt out charge light bulb?

If you experience a feedback situation after installing a resistor instead of a bulb, you`ll know where to look

I could put a light in but I don't have a spot on the dash for it so it would just be a light hanging out there. Seems way easier to use a resistor particularly since it won't burn out.
 

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Eagle Eyes
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Been thinking it over and talking it over with my Dad who has alot of experience on these. We believe that once the alternator is excited it shouldnt give you issues until the alternator stops rotating. You could manually disconnect the exciter wire and it should still be putting out amps. As the table indicates its for the field supply and with your intermittent problems while driving, doesnt seem to indicate that, although who knows.

Perhaps a higher chance of the problem is the other wire, the sensing wire. If that was at fault it would either tell your alternator to stop charging, or keep charging (to point of overcharge). We are not sure which it would do per condition. Those symptoms would make more sense for intermittent operation as you describe. That wire might go to the terminal block, but closer to a battery the better.

I think you said you will be rewiring the charging circuit. With 140Amps I would consider a 6awg wire from the alternator stud directly to the battery, which at 3 foot distance would give you a 2.3%/.33V drop.
 

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Discussion Starter #18
Thanks for the additional thought. I should clarify that the issue doesn't change once the truck is started. It was charging and now it never charges. Intermittent might be misleading and could even be incorrect because the changes have probably happened when I have changed something in the system.

I can do an output check on the alternator as well as checking to make sure the sensing wire is getting power to the connector. Then checking all of the wires. Putting the lamp/resistor back in the line seems like a good idea anyway and will eliminate that as a candidate.
 

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Eagle Eyes
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Yes I think we covered all the bases. I dont think it could be much more. Let us know what you figure out good luck.
 

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"ol` smokie"
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Been thinking it over and talking it over with my Dad who has alot of experience on these. We believe that once the alternator is excited it shouldnt give you issues until the alternator stops rotating. You could manually disconnect the exciter wire and it should still be putting out amps. As the table indicates its for the field supply and with your intermittent problems while driving, doesnt seem to indicate that, although who knows.

Perhaps a higher chance of the problem is the other wire, the sensing wire. If that was at fault it would either tell your alternator to stop charging, or keep charging (to point of overcharge). We are not sure which it would do per condition. Those symptoms would make more sense for intermittent operation as you describe. That wire might go to the terminal block, but closer to a battery the better.

I think you said you will be rewiring the charging circuit. With 140Amps I would consider a 6awg wire from the alternator stud directly to the battery, which at 3 foot distance would give you a 2.3%/.33V drop.
If your speaking of the monitor wire best placed as close to the battery as possible, that`s completely wrong. To monitor load it must be placed at a load source, like a junction block that feeds several circuits. Single wire conversion kits for Si alternators do exactly what you suggest, that`s why they don`t charge for crap. Keep that monitor wire away from the battery and in the thick of draw. Think of all the wire GM could have saved if they would have just jumped the monitor terminal to the "BATT" terminal on the alternator (that`s what the single wire conversion kit does) It doesn`t allow your regulator to sense load
 
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