Intermittent Charging issue - at whits end

So I have a 91 Mighty Max with the 4G64, 5spd manual, 2wd. Amazing little truck. Over the last few months I have had the most pain in the ass issue. The vehicle, from a cold start, will charge the battery like normal at 14.5v. I can get away with 20 min or so drives without issue, but either when I turn my headlights on or I drive for about 30 mins, it will stop charging the battery. It will sit at around 12.8v without accessories, and then when accessories or lights are turned on, it will slowly drain until dead.Things I have done:-Replaced alternator-Cleaned up all grounds, verified continuity and resistance at all ground points-Checked all fused links I honestly have no idea what the issue is. My current thoughts is that it may have something to do with the ECU if that is involved in the voltage regulation, but I can't even determine if it does. I know there are three wires from the alternator, the bat+ connection obviously, a white wire I think is voltage sense, and a blue wire. I think the blue wire might go to the battery light on the dash, my manual says "voltage combinator" and then "gauge cluster" but I am not sure if the voltage combinator is part of the ECU or even something that would cause this issue.So here is my questions1. Does anyone here have an idea what might be going on? Is there a common issue that I should look into?2. Is there a hacky way to fix this? Either tying the blue wire high or tying it low if it is part of the ECU? I don't want to replace or repair the ECU.3. If none of the above are true, is there an easy way to convert this vehicle to a 1-wire alternator? Anything that will just bolt up without modifying belts and brackets and stuff?Thanks!

Also, one thing I would like to add is that the battery light on the dash does light up normally when the key is inserted in the off position, and goes away when the engine is running, even if the voltage is very low.

This can happen when the cables are corroded or have poor connections.
battery cables tend to corrode out from the inside

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Originally Posted by tortron

This can happen when the cables are corroded or have poor connections. battery cables tend to corrode out from the inside

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The battery cables look relatively new. The only thing that looks a tad rough is the engine ground, but I think thats just grimy. The terminals are new as well, I forgot to mention that. I did measure the resistance and there was nothing worse than a few ohms.One thing that I can mention because I just remembered it, if I start the truck and then disconnect the negative lead from the battery, and then run a voltage test between the positive terminal of the alternator and ground, its up at 14.5v. When I re-attach the battery, it goes back down. Any thoughts?

For what its worth as well since I know people will get on my butt about this, but this test was performed accidentally. I knocked the negative terminal off when I was working and i was very excited to see the voltage jump up until I realised what had happened. I believe nothing was damaged as a result as all issues are just as present as they where previously, nothing new, nothing went away.

The alternator needs the battery in the system to regulate charge voltage. I would do an error code check on the ECU and see if anything comes up. Vehicles in the 90's began to have active charging regulation triggered by the ECU. I don't know if the MM fits into this category though. The biggest gremlin in an electrical system is a bad connection somewhere - usually the grounds get funky and no longer have a solid connection but you've stated that you have cleaned them all so that is a good start.

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Originally Posted by geezer101

The alternator needs the battery in the system to regulate charge voltage. I would do an error code check on the ECU and see if anything comes up. Vehicles in the 90's began to have active charging regulation triggered by the ECU. I don't know if the MM fits into this category though. The biggest gremlin in an electrical system is a bad connection somewhere - usually the grounds get funky and no longer have a solid connection but you've stated that you have cleaned them all so that is a good start.

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I'm just popping in to let y'all know that I ordered a new ground cable, it should be here on Thursday and I'll put it in ASAP. Hoping thats the source of the issue. As far as I can tell from everything, the blue wire only goes to the dash light, not the ECU, but I would be happy to be proven wrong on that point, that would help me emmensley in diagnosis, just knowing where that wire goes and what sort of signal it is expecting.

i know you said that you did this already but clean all the grounds again. Start with the ones on the firewall. take the kick panel off the passenger side in the cab and clean the ecm ground. while in there, pull the ecm out. open it and look for cap leek. if its smells like fish than the caps need replacing. make sure the ground cable you are getting is nice and clean and the spot where it grounds to the block is clean as well. also run a ground from the battery to the body. the engine is on rubber mounts and its possible the connection from the battery to the block is good but the block to the body is poor. check the fuse box for bad fuses and clean the connections. there is a ground cable underneath the driver side of the truck that connects the frame, body, and transmission, make sure this is clean as well. its likely that it got ripped off so check for it.

good luck.

I see a cable that connects the block (around where the AC compressor would be) the body, and the battery, but I do not see any location on the drivers side where the transmission, frame, and body are connected, or where they would be connected. There is another ground strap on the exhaust manifold heatshield that looks like it goes to the body. For charging to work though, I would expect the battery to charge as long as the block ground is good and the alternator cables are good, right? Also no other electrical issues when the vehicle is running until it heats up to the point where it stops charging. Radio, lights, blinkers, etc.

Couple of questions 4gco,
1) replacement alternator Amp rating?
2) battery load test readings (non -hand held tester) cold & hot ?
Thank You.

I think you probably have a failing connection somewhere inside the alternator. Heat increases electrical resistance. Typically if the voltage sensing wire has resistance, you'll get an overcharge condition.
claych's suggestion to load test the battery hot and cold is spot on. I have seen batteries intermittently fail when they get hot.
If it's not the battery, I think it's probably failed windings or regulator/rectifier in the alternator. Ground and charging cables typically don't get hot enough to create enough resistance to stop charging. Also, when the charging cable gets shitty, you typically see a higher voltage at the stud on the back of the alternator. Have you checked to see if you have the same voltage at the stud on the alternator, and at the positive battery post?

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Originally Posted by Giovanni89

I think you probably have a failing connection somewhere inside the alternator. Heat increases electrical resistance. Typically if the voltage sensing wire has resistance, you'll get an overcharge condition.
claych's suggestion to load test the battery hot and cold is spot on. I have seen batteries intermittently fail when they get hot.
If it's not the battery, I think it's probably failed windings or regulator/rectifier in the alternator. Ground and charging cables typically don't get hot enough to create enough resistance to stop charging. Also, when the charging cable gets shitty, you typically see a higher voltage at the stud on the back of the alternator. Have you checked to see if you have the same voltage at the stud on the alternator, and at the positive battery post?

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I have put two new alternators in there now. If that was the issue, I think one of the new alternators would have fixed it. Voltage is the same on the stud as it is at the battery post. I just put a new ground cable in there and the issue is persisting. Battery also tested good by the shop, not sure if they are pulling by dick or not though, because they have an incentive not to replace the battery.

Not sure what the exact readings are, but its a 750CCA battery, about a year old, tested good at the two shops I brought it to. I don't own a tester. Alternator is 50A

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Originally Posted by Giovanni89

I think you probably have a failing connection somewhere inside the alternator. Heat increases electrical resistance. Typically if the voltage sensing wire has resistance, you'll get an overcharge condition.
claych's suggestion to load test the battery hot and cold is spot on. I have seen batteries intermittently fail when they get hot.
If it's not the battery, I think it's probably failed windings or regulator/rectifier in the alternator. Ground and charging cables typically don't get hot enough to create enough resistance to stop charging. Also, when the charging cable gets shitty, you typically see a higher voltage at the stud on the back of the alternator. Have you checked to see if you have the same voltage at the stud on the alternator, and at the positive battery post?

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I have put two new alternators in there, I highly doubt that is the issue. Battery voltage is the same at the poston the alternator as it is at the battery terminal.