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I did get the alt tested just to rule it out and it tested fine. I installed it back in the jeep and it was showing low 13 volts on my meter at idle, then it went down to high 12s after I turned on the headlights.

I'm beginning to wonder if my ground is not adequate. I cleaned up all the grounds I could find except the big negative battery cable. Its attached below the AC compressor IIRC and was difficult to access so I left it alone since it seemed fine. But I had a weird experience with jumping the jeep with my wifes van that confused me.

This didn't make any sense to me, but I hooked jumper cables to my wifes van and to the jeep battery terminals. My meter on the jumper cable clamps on my wifes van's battery read 13.6v. If the cables were unhooked from the jeep then the other end of the cables would read the same as expected. However once I hooked them up to the Jeep terminals, the cables would read 12.6 again.

I could literally remove the negative cable, put it somewhere else on the body (or remove them completely) and it would show 13.6 (same as the van), but whenever I put it straight to the battery it would go to 12.6. It doesn't make sense...the jumper cable is just a large wire and it should read the same. So I'm wondering if my engine to chassis ground is not good enough. After messing with it I finally got it to show right around 13V and the Jeep started up great. I let it run for about 15 minutes before shutting it off to see if it had charged the battery and the Jeep fired right up again. Again, I know bad grounds can do funny things but I have never encountered this issue with any old car I've resurrected.

I have two meters; one from HF and a Fluke. However, the Fluke doesn't show me the decimal points of the reading so it will only show 13v or 12v whereas the one from HF will show 13.46 or 12.68 for example. My HF meter is older and the probes are bent/worn out so I want to get a fresh meter and see what it's saying.

I know dumb questions have dumb answers...... And I do not want to be pouring salt, but gotta ask.

Are you sure the battery is fully charged? Is it 12.6 not running? If it is not that is part of your problem.

AlexJordan22 wrote: ↑Thu Aug 17, 2023 8:04 am I could literally remove the negative cable, put it somewhere else on the body (or remove them completely) and it would show 13.6 (same as the van), but whenever I put it straight to the battery it would go to 12.6.

I don't understand which cable here. You are removing the negative battery cable and running the alternator without a battery in the circuit? Not sure that's a good idea.

You could take your jumper cable and bridge the battery negative to the engine, or the alternator bracket, to get a better ground. Note you are supposed to have a ground cable from the chassis to the body steel, and from the engine to the chassis. The ground the battery to the engine because that's the best path to the starter motor.

Exactly when did you see 13.6 volts? With the jumper cables between the Jeep battery and the van battery, Jeep engine running? This puts the two batteries in parallel. Maybe you better describe what you did with more detail.

Suggest you take the battery out of the van, put it in the Jeep and see what you get.

I have two meters; one from HF and a Fluke. However, the Fluke doesn't show me the decimal points of the reading so it will only show 13v or 12v whereas the one from HF will show 13.46 or 12.68 for example. My HF meter is older and the probes are bent/worn out so I want to get a fresh meter and see what it's saying.

Never heard of a Fluke meter with that low a resolution. The used Fluke 27s you can buy from eBay are good, and not too expensive. They are ruggedized, and the military bought thousands of them that are still around.

Yeller wrote: ↑Thu Aug 17, 2023 8:42 am I know dumb questions have dumb answers...... And I do not want to be pouring salt, but gotta ask.

Are you sure the battery is fully charged? Is it 12.6 not running? If it is not that is part of your problem.

Battery is not fully charged as my charging system has been not working properly. Not running my battery voltage was mid-11 volts so that could have an effect here. I'm used to seeing 13+ volts on a battery with a working charging system regardless of how dead the battery is, but this one is weird.

I don't understand which cable here. You are removing the negative battery cable and running the alternator without a battery in the circuit? Not sure that's a good idea.

The negative jumper cable. The battery terminal was still attached. I moved the negative jumper cable to different ground locations which brought full van voltage into my system. I couldn't get full van voltage into my system with the cables hooked solely to the battery.

You could take your jumper cable and bridge the battery negative to the engine, or the alternator bracket, to get a better ground. Note you are supposed to have a ground cable from the chassis to the body steel, and from the engine to the chassis. The ground the battery to the engine because that's the best path to the starter motor.

Yes, that is what I did. Moved the negative jumper cable to different ground locations.

Exactly when did you see 13.6 volts? With the jumper cables between the Jeep battery and the van battery, Jeep engine running? This puts the two batteries in parallel. Maybe you better describe what you did with more detail.

I got 13.6 volts either when the cables were not attached to the jeep battery at all, or when I put the negative jumper cable on a different ground location on the body. I did not try it on any part of the engine to see if that would work too. I tested the voltages at the jumper cable clamps themselves. When the jumper cable clamp was hooked to the body, I could read full van voltage at the cable clamps. However I still did not see those voltages at the battery terminals for some reason, making me think it's a ground problem.

Suggest you take the battery out of the van, put it in the Jeep and see what you get.

Yes, perhaps that would help. The current battery in the Jeep is a newer Interstate from Costco with a date sticker of 4/23 but it wouldn't hurt to try a fully charged one.

Never heard of a Fluke meter with that low a resolution. The used Fluke 27s you can buy from eBay are good, and not too expensive. They are ruggedized, and the military bought thousands of them that are still around.

Yeah...not sure why it does that. This is the one I have, it's not like a standard multimeter. Great for home stuff but not great for car stuff.

I'm still dealing with this problem. I have traced my wiring and found that the brown wire has continuity and shared voltage with the other wires it's connected to (power wire at the coil, etc). However with the key on engine off the voltage on that wire is just 5.5 volts. Battery is at 12.3 at the time I'm testing. I don't think I'm getting enough power to the resistance wire to start the alternator.

If I remove the connector from the coil then the voltage on that wire goes up to battery voltage. Does that mean anything? The Jeep will start fine if it has enough juice to do so but I'm not getting any output from the alternator.

tgreese wrote: ↑Sun Apr 16, 2023 11:50 am No, it does not work like that. The electrochemical potential of the lead-acid cells is 12.6 volts. The battery has an internal resistance which creates a voltage by Ohm's law when charging; voltage equal current times resistance. These voltages add. The voltage regulator is just that; it acts to regulate the charging voltage of the battery. That regulated voltage should not exceed about 14 volts, even if the battery is completely flat.

The charging current goes up and down in response to the load and the condition of the battery. The feedback is by voltage, acting on the internal resistance of the battery. The voltage regulator maintains that voltage at a constant value.

My 150amp ppwer master and 94 amp 12si always charge above 14v. The 12si on a near dead battery at 12.4v charged at 15v for a min or two before dropping down to around 14.6. Havent checked it yet at full charge.

But my direct wire to battery digital voltage gauge on my 150a on my 90 always is around 14.2 or 14.4v.

The two wires on the pin of the alternator. To simply put. One is a wire which tells the alternator how much to charge at. The other tells the alternator to start charging. I personally would cut wire number 1 the smaller wireand run it directly to 12v keyed and see if it starts charging properky. ( it wont shut off though youd have to unplig that wire after to get it to shut off but at least you can see if thats the problem

I ran a wire from the battery positive to the #1 terminal on the alternator and I got no change at all.

Today I went poking around and found that with the jeep running and the 2-wire plug hooked up it is charging but only at 12.5 volts. If I unplug the 2-wire plug while running the voltage drops to just below 12 volts (what the battery is at). However when I plug the 2-wire plug back in it momentarily shows 14+V before dropping to 13 and then 12.5. its like the alternator is working fine but the voltage regulator is dropping it down to 12.5 for some reason.

If I run it with the blower motor and headlights the voltage at the battery is 11.2V. This is the 2nd napa alternator Ive installed and the last one was doing the same thing. I assumed it was the voltage regulator but now I just don't know.