Alternator or Voltage regulator issue? - Diesel Forum - TheDieselStop.com
7.3L IDI Diesels (Not Power Strokes) Technical discussion of topics related to vehicles powered by the 7.3 Liter In-Direct Injection Navistar engines.

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post #1 of 10 (permalink) Old 06-24-2019, 08:51 AM Thread Starter
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Alternator or Voltage regulator issue?

Hello everyone. Longtime reader, first time poster. Here is my issue:
I've got a 91 F250 7.3 idi that doesn't get used very often. Last week I went to start it after sitting for 2-3 weeks and the batteries were dead. I jumped it, ran fine, charged the batteries, found out one was bad so left the one good one in to do what I had to do. The truck started fine until the next day, the battery was dead again. Checked for power draw and it is pulling 3 amps on my cheapo meter when placed between the battery and neg cable. This is with the ignition off and key removed and door closed. It also makes a small arc when connecting or disconnecting the cable. Charged the battery and drove the truck and noticed the volt meter in the dash went from upper-middle while idling to the H while driving. I parked it and pulled the battery cable again. I've done some reading and it sounds like it could be the alternator pulling the parasitic load, but I also read it could be the voltage regulator causing the overcharge issue. Have any of you had this happen before or any ideas which part is causing both issues?
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post #2 of 10 (permalink) Old 06-24-2019, 07:24 PM
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It sounds like you have two separate issues. Volt meter over charging, and alternator discharging through a leaky diode. You'll need to replace the regulator, but the alternator will work just fine as long as you unhook the batteries while it isn't running. Kind of a PITA, but poor folks got poor ways.

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post #3 of 10 (permalink) Old 06-25-2019, 05:43 AM Thread Starter
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Thanks for the info. I was hoping it was one and not both and if both, which one may have caused the other. Didn't want to buy any alternator if the voltage regulator caused the alternator issue or vice versa. Plus, just spent $75 on one of the batteries because NAPA's "6 year warranty" suckedn. I bought the current ones 3 years ago. I guess I'll start cheap with the voltage regulator...
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post #4 of 10 (permalink) Old 06-25-2019, 09:16 PM
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Diodes can and do wear out. The regulator stuck on full charge, it happens from time to time. the old regulators with the two separate coils, and contact points were real bad about that, but I've seen it happen with the solid state ones from time to time as well. You can replace the regulator first, then a little later the alternator without doing any damage...if you don't let it go too long. The regulator overloading the alternator like that will eventually short it out. Diodes, brushes, and bearings are surprisingly cheap, and most rebuild shops don't overcharge to rebuild one either. If I have the time, I usually go that route on everything I own.
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35 years a mechanic on large stationary engines natural gas and diesel fired, and other industrial equipment.
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post #5 of 10 (permalink) Old 06-26-2019, 08:40 PM
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Using the alternator to charge dead batteries is extremely BAD for the batteries AND the alternator - especially the problematic 2G. You should always try to use a 110VAC regulated charger to recharge dead batteries. And you should swap in a 3G alternator ASAP. Click these & read the captions:


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post #6 of 10 (permalink) Old 06-27-2019, 07:12 AM Thread Starter
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I've been reading about the 3g swap and checking to make sure that is the way I want to go. Does the tach and volt gauge still work when you do the swap using the 3g and a crown vic harness?
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post #7 of 10 (permalink) Old 06-27-2019, 09:51 AM
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I thought all alternators had the regulator's built in. Also alternators dont really charge batteries very well. Mostly they maintain them. You don't use much battery to start the truck so the alternator can recharge the battery quickly. However they can charge a battery if you don't use the accessories like lights and radio. Alternators get hot if thy have to charge the battery and that is hard on the diodes. and when the diodes go bad than it's junk.
Usually if you have a battery go dead and the other one is still ok than chances are the alternator is bad. If you just replace a battery that is more than 5 years old and leaking than the alternator will still be ok. If the battery is completely dead than chances are the alternator is junk also. I wouldn't rebuild a alternator. Sure you can but how long will it take to replace the diodes. They don't just plug in. Whet you have when you are done is a old alternator with new diodes. Best to replace the whole thing.
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post #8 of 10 (permalink) Old 06-27-2019, 06:57 PM
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Quote:
Originally Posted by Stroking it View Post
I thought all alternators had the regulator's built in. Also alternators dont really charge batteries very well. Mostly they maintain them. You don't use much battery to start the truck so the alternator can recharge the battery quickly. However they can charge a battery if you don't use the accessories like lights and radio. Alternators get hot if thy have to charge the battery and that is hard on the diodes. and when the diodes go bad than it's junk.
Usually if you have a battery go dead and the other one is still ok than chances are the alternator is bad. If you just replace a battery that is more than 5 years old and leaking than the alternator will still be ok. If the battery is completely dead than chances are the alternator is junk also. I wouldn't rebuild a alternator. Sure you can but how long will it take to replace the diodes. They don't just plug in. Whet you have when you are done is a old alternator with new diodes. Best to replace the whole thing.
What do you think happens when you go to the parts store, and they sell you a reman. alternator or starter. If you know what you are doing diodes can be replaced in approximately an hour, and with replaced brushes, and bearings, you have a perfectly good alternator. it is quite a bit cheaper than buying a reman at the parts store, and you know what was done to it.
As far as built in regulators, some do some don't. Diodes, brushes, and bearings are all that really ever goes wrong with an alternator anyway, unless it is subjected to a direct short or something like that.

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post #9 of 10 (permalink) Old 06-27-2019, 08:42 PM
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Quote:
Originally Posted by Comander420 View Post
Does the tach and volt gauge still work when you do the swap using the 3g and a crown vic harness?
According to this:
BBB Industries- Premium Alternators, Starters, Power Steering Products and Turbochargers | TSB and Wiring Diagrams Database Results
...the RPM sensor is a discrete component - it's apparently NOT part of the alternator. So yes - 3G is certainly the way to go.


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post #10 of 10 (permalink) Old 06-28-2019, 09:17 AM
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Quote:
Originally Posted by oldrebuiltdodge View Post
it is quite a bit cheaper than buying a reman at the parts store, and you know what was done to it.
As far as built in regulators, some do some don't. Diodes, brushes, and bearings are all that really ever goes wrong with an alternator anyway, unless it is subjected to a direct short or something like that.
Right, I've overhauled many back in the day, maybe a diode (they're easy to check and replace), brushes, bearings and it's good as new.

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