Voltage varies

[Kenneth Cole]

Hi
My 1991 humvee's voltage some days stays in the green all day but sometimes it will drop into the yellow for a few minutes then go back into the green and then back to the yellow. Is this something to do with the voltage regulator?

Thanks for the help
Ken

 

[Bulldogger]

Something is wrong, definitely. I've had this happen now and again, as so many of us have. First check your wiring connections, everywhere connected to the alternator, especially the battery box and don't forget there is a contact post on the outside of the battery box where the +24V battery cable "leaves" the box for the alternator. It's a two sided post, NOT a cable pass through as in civilian vehicles. Mine was loose on the outside post, and some numbnut used a nut with the wrong thread pattern on one side of the pass through as well.
Also, someone will ask: "Do you have a grounding harness?" so it might as well be me. Checking and cleaning ALL grounds, including those behind the instrument cluster is also a VERY good idea. The supplemental harness is favored by many. Plenty of threads about it here on the site, and some instructions to make your own (I did).
If all that doesn't help, then possibly the regulator is beginning to fail, or just needs a tweak to clear some smudge on the adjustment contacts inside. To give you more advice on the regulator, we need to know what Amperage alternator you have as well as any custom electrical stuff you might have installed. My knowledge is currently limited to the Gen Zero 60A version, though I have a 200A setup being put into my truck now as part of a powertrain swap.

Bulldogger

 

[papakb]

If you have the 60 amp Leece-Neville or Prestolite alternators you have a fairly common problem. It can be caused by a couple of things most commonly worj brushes, a blown resistor, or a bad regulator. The first thing you want to do is disconnect the battery negative terminal for safety. Then remove the regulator assembly by removing 6 screws on the rear cover. The regulator can then be pulled straight out the back. Brush wear is indicated by excessive carbon dust in the alternator housing and it's a simple job to replace either the brushes or the entire brush holder. Make sure to blow out the housing before you reinstall the regulator cover. Put a little vaseline on the O ring seal to make it slide in easier. The 50 ohm, 50 watt resistor is replaced by removing 2 screws and a couple of wire leads. The regulator itself is a potted assembly and you can't do anything with it other than replace it as a unit.

They're commonly found on online auction sites for around $60 or you can buy one directly from the manufacturer, Southern Automotive here:

Alternators & Generators - - 2920-00-909-2483 Alternator 60 Amp

southernautomotive.com

 

[Kenneth Cole]

Thanks for all ideas. I will check all of those out.
Ken

 

[TOBASH]

If it only happens when you start the vehicle on cold days, the PCB or EESS might be cycling the glow plugs. That temporarily consumes power and the Voltage drops into the yellow. It can cycle on and off for a few minutes with some units, and when she warms up the Voltage returns to green and stays there. My old SSI Technologies PCB is known for this, but there are several others that do this too. If it is the PCB or EESS cycling the glow plugs, it is normal and not an alternator issue.

 

[papakb]

This time of year in Wyoming you can expect to see the glow plugs cycle for up to 10 minutes on a cold start until the engine starts to warm up. During normal cycling the voltmeter should only dip into the yellow for about 10 seconds at a a time. On warm summer days this might only last for 1 or 2 cycles. This cycling is referred to as "afterglow". If the voltmeter is staying in the yellow for longer periods it'a an indication of an alternator problem.