M998 Electrical Questions / Problems

[williec30]

I'm the proud owner of a 1988 M998 and have used this board many times - thanks to all who have contributed.

I have what is believed to be an electrical issue - but I'm really not sure. I'm new to the HMMWV but have been working on vehicles since I was a kid. After hours of research, I'm finally posting to get the proper help. There are multiple symptoms, it's unclear to me if they are linked.

1. A month ago my windshield wiper motor started to work intermittently. It has now stopped working completely. I used a voltmeter to test the electric and it's reading 21v at the plug. It's understood the motor itself could be faulty, however, is this voltage correct?

2. Second related electrical related issue is the tail lights. When the main switch is set to Stop Light, everything works as expected. However, when I move the switch to Serv.Drive my tail lights no longer light up when the brake pedal is applied. The turn signals work fine, but no solid lights on breaking. All the light bulbs have been tested and confirmed working.

After reading and doing more testing this afternoon, here is what I observed when the motor was running on the Alternator and Voltage Regulator:

• The main battery connection to the alternator is reading 24v
• The red contact on the voltage regulator reads 28.5v
• The white contact on the voltage regulator reads 11v
• The yellow contact on the voltage regulator reads 0v

I appreciate the help in advance, pictures attached.

 

[Coug]

1) So the wiper motor has contacts under the windshield that are notorious for bad connections. Fold the windshield down and clean the connections. Some people use a small washer or similar in there to create more pressure on the connection.

21V is too low, you should see full battery voltage (25V when off, 28-29V when running)

2) I can't help much on this, but it isn't uncommon for one circuit to ground itself through another circuit if something isn't working right, so once again it sounds to me like bad/corroded connections somewhere in your electrical system.


For the bottom part, if I am understanding correctly, you are saying that you are measuring 28V at the generator, but only 24V at the battery? If that's the case you need to go through the truck and check all of your cable connections and grounds, because you shouldn't have that much voltage drop between the generator and batteries.
(I think you can also run a wire from the front of the regulator to the positive terminal of the rear battery to keep it charged up better, and be able to run some 12V accessories. Unless you rotate the batteries regularly, they tend to have the forward battery charge more than the rear, causing premature failure on them both, but someone with a little better understanding on this will hopefully chime in soon)

 

[Bulldogger]

For the lights, I agree there could be a grounded or chafed wire somewhere. One thing that helped mine was to unscrew the connector from the back of the light control box, clean it and replace it. Could just be some corrosion on the pins.

BDGR

 

[papakb]

Something to keep in mind is that the taillights on military vehicles ground thru both of the mounting bolts. That means they both need to be clean of corrosion and tightened properly. There may or may not be a link between them. Since the HMMWV body is an unreliable ground AMG included ground wires in their harnesses and they both need to be there.

 

[Augi]

Try changing your main light switch. There are a complex set of contacts in there and sometimes if one is burned it will do wierd stuff.

Augi

 

[williec30]

1) So the wiper motor has contacts under the windshield that are notorious for bad connections. Fold the windshield down and clean the connections. Some people use a small washer or similar in there to create more pressure on the connection.

Thanks for the help here... the contacts were clean but the rubber gasket has deteriorated to the point where the contact is no longer tight between the two. The washer worked great as a good stop gap.

Still tracking down the rear light problem as there seem to be wires frayed all over the place.