Alternator Questions

[kzeyus]

Working on a 1986 M998 with 6.5 GEP and 200amp dual voltage alternator. Been working great, but on thirty mile trip to town this week, voltage dropped into yellow near the end of the trip. I have the 14v tap routed to the + post of rear battery and 12v accessories such as radio and emergency lights running off this circuit with a breaker near the battery. Have a battery cutoff switch connected between the negative of the rear battery and the ground. When volt guage went into the yellow, the digital voltmeter on the 12v circuit dropped from 14.8v to 12.5 or so. Stopped the motor , switched off the batter disconnect and checked the belts etc. Started back up and drove home, volt guage in yellow and 12v voltmeter dropping to about 11.5 volts by the time I got home.

I have noticed that with the 12v tap added, even with ground cutoff disconnected, the 12v tap acts as a ground and 12v from first battery flows to dash lights etc even when main ground disconnected. Is there a better way to do this - not sure it's really a problem as you don't have enough batt to start the truck and that's the main reason for disconnect so as long as it doesn't hurt ESSS box or something.

Next day, I disconnected both batteries and tested for a bad battery, both look to be good. Reconnected batteries, started truck and both volt meters showed normal - in the green and 14.8 v on 12v guage. Checked voltages at alternator and all seemed proper. Let it run for an hour and charge batteries, went for a short drive and all seemed fine. Today went for a 10 mile drive, voltage dropped out about quarter of the way there. Stopped truck, turned off batt disconnect and took care of business, when I started up 15 minutes later, proper voltages again and all the way home. I did notice 12v voltmeter was now reading 15v instead of normal 14.8

Next plan is to track grounds and make sure all are good and tight as soon as I get a chance, especially on the added disconnect switch. Considered heat, but don't think that's it as was pretty cool when it failed this morning.

Anyone have other suggestions to check? Is it a sign of a failing regulator or generator?

 

[erasedhammer]

Don't know much about electrical work, but by battery disconnect do you mean a relay?

My setup is a 24v relay that I have connected from the post of the rear battery through the relay then up to a circuit breaker for my 12v connections. The ground wire for my 12v system is run directly from the - post of the rear battery up to a stud on my radio deck where all my 12v things are mounted.

I have my relay wired into the ignition switch so the circuit for 12v is opened when the truck is running (plus I have the keyed ignition so I also have it hooked into the acc position on that switch).

So you have a switch to disconnect the rear battery - post from the main ground shunt?

 

[kzeyus]

Yes, I have a 'Those Military Guys' battery disconnect wired to disconnect the ground. Later added the 12v tap, but a relay there might be a good idea so it does not act as a ground for the front battery.

Main problem is alternator working then randomly not working. Raining cats and dogs here so not able to crawl under and check ground connections or recheck alternator by the TM right now. If I can test when not getting the proper voltage rather than when working may tell me more.

 

[Action]

When folks have electrical issues with stuff added on, I would suggest putting it back to stock and see what it does.

 

[kzeyus]

Finally got chance to work through it and found the battery terminal on the + side of the front battery would not tighten as far as it should causing a poor connection. It was tight on the battery, but lead wire was not making good connection all the time. Replaced it and now seems to be back in order.

 

[M99821T]

Finally got chance to work through it and found the battery terminal on the + side of the front battery would not tighten as far as it should causing a poor connection. It was tight on the battery, but lead wire was not making good connection all the time. Replaced it and now seems to be back in order.

Did that fix hold up?

 

[kzeyus]

No, wasn't the main culprit. If I turn off the disconnect routinely, the rear battery slowly works it way up in voltage. When it gets to 15.6 (with truck running) volt regulator shuts off alternator. I've found if I'll leave disconnect on at night on occasion so there's a little drain, battery voltage stays down where it should and all works well.

 

[M99821T]

Yeah, I guess one of my 12V accessories is drawing too much current when off. I let mine sit of a couple weeks with everything connected. The rear battery was down to ~5V, but the front one was 12.5V.