electrical headache

[pybyr]

Lately I have been getting erratic readings on the OEM dash voltmeter on my 89 bricknose.

I'd been hoping it was the gauge being flaky, but several times after the truck has sat for several days, I have had batteries that faded into nothing and failed to start the engine.

Last week I ran the engine to 2000 RPM and measured 14.2 volts at the batteries- perfect. At that time the dash gauge was reading where I'd expect.

A few days later the gauge was reading low again, followed by a day of sitting followed by RUH Ruh ruh uh.

Last January I had alternator troubles and replaced the Alternator (Leece-Neville 160 amp)'s onboard regulator and internal diode trios [the diode trios had failed); I also checked all the windings with an ohmmeter, brushes looked great, cleaned and shined slip rings.

Batteries were supposedly new soon before I bought the truck, which was two years ago. Both were new, both replaced at the same time.

Any suggestions of what to check to figure out what is going on?

Thanks!

 

[lotzagoodstuff]

Not trying to oversimplify, but have you thought about maybe one battery starting to go south? Sounds like your charging system is working well, but when your truck sits you have low voltage. I would see if you can get somebody to load test both batteries before I moved on to any more troubleshooting, but that's just my .02.

Good luck, electrical is almost never very fun, but I'm sure you'll get it sorted out.

 

[franklin2]

I would disconnect the driver's side negative battery cable and then take the negative battery cable off the pass side and hook the alligator clip from a testlight onto the negative cable end. With the truck off, touch the probe of the testlight to the negative battery terminal, bridging the negative connection with the testlight. If the light lights up, then take the wiring off the alternator, and then try it again. If the light goes out, your diode pack in the alternator is shot again, and draining the battery.

 

[pybyr]

I should have mentioned-

My truck's heritage as a military/ firefighting truck means that there's a battery isolator and a big disconnect switch that allows off, battery 1, battery 2, or both.

I nearly always set it to off when parked; it was definitely in the off position over the weekend when the batteries went flat.

Can't figure whether the inadequate state of battery reserve is due to lack of charging or bad batteries or some sort of phantom load from some problem that's occurring even when the master switch is off.

Guess I need to take some readings, and maybe get the batteries checked.