Alternator troubleshooting. 6V92

[stoker]

I have 1982 GMC RTS with a DDEC 6V92. On a trip last week the batteries went dead and the engine stalled. After charging the batteries it runs fine. The alternator is not charging. I am trying to find out what signals and outputs the alternator should have. I did change one of the switches on the rear start panel last month. I wonder if I installed that new switch wrong and that led to no alternator output. Any advice would be appreciated.

[Oonrahnjay]

I have 1982 GMC RTS with a DDEC 6V92. On a trip last week the batteries went dead and the engine stalled. After charging the batteries it runs fine. The alternator is not charging. I am trying to find out what signals and outputs the alternator should have. I did change one of the switches on the rear start panel last month. I wonder if I installed that new switch wrong and that led to no alternator output. Any advice would be appreciated. 

    Is it one of the big DN50 oil-cooled alternators?  (It's likely to be but just checkin'.)

[luvrbus]

Turn the switch on and take a screw drive touch the field terminal it should be magnetized if not check the charging relay in the panel it will be a 6v relay then check all grounds from the alternator to the regulator

[stoker]

It is a 50DN

[stoker]

Any ideas where the charging relay is?

[stoker]

The alternator has 4 terminals

The big output terminal has battery voltage at all times. It does not increase when the engine is running.
The F1 terminal is ground at all times.
The F2 terminal has a strap to full time ground.
The reg terminal has 13 volts with engine running.

[Lee Bradley]

12 or 24 volt system?

[luvrbus]

Should be 24 v the alternator relays are 16 and 17 in the drivers area if you have 13 v on the R terminal the relay is working sounds like the alternator needs attention.Did you install the DDEC 1982 is early for a DDEC engine

[Jim Eh.]

You could start with regular maintenance of the battery cables at the battery posts. Clean, apply dielectric (or whatever magical potion you wish to use) and make sure those suckers are tight. Needs to be done periodically anyway and that is the easiest place to start.

[stoker]

It is 24v. The bus is an ex Avis airport shuttle. The DDEC was installed as part of a Blitz Bus overhaul in 1991.

[stoker]

Working on old cars years ago, I remember "full fielding" the alternator. I think it was jumping the field post to the positive post. Can I try to jump F1 to hot?

[Lee Bradley]

Put your volt meter on the battery terminal and connect 24 volts to F1 and the battery voltage should increase. If it does, time to check the regulator. If it doesn't, time to check the alternator.

[bevans6]

The F1 terminal is the input to the alternator field coil from the regulator.  F2 just grounds the field coil.  R terminal is a half voltage half rectified output from the alternator that goes to a relay somewhere, and isn't important.  The big thing is the F1 terminal - the regulator should be sending a voltage out to the alternator on that terminal to turn the alternator on.  The regulator toggles that voltage on and off at high frequency to switch the alternator on and off to regulate the output voltage.  To test the alternator, as mentioned already get a jumper and apply battery voltage to the F1 terminal with the regulator wire disconnected, and see what comes out of the alternator.  If nothing comes out (stays at battery voltage) the alternator needs help.  If the voltage jumps up high (you are forcing the full output of the alternator) then the regulator needs help.  It could just be a bad connection at the regulator, ground or battery sense.

Brian

[luvrbus]

Some of the RTS models had a air activated relay to tell the alternator to start charging

[fortyniner]

If you replaced one of the rear switches, I would start looking for a problem there. I know my old GM has an engine control switch that will disable the alternator and the front start switch when its on the off position.