Ok, I have an F800 with the 7.8L diesel and the batteries seemed to be dead. Put two new in and then found that wasn't the issue, checked alternator and was not putting out any voltage when running.

Installed new alternator and still nothing. Had alternator tested, showed good. Put in new voltage regulator and no difference.

It appears (from what I can figure from this thing) that the field terminal is not getting any voltage when the key is turned on, and apparently that voltage has to be there to "excite the field" to get the thing generating (am I right?).

Any leads on where to look next? Is there some fuse burnt I am not seeing? Maybe just a bad wire? I cannot find a schematic which would really help me out.

Any help is appreciated! Thank you,

Nick

 

...and if it helps at all, the gauge on the dash for the charging status of the electrical system just shows the needle in the middle all the time, even when not running.

 

I know these things will drive you nuts. I have one, same truck couple
yrs newer- I jumped the field terminal to batt +, truck charged to max, ok
thats good, there is a couple of fusable links around or near the regulator,
they were good, new regulator- imparative that the reg. wire connector
is good and clean they tend to get all greened up. Then I remember
back in the day we use to put the old style Delcos with reg in getting rid
of generators. Problem was the field wire that goes to ign switch to
energize it when ign. switch is turned off the engine still runs. Solution is
to put a diode in line on the field wire. Ok instead of a diode we just use
a #57 dashlight bulb thats the trick. Just by chance is this truck using
that idea. Or by luck or by chance do I have this bulb on the dash, I
looked and looked and it does a little red battery icon with with a burnt
bulb new bulb and it charges. I do not know if that little bulb is really
neccesary field trigger or the truck simply gave up playing games with
me but it now charges.

 

Thank you for the ideas/info. I will check these items and post back.

 

Well I got it! Jumped the 12v +ve to the field and it started charging. So I traced wires around the voltage regulator and wasn't getting voltage on the "a" terminal on the regulator. Traced it to a solenoid mounted in a box near the alternator and found a wire corroded through completely. Hooked it up and now it is making 13.7 volts at idle.

Thanks for the help!

 

That's great!
Back to reliable driving.