rear alternator light

James Bennon · #1 04-14-13, 10:59 PM

Let me preference this question with the fact that I have read all the threads on the above subject and have not found any reference to a similar situation.

Problem: Rear alternator light and discharge light stays on at all times when battery switch is on.

If front alternator is turned off ammeter shows positive amps and JPI 760 reads 28.4+ volts from rear alternator, only indication that anything is wrong is the fact that the alternator light is on. Full field test of rear alternator checks out as normal. Voltage regulator checks out as operable. Found a broken wire on capacitor and replaced---no change.

Only thing that has changed when this issue came up was that we had completed a major on the front engine.

Any suggestions??

hharney · #2 04-15-13, 09:10 PM

What flavor of Skymaster do you have?

James Bennon · #3 04-15-13, 10:54 PM

1973, T337G, s/n P3370018, Pressurized

Mark Hislop · #4 04-23-13, 01:21 PM

More than likely is that you've got a blown diode on the little signal processing board on the top center of your firewall. Or possibly a bad relay on the same board. The diodes are very easy to replace. I would disconnect them one at a time and check them first. There are a total of 4 of them, 2 for the front engine, and 2 for the rear engine circuits. There is one relay for each circuit. There is also a fuse for each circuit.

James Bennon · #5 04-23-13, 07:07 PM

Thanks Mark.
When you converted to 400 series alternators what was eliminated on the old parallel board?

Mark Hislop · #6 04-23-13, 07:11 PM

Although I could have eliminated the parallel board (I installed a volt/ammeter) I left it in. I still like having the "idiot lights" in my scan.

James Bennon · #7 04-23-13, 07:18 PM

So I would still need to fix issue I am now having?

Mark Hislop · #8 04-23-13, 10:03 PM

I don't think you have a real issue. It sounds like your electrical system is working. You just have a metering problem, and you are getting false indications. As far as I know, the parallel board only monitors the current flow of the two alternators and compares them against each other. As long as you have a volt meter and an ammeter, I believe you can remove the board.