[GMCnet] No Charging, light off in RUN, light on in ACCESSORY

[Dan Wallace]

Every few days, I take the Chief out for about 15 miles - Up and down the freeway -- to see what breaks. When I finally get it ready to camp most of
the running bugs should be swatted.

Last night, the coughed and died a couple of blocks from home. The battery voltage dropped below the Pertronix module's ignition level.

To get it home, I fired up the Honda and jumped the systems.


Here's what I found:

Engine running at the stock isolator:

Nothing worth counting (~0 VDC)at the alternator terminal on the isolator (Random, spurious voltages due to the high-impedance DMM)
12.6 VDC at the house side
10.8 at the chassis side

The GENERATOR light:
Illuminates in the ACCESSORY position
Does not illuminate in the RUN position (engine running or not)

I might not have tine to look at it today, but as I eat my lunch, looking at the schematic, here are my guesses:

1) The exciter wire lost connection with the alternator.

OR

2) The field or (field connection)in the alternator is open.

Here is my reasoning:

The generator is not charging because the field is not excited.

The GENERATOR indicator illuminates in ACCESSORY, because the ACCESSORY feed to the bulb is ground seeking through the bulb (and resistor) to
un-powered loads connected to IGNITION 1.

The GENERATOR indicator does not illuminate in RUN, because, without the field load, the ACCESSORY and IGNITION 1 feeds are at the same potential. So,
no current flows through the bulb (and resistor).


I ask those of you with more experience, other ideas?

I'll post what I find, but it may be a few days.









--
Dan Wallace
Detroit Metro
1974 Canyon Lands 260
Headers, 3.55, 16"
Floor plan 26-9


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[gene Fisher]

Do you have an APC cable

On Mon, Aug 14, 2017 at 8:49 AM Dan Wallace <danwall...@gmail.com>
wrote:

--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
http://gmcmotorhome.info/
Alternator Protection Cable
http://gmcmotorhome.info/APC.html

[Dan Wallace]

I don't know. I haven't looked at that part of the electrical system yet.

It is likely, because the PO installed most of the "tricks."

[Ken Burton]

It sounds like you are right on with your diagnosis. If you have an APC in line with the alternator, disconnect it temporarily and try again. I had
a problem with the connections loosing tension on the APC connector on the back of the alternator. A little bit of tightening on the connectors took
care of the problem. The APC was installed for years before the failure appeared. I'm assuming many, many hot and cold cycles caused it to loose
tension.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[roy keen]

If it isn't an external wire connection I would suspect you have an internal regulator gone bad or a bad dio treo in the alternator also check the
brushes for wear while you have it apart.
--
Roy Keen
Minden,NV
76 X Glenbrook

[John R. Lebetski]

I think you are on the right track. Spray the connections to the plug going into the internal reg and or APC. Your house battery is dead. Charge it
for a day with an external charger then see if it holds charge, then load test it. If either battery is failed in the mode where they get hot when
attempting charging, that can take out your alt or isolator.
--
John Lebetski
Woodstock, IL
77 Eleganza II

[Johnny Bridges via Gmclist]

No alternator output. Easiest cause - poor connection at the two-wire plug on the rear side of the alternator, clean it and bend the tabs to give
better contact. Next is bad regulator or diode oak in the alternator. Both cheap and easy to replace but you hafta unship the alternator from the
coach. A good parts house should be able to supply you the diode pak and the regulator. Ad a set of brushes, they're cheap. Take it apart, replace
regulator and diodes (keep the takeouts, one or both might be OK). Take some crocus cloth to the slip rings, then note how the wire in the brush
assembly keeps them out of the way until the thing is reassembled. Greas the back bearing and put it back together. Option - buy a lifetime warranty
rebuild from Advance, Keep the receipt and every time it quits (it will) go get another one.

--johnny
--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased

[Dan Wallace]

Thank you all for the feedback!

The alternator field connection was open -- I did not open the case to find a root cause.

It looked pretty corroded inside, and since it was pretty noisy last week, it may have an open output diode too.

Also, it was kind of "rumbly" when I spun it by hand.

The resistance in the Chief's bulb/field tickler, remote sense, and output circuits are all acceptable.

So, I went to O'Reilly's, gave them $105, and they gave me a limited lifetime of charging bliss.

While I have it out, I will install a tickler circuit diode for resistor protection.

The Chief has less than 38k miles on the clock, but as you know, for many components, sitting is worse than operating.

--
Dan Wallace
Detroit Metro
1974 Canyon Lands 260
Headers, 3.55, 16"
Floor plan 26-9

[James Hupy]

You might want to rephrase that to" a lifetime of changing bliss" with
alternators. (Grin)
Jim Hupy
Salem, Or
78 GMC ROYALE 403