[rheavn]

"One, does it make a difference, with the gen running, if you put the main engine on high idle?"

No

"Two, as a quick piece of info, can you temporarily remove the fan belt from the gen motor. You don’t want to run it long this way but it will disable the gen motor alternator."

I can & will and report back the results

"Can you get your voltmeter probe on the back side of the alternator and read the voltage with the engine running?"

No I can't, but I can test voltage at the alternator lug at the battery isolator.

[bestgenman]

As an alternative, unplug the “control” wire on the back of the generator alternator.  It’s a plug in connection.   This keeps the belt in place and Undisturbed.

[rheavn]

Good tip...............................................thanks

[rheavn]

Here are the results of the tests........................................................

Power post shut off.

Started coach engine, put on high idle and alternator worked fine showing 14.2 volts on the VMS--15.06 volts at battery isolator center alternator post.

Turned on genset.  Once it transferred in, the voltage began to drop until it reached battery voltage of 12.4V.  "Check alt" warning light lit.  Voltage at alternator post on battery isolator showed less than 3 volts.

Shut off genset.

Check alt light went out.  Voltage on VMS went back to normal 14V.  Voltage at alt post of batt isolator went back to 15V.

Pulled genset alt control wire.  Restarted genset.  Once it transferred in VMS voltage dropped to batt voltage and check alt light lit.  Voltage at battery isolator down to under 4 volts.

Shut off genset and alternator voltage returned to normal.

Any ideas......................................................

[Richard]

Hmmm,

What is house battery charging doing during this time? If you disable the house battery charger, what happens.

Seems to me, that you have eliminated the generator alternator as a cause.
Also, you have confirmed the alternator is working as it should.

Do you have an echo charger to keep the chassis batteries charged from the house batteries?
What is your house battery voltage? I am just thinking out loud about how the bulk charge voltage on the house batteries could trick the voltage regulator on the alternator IF the two banks were connected in some way.

[rheavn]

"What is house battery charging doing during this time? If you disable the house battery charger, what happens."

Never shut it off, but I'll give it a try.  Remember though the charger is on when I am connected to a power pole and if I start the engine, no problem.

"Do you have an echo charger to keep the chassis batteries charged from the house batteries?"

Yes

"What is your house battery voltage? I am just thinking out loud about how the bulk charge voltage on the house batteries could trick the voltage regulator on the alternator IF the two banks were connected in some way."

The only place that I think they are connected is within the battery isolator as I understand the alternator charges both the house bank and the chassis bank as one drives.

With the charger off, the house batts voltage is 13.35V

[Richard]

The scenario I am imagining is that when the Xfer switch cycles, it briefly interrupts power to the inverter/charger. When the charger comes back on, it goes into bulk charge mode. But this is only in my imagination. And the echo charger would have to then pass the higher voltage from the bulk charge to the chassis batteries.

And the isolator should not allow the banks to talk to one another, because it is in practicality two great big diodes.

And you said merge switch is off.

[HoosierDaddy]

I dont have an answer but I will offer a couple thoughts that might spark an idea.:
Have u checked voltage at the main alternator exciter wire with it disconnected from alternator while gen is running?
Could u introduce a non chassis 12v source to the main alternator exciter (control) lug?
I'm wondering if there is a back-feed that is causing your exciter wire to not show 12v.

[bestgenman]

I’m wondering if the diodes in the generator alternator are shorted ?   Maybe that is a stretch but if you unbolted the output lead on the alternator it would rule out any possible interference from the generator alternator.  

Something is pulling a lot of amps, I would think.  The voltage would not be dropping unless there is a large current flow OR, a voltage higher than the set point on the chassis battery system is shutting off the main engine alternator’s regulator.

I’m running out of ideas.

[rheavn]

HoosierDaddy,
Newer alternators did away with the exciter wire.

Yes, the merge switch is off.