Larry,

Yes, your charging system has failed, but don't go buying parts before finishing the diagnosis.
You have a meter? If no, go get one.
Measure the battery voltage.
Fire the engine. Measure again.
Measure between the center of the isolator and the plate. Should be 14+ with the engine running. Less than that check alternator connections.
Engine Off. Headlights on.
Measure between the battery + and the terminal on the boost contactor. Same as the battery?
Same for the battery - and the aluminum ground plate where things are mounted. Same? Ok connections are good.
Engine off.

If you were good to there, pop the wires off the islator and set the meter to resistance. May be marked as Ω.
Check between the center and both others one way and then reverse the test leads and do it again. If you don't get a difference one way and not the other, things are good.
Isolator failures are rare, but do happen.

Good luck Be sure to tell us what you find.

Matt

Thinking about the role of the isolator and the fact that my battery was down so much after behaving normally just last week, am I right thinking a failed isolator could have allowed my house batteries to draw down the chassis battery?
Larry

If you were good to there, pop the wires off the islator and set the meter to resistance. May be marked as Ω.
Check between the center and both others one way and then reverse the test leads and do it again. If you don't get a difference one way and not the other, things are good.
Isolator failures are rare, but do happen.



Matt

Larry,

An Isolator allows current to flow from the center post (Alternator) to the outer posts (Chassis and House batteries). It blocks current from flowing from the battery terminals to the alternator or to each other. Therefore the house loads cannot discharge the chassis battery, but the alternator can charge both batteries.

Since you see ~15 volts at the center post (alternator) but less than 12 volts on the chassis side, that indicates to me that the Isolator is faulty. The battery terminals should be about 0.7 volts lower than the alternator terminal with the engine running. That's the only test you really need to indicate a good/bad isolator.

Isolators are widely available, I'd suggest Princess Automotive or Canadian Tire for us Canadians. Get one rated at at least 90 Amps, 120 or 150 amp models give you some more robustness.

The center terminal is generally the Alternator. It doesn't matter which battery connects to the other two terminals.

Possible scenario. Your failed battery may have taken out one diode in your isolator. The battery fail you experienced is typical with today’s batteries. My take is in the old days they would gradually start to crank slower over a period of time. Today the end of life curve is dramatic.

Did you get the isolator that has the aluminum fins? or the one in the plastic case. The plastic case unit is a combiner. It connects the batteries together when it detects that either battery is getting a charge voltage, either from the alternator or the house power converter. Nothing wrong with the combiner, but you do lose the higher voltage from the alternator which is used on high-speed fan to give you more air flow from the dash heater-A/C unit.