Battery Isolator on RV

Hello Folks. I've got a '79 CHEV VAN 20 series - camperized. Today wouldn't start after short run - dead battery , battery new 1 month ago. I had alternator tested on machine : max. current 33 Amp at full load
( rated for ~ 65 A ) I replaced alternator but I'm suspicious of Isolator + wiring to + from it. Alternator output ( hot ) wire goes to Isolator then back to original output wire ( isolator is an aftermarket conversion ). Another terminal on isolator runs presumably to Auxillary battery for lights + furnace inside, QUESTION : presumably isolator is a diode to prevent Auxillary battery from draining main battery i.e. allows current to go one way only to auxillary. HOW CAN I TEST ISOLATOR ? Isolator has tree terminals on it : from alternator ; back to splice into original alternator output wire ; to aux. battery.
I trust this long winded explanation is adequate. Please reply to NG. Thanks- -JS
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Seems to me that you could test battery voltage, main + aux. battery, start engine, re-test with engine running, both should be higher. Shut off engine, disconnect one terminal of main battery, turn on key, see if radio plays.
No radio = isolator is keeping aux battery out of circut Radio plays = possible short in isolator
Re connect main battery, disconnect aux. battery, try a light in camper No light = isolator OK Light lights = short in isolator
This is best a guess as I've got to offer, knowing Chevys, I'd take a close look at grounds, they are skimpy at best, problematic at worst. A heavily painted alternator bracket can cause all sorts of Chevy ground hell !
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question snipped-for-privacy@home.org (john) wrote in

You can test the diode with a digital meter or even an analog meter if you have one or maybe get a friend who has one to test the diode. A diode should read about .6 or .3 volts in one direction and open in the other. Disconnect alternator and battery connections before test. With an analog meter it should read have low resistance in one direction and when you reverse the test leads it should be at least 10 times greater.
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wrote:

Most all digital meters will read .6-.7V or .3V depending on the diode type and the current is supplied by the low voltage battery in the meter, if the meter is connected to a live charging ckt in the diode switch position or ohms position on an analog meterthe meter may be damaged. DISCONNET all leads from BATTERY and ALTERNATOR before performing Diode test. You can read the voltage drop across any diode while the circuit is charging but this should be done in the VOLTAGE mode and not the Diode mode. always start on a much higher scale than the expected voltage or use autoranging on a digital meter.
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You mentioned that;

A diode does not produce any voltage.
And the first and simplest test without disconnecting any wires should be just connecting a voltmeter across the center post of the isolator and the other lead to each of the other 2 isolator posts. With the engine running. So as to confirm all at once,
1) A series current path from the alternator output thu the isolator then to the batteries. This will prove that the alternator is working and that everything is connected. 2) And if one of the diodes are open by not having a voltage drop across it. This proves that the diode is bad and tests it under actual current conditions.
And incidently I am betting that you will always find silicone diodes with their inherent cold .7 volt drop in high current circuits other than diodes with germanium junctions (.3 volt types) with their known reverse leakage, and temperature instabilities..
MIKE......
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Michael wrote:

<snip> Actually, you should read the applied voltage across an open (12VDC). A short would normally read zero.
Just a nit-pick, Mark Sparge
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On Tue, 15 Jul 2003 10:08:47 -0500, "Mark Sparge"

Yes, Yes, you are correct!! It's back to the drawing board.
Mike....
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