Instrument isolation problem?

[In Memory Walter K]

I have 3 banks of batteries. I did so when I put in diesels because on startups some of my instruments would shut down from the electrical strain.
I devoted the 3rd bank to my electronics and isolated them from the engine startup ones with a solid state finned piece (forgot what it is called) that allowed my alternators to charge all of the batteries but the current couldn't drain backwards and drain the others if any of the batteries went bad. The system has worked perfectly for years now. Never had an instrument shut down on startups again. Suddenly today I discovered my Northstar GPS and depth recorder started to shut down on startups. I have to assume the solid state Isolater (is that what it's called?) has died inside and the diodes that kept the current from running back to the other batteries are not functioning properly. I will check the status of all my batteries tomorrow, but even if my instrument bank is bad, I don't think the gps and depth recorder should have shut down on startups if the isolater(?) was functioning properly. Being solid state, I can't figure out a way to check it. Buy a new one? Am I correct in my assumptions? Any recommendations? Thanks. Walter

[ScottD]

Walter, I think your talking about a diode on a heat sink. It sounds like the diode (or diodes) have failed. To check it, just use your multi-meter on the ohm setting and read continuity across the device, switch leads and repeat. Your resistance should be infinite one way and low the other, actual value of low restance is not so important. If it's low both ways, it's bad, however if it passed that test it still could be leaking under higher current and still be bad. Bottom line is, the test can tell you it's bad, but not necessarly if it's good. One more thing, when working it will drop about .7 volts across it, keeping the electronics battery from getting a full charge. It's sometimes good to give it a full charge with a shore charger.

ScottD