Testing for isolated alternator?

[heintje23]

Ow do you test with a multimeter if an alternator is isolated? Measuring resistance between ground cable and alternator casing? What should be the reading?

[DeepFrz]

Some resistance greater than 0 ohms. Try reading the resistance with the probes first one way then the other. Of course you have disconnected all power to the alternator, right?

[heintje23]

Uhm no. All cables are still connected. You mean only measuring between the terminal and alternator casing?

[Maine Sail]

Originally Posted by heintje23

Ow do you test with a multimeter if an alternator is isolated? Measuring resistance between ground cable and alternator casing? What should be the reading?

Should read open circuit or open lead (no continuity) when the DVM is placed across the alternator ground STUD and the alternators CASE...

[heintje23]

Ok.

And with the cable connected? Should it make a difference?

[S/V Antares]

An Isolated Alt will have a Neg stud (B-) that is isolated from the case. Probe case to stud and you should have an open curcuit.

[FlyingCloud1937]

Originally Posted by heintje23

Ok.

And with the cable connected? Should it make a difference?

To test for an isolated ground. The B- cable going from the neg stud on the alternator must be disconnected. Then as mentioned by others, measure between stud and case.

If you leave the neg cable connected, a ground circuit is formed due to the neg of the battery being connected to the engine block, as well as the neg of the alternator.

An isolated alternator provides a low resistance path to the battery, as long as the cable is sized for the proper voltage drop. It also eliminates circulating currents from the alternator to engine block. High output alternators can cause electrolytic corrosion at bearing surfaces, ans head gaskets. That's why all high output alternators on boats should have an isolated ground.

Any alternator can be converted to isolated ground, with a little work.

Lloyd