Alternator Bench Test

[DPDISXR4Ti]

Have a used alternator on the shelf but not sure if it works? Did you pick one up from the junkyard but want to make sure it works before putting into service? Dean recently posted this in a thread regarding using a Nissan spec alternator in a Scorpio and I thought it was worth having as it's own topic here in the FAQ. I've made a couple amendments, but it's pretty much as he posted.

Summary: One side of the test bulb goes to +12v. The other side of the bulb goes to the lamp (D+) connection on the alternator. With the alternator NOT spinning the bulb will glow. When the alternator spins the Lamp connection side will rise to +12v. You will then have +12 on both sides of the bulb and it will go out.

Lets review the alternator bench test...

On the bench we have...
A car battery
An alternator
several big clip leads
drill rigged with a socket that can spin the alternator (often 24mm)
A Digital Multi-Meter
12v light bulb. Preferably with two wires soldered to it. A spare instrument cluster bulb works fine (LED bulbs will not work)

1) connect the case of the alternator to the battery negative terminal with a clip lead (or jumper cable)
2) connect the output stud of the alternator to the battery positive terminal with a clip lead.
3) connect the S (sense) of the alternator to the battery positive terminal with a clip lead.
4) connect one side of the 12v light bulb to the alternator LAMP , the other side of the bulb to the positive terminal of the battery. Note that the bulb glows at this time.
5) hook up the DMM to measure the battery voltage.
6) spin the alternator with the drill. Note that lamp goes out. Note that voltage across battery rises slightly.

[DPDISXR4Ti]

Here's a good video that describes how to do Diode test. He's doing this one with the alternator attached to the engine, but he basically turns it into a bench test by disconnecting the power to the alternator.

https://www.youtube.com/watch?v=dhf6IdBbE3Y

[DPDISXR4Ti]

Some real-world data-points testing a couple original Bosch alternators from Scorpios...

Diode test yielded .948 & .931 microvolts, which is probably within spec.
For the voltage test I couldn't spin the alternator up fast enough with my drill to yield any charging voltage and turn the bulb out. With the ~2005 Nissan spec 110A Mitsubishi and 130A Hitachi alternators I was able to generate 12.95 and 12.75V which readily turned the bulb out. I think this is a function of the older Bosch alternators not being very good at making power at low RPM's. A faster drill should yield different results ...or perhaps persuade you to upgrade to a newer technology alternator.

[Ed Lijewski]

Alternators have cut-in rpms above which they're sufficiently excited and current is produced.

I've observed that OE Bosch units excite sufficiently at low starting rpms, 5/6/700 rpms.

Rebuilt units I've installed need rpms at/above 1k to get sufficiently excited.

No idea why this difference.

YMMV