Alternator testing procedure

[Twin pipe]

Whats the best way to test output, ? is it a case of removing the big red wire, running engine and holding tester on the big red terminal and taking a reading in my case it reads 3.5 volts? [my battery light stays on] i guess it should read 13/14volts?

[HarryW]

Generically (as I've never measured it on a cerb); Just measure the battery terminals or the big red wire, leave it connected, on the back of the alternator (battery feed), not running should be 12v(ish) (battery voltage) running it will be 13.4v (ish) (charging).

[Steve_T]

You can just test with the car as is - no need to dismantle/disconnect anything. Put the test meter across the positive battery terminal and an earth on the engine - air con compressor or whatever, with the engine running. You should use ~14.5v to charge a sealed battery, but I don't know exactly how much resistance there'll be between the battery neg and the earth that you touch your test meter against. What you see see on the meter's display will very likely be lower than this figure - mine read around 13.4v last time I checked too.

Steve.

Edited by Steve_T on Friday 29th June 17:09

[Twin pipe]

is there a signal / wire to activate the alternator charging?

[HarryW]

Twin pipe said:

is there a signal / wire to activate the alternator charging?

Don't know, but the alternator excitation feed is often in series with the ignition lamp on the dash but as your is lit I doubt its that.