testing the alternator

[johnnymo08]

i was just wondering (curiosity) how you test the alternator.........i think my Cadillac devill has a bad alternator because it dies sometimes and can only be started by jumping it....

 

[Wicked_Sludge]

start the motor and check the voltage at the battery, you should get at least 14 volts. if not, bad alternator (or wiring)...if so, turn the headlights and blower motor on high and check again...should still be above 14...if not bad alt.

 

[jlloyd]

You can also start the car and then disconnect the batt. If good,it will continue to run,Jim

 

[MAKG]

That's much less reliable.

Wicked Sludge's method is the right way to do it.

If there is any doubt, an alternative is to bring it to the auto parts store for testing.

 

[eli]

Check also the A/C voltage out to check to see if the diodes work in the alternator, the reading should be .5 or less

 

[Bob Ayers]

You can also start the car and then disconnect the batt. If good,it will continue to run,Jim

Don't ever disconnect the battery while the engine is running! You risk damaging expensive electronics (like the PCM, etc.) from the huge voltage
spike that can be created from the Di/Dt.

 

[Ranger5.0]

wikked sluge? why would an alternator put out max voltage/amperage @ idle?

 

[Wicked_Sludge]

huh? it shouldnt...

but if its not putting out the 15-20 amps it requires to run headlights and a blower motor at idle then its not functioning properly.

 

[bottledgt]

wikked sluge? why would an alternator put out max voltage/amperage @ idle?

it should be at least 13.5 at idle. and probably not show much more than that revved up. thats why theres a voltage 'regulator',to keep it from going to high

 

[red85]

NEVER disconnect a battery while the vehicle is running. You can fry your PCM by doing this. If you're not sure how to test that alternator, take it to AutoZone. If you need to replace it, get a good quality reman with at least a 1 yr warranty. you may also want to have the battery load tested if that alternator is bad. Batteries are designed to start the vehicle and that's it. The alternator is what keeps it running.

 

[Ranger5.0]

juss asking. They trained us to test alternators @ max output befor condeming them, thats all....bottledgt, I know what a voltage regulator is, and even how it works, im not an idiot lol

 

[shadetree]

While spikes occur, and can damage the electrical system, the main danger comes from the fact that the battery acts as a filter for the pulsating dc in the alternator. Pulsating dc voltages can reach 40 volts or more. The battery filters, and smooths out these pulses to insure a safe, constant voltage source for the system. Removing the battery cable removes the filtering effect endangering the system.

However, in a pinch, on occasion, I have removed the cable to check the alternator, but the circumstances were special, and I do not do it as a matter of routine. So far, luck was with me, and no damage occurred. shady

 

[eli]

The alternator triple diode acutely cuts down the A/c voltage and since typical alternators are 3 phase it generally should come out of the alternator pretty smooth in D/C fashion and the regulator controls out put voltage, if your getting 40 volts out of your alternator you have a problem. The battery is there to absorb small spikes and store power

 

[Bob Ayers]

The alternator triple diode acutely cuts down the A/c voltage and since typical alternators are 3 phase it generally should come out of the alternator pretty smooth in D/C fashion and the regulator controls out put voltage, if your getting 40 volts out of your alternator you have a problem. The battery is there to absorb small spikes and store power

The diodes in the alternator converts AC to pulsating DC, NOT smooth DC.

Here are a couple of slides from a semiconductor device course I developed a
few years ago. These explain half-wave, and full-wave rectification: