Guys,

We were wondering how you test an alternator? Where do you connect your multimeter? What are you looking for?

We have tried measuring the resistance from the ground of the car (at the rear of the alt.) to the positive (red) wire on the harness that connects onto the top and got a resistance of 1600 ohms. this seems pretty high for a big long bit of wire, effectively.

How do you measure things while the car is running?

Many thanks for any help or suggestions!

 

the simplest way to test the alt is to remove it and take it to a parts store (who will generally test it for free). if it is good, reinstall it. if it is bad, well you need it for the core anyway so its already out. just my opinion (my alt is just a couple torx bolts, a 10mm wrench, and a wire harness, it barely takes 10 min to remove)

 

To test the gen simply measure the voltage across the battery terminals with the car off. Typical resting volt is approx 12.4-12.6 volts, anything lower that 12.4 means the battery needs charging. Start the car then measure the voltage across the battery, it should read at fast idle with the accessories off approx 13.5-14.8 volts. You can also read off the rear lug on the back of the gen, it ties to the battery directly through a fuse link. Ground for the meter can be the generators case. There is no need to remove it from the car if you have a decent voltmeter. Total time to test it, about 2 minutes.

 

Testing voltage only checks half of the alternator. You can have a bad alternator put out 14v. It just might not put out the current need to run all of the accesories. Thats why they hook it up to that machine at autozone. It measures max current production too.

 

Not really a true statement. Anytime you normally see a problem with a gen, output voltage will be affected. Remember, voltage and current are related. You can have a situation where you lose a diode which is very common in the stock Delco rectifer packs and the gen regulator will full field to make up for the lost phase but even in that case the output voltage is generally lower than the regulator target voltage which for most Delco/Delphi regulators is 14.8-14.9 volts. Placing a load to measure current is much the same as using a battery and you are correct in saying that the current is a factor to know. The load they use will also tell if a diode is breaking down under load. For field testing in most cases a voltmeter is all you need to ascertain what the gen is doing. It also comes from a little experience in seeing many of them and how they behave. In most cases with a burnt diode in the Delco rectifier they will full field but output is usually less than 14 volts with a battery as a load. They will continue to full field until the regulator reaches about 280 degrees and then it shuts completely off.