Is there a way to test to see if an alternator is good using a multimiter? or does it require special equipment. Doesn't Autozone do this testing for free perhaps?
There are a couple of ways to check an alternator. First, to see how well the diodes are blocking the a/c current. Check the positive output to case ground for a/c voltage. anything over 300 mv fails. second you want to do an alternator out put test. usually i put a amp meter on the output wire, zero it, and apply a load to the alternator ( like an old style bread toaster, I mean battery tester. if you know what i mean you know what i mean.) So if your DMM has a amp clamp then your in business but if not then no. You also want to look for 13.8 to 14.5 volts DC output voltage.
I've always done the "not recommended" method of starting the car, turning off stereo, A/C, lights etc and pulling the battery cable off. If the car stays running your alternator is most likely OK. If it dies, time to get a new one. I would go with Chris A's suggestion, safer and more reliable results.
Like 4PASNU said, not recommended, but if ya start the car, pull either the - or + off the battery, if it stays running, your alternator is prolly okey. I usally go 1 step more and press the brake pedal, it requires a little stain on the alternator, and if the car shuts off, id replace the alternator. Autozone can test the battery and alternator with them still in the car.
-Jeff
I'm not knocking anyone's head here, but there is a reason it's "not" recommended. The alternamatator needs the battery to resist it's output. WHen you unhook the battery, the alt. is allowed to output full voltage. "No, it's not!", you say. "Because of the voltage regulator." Well, w/o the battery on there, the alt. can make WAY more voltage than the diodes and regulator can sustain. BUH BYE diodes. I think it's something like 30V (But I can't remember.) So, ya, just don't do it. :lol: That's about all I can tell you. The tests are free at Wal-Mart, Auto Zone, Adv. Auto (I think they test them.) So, don't risk it, because if you are lucky enough to have a Delco, and you fry it by unhooking the bat. you will be pissed when you buy a shitty remand. unit, and it's half the one you had. (Adv. auto remand. alternators suck my ass.) :evil:
I would sure like to know where you got your information from. There is no way an alternator puts out even close to 30 volts when you unhook the battery. Electronics is what I do for a living and even though I've never tested the output of an alternator with the battery unhooked I would bet large amounts of money that it would be no more then around 15-16 volts, certainly not 30. The regulator on the alternator is for just that, regulating the voltage output to within the tolerances of the equipment running off of it. If what you say where true when you unhook the battery and that voltage where to jump up to 30 it would fry half the electronics in the car. Another thing to consider is the load on the alternator. Voltage output from the alternator is dependant on the load draw from the equipment in your car. Just because you unhook the battery doesn't take away the draw from the car itself. If I'm wrong on this and someone car prove it to me I'll gladly admit it but like I said about, I'd bet that I'm not. I stand by my original statement that it is still not recommended but in a pinch it usually does the trick.
take it out of the car, and take it to pep boys. they test it for free.
Hey, I'd love to try and prove you wrong, but I'm not going to destroy my alt. to do it. :lol: Like I said, I can't remember how high the voltage is, but it is enough to roast the diodes and VR. If electronics is what you do for a living, and you unhook a battery to check and alternator. Don't ever, EVER, come near my car. J/K The point I was trying to make was simply DON'T DO IT. That's all.
Edit: I found an article on ChevyTalk (another forum I visit) a segment about bad GM side post terminals, and it is as follows:
http://subtopics.freeyellow.com/Batteries-Charging.htm
....buddies battery/charging info page
TEXT from that article:
That bad battery post design can/does kill alternators, batteries, starters, battery cables, and wreck havoc with other electrical circuits.
Think many don't know what Traveler and I are talking about. Here's an example of what is happening and the cause'n affect:
Take the battery positive cable lug and unscrew it from the battery (disconnect it). Manually hold the cable lug to the battery post. Start the vehicle. Now touch, remove, touch, remove the cable lug to and from the battery pos terminal. THAT is EXACTLY what happens when that stupid lead spacer crushes and allows the bolt to bottom into the battery pos post.
The alternator sees the battery, charges (provides oltage/current), then the connection goes away. Voltage at the alternator goes up (maybe several hundred volts, instantly...or almost that fast). Then when the contact is made again, the alternator will see huge load (maybe hundreds of amps, instantly...or almost that fast). This is called load dumping and will kill a good alternator in short order.
Above load dumping will also kill the battery in short order too. Over voltage and over current. Also will have high resistance terminal, meaning voltage drop, meaning current used at that spot. That current usage turns out to be heat...high enough to melt the battery case, which will allow battery acid to leak into the joint *AND* down the cable (cable acting like a wick and cable insulation acting like a hose...carrying battery acid down to the starter...starter don't like battery acid).
There are many places online supporting this theory. I just passed along info.
Quote from: 4PASNUI would sure like to know where you got your information from. There is no way an alternator puts out even close to 30 volts when you unhook the battery. Electronics is what I do for a living and even though I've never tested the output of an alternator with the battery unhooked I would bet large amounts of money that it would be no more then around 15-16 volts, certainly not 30. The regulator on the alternator is for just that, regulating the voltage output to within the tolerances of the equipment running off of it. If what you say where true when you unhook the battery and that voltage where to jump up to 30 it would fry half the electronics in the car. Another thing to consider is the load on the alternator. Voltage output from the alternator is dependant on the load draw from the equipment in your car. Just because you unhook the battery doesn't take away the draw from the car itself. If I'm wrong on this and someone car prove it to me I'll gladly admit it but like I said about, I'd bet that I'm not. I stand by my original statement that it is still not recommended but in a pinch it usually does the trick.
Fair enough that it may work in a pinch, but in that same pinch it may destroy the alternator. The battery is a large resistor in the electrical system. When that load (even if fully charged) is removed then the regulator can't respond fast enough to the voltage demands and the voltage spikes. Then you hook the battery back up and it pulls it down again. Not good for it at all puts a huge strain on the reg/diodes.