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How to bench test an alternator / diagnose a starter

Started by Casey, 14 December 2011, 04:55 AM

Casey

The battery wouldn't charge in one of my cars, making it pretty useless.  The alternator in the car is a remanufactured one installed quite recently by the previous owner - it's still shiny and new looking.  Nonetheless, I assumed that was most likely the problem, and decided having spare alternators around wouldn't be a bad idea, so I bought two used ones - one from a junk yard (never know what you're going to get) and one from a private seller that I got a bunch of interior parts from, who claimed it came out of a working car so it was known good.  The latter had the wrong pulley on it for the engine, so I decided to try the junk yard one before changing pulleys.

First off though, I tried connecting a multimeter to the plug pins on the old alternator with the engine running.  I'm not sure if I did that right because there are three pins and I'm not sure why that's the case instead of there being two, but I couldn't get any significant reading from them - so it seemed the alternator was dead.  I didn't think to try running the headlights with the battery disconnected with the old one...

Put the junk yard alternator in, and /did/ test running the headlights with the engine running and battery disconnected, and they worked, although a little bit dimmer than usual.  I assumed this meant the alternator worked, and didn't do a multimeter test again (my consistency sucks, I know...).  Put a battery in (which had been sitting for a couple months but had been fully charged in the interim maybe two or three weeks before), which was strong enough to start the car without jump starting (took a few tries though).  Went driving around for a good 45 minutes, which I figured would be enough to get a good charge on the battery.  Stopped and ate some late night dinner.  Came out and the car wouldn't start.  When I turned the ignition I'd just hear a soft click and that was it.  Had to push start the car, which I wasn't too excited about.  When I got back to the garage, I noticed that the headlights were dimmer than they had been, though they would get brighter when I pressed the gas.

This seems to indicate to me that the alternator is working, but I wonder how well.  I'd like to know how to better test them.  Furthermore I'd really like to be able to test the ones I've got that aren't in the car so that I know their condition.  I'm a bit worried the starter might be bad, since even with a brand new battery or a 200amp electric jump starter, it often just makes a soft click when I turn the ignition and I have to try numerous times to get it to start, but I'm not sure that's the issue or not.  I've got a truck battery that's too big for the car, but it will *always* start with that connected.  Is there a good way to test the starter in isolation?

The glow plug light never turns off in this car, so I wonder if there aren't bad plugs shorting out and draining the battery or something.  I haven't tested them yet because it's a pain, though the relay strip fuse is good - I'll probably get to that and the injectors this coming weekend.  In the meantime though I'd really like to learn how to better diagnose things...

Casey

Different pulley styles - four cylinder engines use the single-belt pulley, 5- and 8-cylinder (and presumably 6-cylinder) engines use two belts.  Not sure why as they appear to be the same alternator - maybe you could replace the main pulley on a 4-cylinder engine and use two belts?  On the other hand the single belt style is a thicker one:


Three different alternators - the one on the left has a different vent style and is the one I bought from a private seller.  The middle one is from the junk yard and has a mysterious cut-off wire screwed on on the right side - it's what's on the car now.  The one on the right is the one that was on the car originally.  Three different styles of voltage regulators!:

Jakob

You should be able to tell if an alternator is working by connecting a multimeter to the positive and negative terminals of the battery. When the battery is charged but the engine isn't running, it should read about 12.7ish volts. If the alternator is working, when the engine is running it should read from 13.5 to 14+ volts. Other people might know more exact figures.

Not sure about bench testing the alternators but you can bench test a starter by removing it from the engine, placing it in a vice on the bench (or some other way of securing it so it won't jump off when you engage it), then attaching negative to the body of the starter, positive to the bolt terminal on the back where the positive cable bolts on (as opposed to the earth) and then running a positive cable between the positive terminal on the starter and the positive terminal on the solenoid. I hope this makes sense since I'm typing from memory without a starter in front of me, and my memory sucks :-P.

This should tell you whether your starter is working properly or not. If it just makes a 'click' it sounds like the solenoid might not be working.



Casey

Quote from: Jakob on 14 December 2011, 05:32 AM
This should tell you whether your starter is working properly or not. If it just makes a 'click' it sounds like the solenoid might not be working.

Hmm, I've heard of selenoids before but never seen one.  Is it something replaceable independent of the whole starter?  I'd assume that's less costly.

Jakob

The whole starter looks like two cylinders. One big one, and one small one attached to the side. The solenoid is the small one. I don't know if it is easy to replace for a home mechanic, but I'm sure most autoelectricians would be able to do it.