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Here are some basics for checking the alternator internals. The majority of the time the problem will be a shorted or open diode. A lot cheaper than a new alternator.

Tools needed: 1/4, 5/16, 11/32, 7/16 nutdrivers or sockets,meter,test light.


Before you tear into the alternator,do a few simple tests. With the key off and using a test light or meter unplug the 2 wire connector and check the red wire at the plug and the red wire at the output terminal.Both should show battery voltage. With the key on check the brown wire.It should have voltage. With the key on use a jumper to ground the BROWN wire.The charge light should be on with the BROWN wire grounded.


Make a mark across the 2 alt. halves so you can get the clocking right when you put it back together. You don't have to remove the fan and pulley to split the case.


Using the 5/16 take the 4 screws out of the case. Pull the 2 halves apart. The brush springs will probably fall out when you split the case. The stator is attached to the back half of the case. If the case won't pull apart by hand pry it between the edge of the stator and the front half of the case.


Using the 11/32 remove the 3 nuts holding the stator wires to the bridge and remove it. I do the stator and rotor tests at this point because if either one is bad it's time for a new alt. A shorted or open rotor or stator is rare but it happens.


Using an ohmeter or continuity tester hook one of the leads to one of the stator wires. Hook the other lead to each of the other two wires.The reading should be the same across any two wires. Leave one lead on one of the 3 wires and connect the other to the stator frame. If it has continuity here the stator is shorted to ground. The alternator I used is a whopping 32a off a Chevette. Higher amp alternators will have larger gauge wire or more windings and the resistance readings will be different.It read 3 ohms across the leads. Don't be too concerned about the ohms. The main thing is that the windings have continuity and are not shorted to the frame. Same goes for the rotor.







Hook the test leads to the slip rings of the rotor. It should show a low resistance. This one read 2.4 ohms. Hook one lead to to either slip ring and the other to the shaft.If you get a reading here the rotor is shorted to ground. If it checks good clean the slip rings with some Scotch brite or fine sandpaper.







Hook the negative lead of the tester to the long terminal of the diode trio. With the positive lead check each of the three terminals. If you're using an ohmeter the reading will be in the 1-2 megohm range. If you're using a continuity tester it should show continuity. If you're meter has a diode test function it should show about .5-.65. The readings should be the same or close. Reverse the leads and do the same test. With an ohmeter- infinite or very high megohm range. With diode tester- no reading. No continuity. What you're looking for here is large differences between the diodes.If theres continuity in both directions the diode is shorted. If theres no reading in either direction the diode is open.








The tests for the bridge rectifier are the same as for the trio, except there are 2 sets of 3 diodes. When checking these out hook the test lead to the flat diode leads instead of to the stator wire studs. The other lead goes on the heat sink. If you use the meter diode function these read about .4-.5.









This diode is shorted.







The brushes are about an inch long new. If they're not worn too much they can be reused. Put one of the springs into the brush holder.Push the brush in on top of the spring.Slide a toothpick thru one hole in the holder to hold the brush in and then put the other spring and brush in and slide the pick all the way thru so it holds both brushes.





Put a little bit of bearing grease into the rear bearing. Put some heat sink grease on the back of the regulator and set it in place. Same grease as the ignition module. I don't know any way to home test the regulator. There's a test hole on the back of the alt. that full fields the rotor when it's grounded,but if there's a bad diode or brush set grounding it won't give a true reading. The dealers used to have a tester and some of the parts stores can test it. I use the SWAG method. If I don't find anything else wrong inside I'll replace the regulator. Line the toothpick in the brush holder with the hole in the case and set it in place. Put some heat sink grease on the bottom of the bridge and put it in place. Install the 2 screws and 1 stud in the bridge. Install the diode trio over the bridge studs and then the 3 screws into the brush holder.There are 2 insulated screws and 1 uninsulated. The arrow points to the uninsulated one. Install the 3 stator wire terminals over the bridge studs and tighten the nuts.









Line up the case marks and put the halves together. Once the screws are tight pull out the toothpick. Done.

For years I used a self powered test light to do all of these tests except for the 2 wire connector. For the diodes reverse the leads as you would with a meter.The light should come on in one direction only. For the rotor and stator the polarity doesn't matter,


 
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