plonka

My alternator was working fine except the return signal that kills the gen light after start up is faulty. So although my alternator works fine, the gen light was always lit. I bought a reman duralast alternator, installed it, but after driving a mile or so, the belt came off. It did this 3 times. After comparing the alternators side by side, I found that the pulley on the new one sits 1/8" lower than the original. I put the old one back on, no issues at all except that light staying on. Anyone else ever have this issue? and how did you remedy the problem? Cheers!

cherokeepeople

assuming your dealing with the car on your avatar.pull the alternator off and take it apart,you have a bad diode,it will either be in the diode trio or the rectifier bridge.pull the trio and test it if it tests o.k. then replace the rectifier.

Sugar Bear

X2

oldcutlass

or, you can take the pulley off the old one and install it on the new one.

plonka

I will take your advice, but since I have no charging issues, wouldn't the problem lie in the resistor, since it is responsible for the dummy light. Please correct me if I am wrong as this is not my area of expertise.

cherokeepeople

if memory serves me correctly and it's been 20 yrs since i have had one apart,but the light is controlled thru the diode trio.
one thing,before you take it apart we used to take and mark the cases,just use a fine line marker and scribe a mark across both halfs and the winding.this way you get it back together and everything lines up.if you don't know how to test the trio just post and i will tell you how

67442nut

Tell us how, anyhow.

stellar

If you mean this resistor, then No it isn't your problem.

cherokeepeople

to test the diode trio and the rectifier your checking the flow of current thru the diode.it is to go thru one way only if it flows both ways it is defective.
to test:you need a one way voltmeter.by this i mean with the positive lead on positive and neg on neg across a battery you get 12 volts or whatever,if you reverse the leads your meter should read zero.most of the new digital meters read a - neg voltage doing this.my dads old meter is the only one i have ever had that would do this.it's a big old thing and has no battery in it.just thinking you could probably use a voltmeter gauge made for a car like an autometer or s-w gauge.
take the trio it has 3 eyelets on the top and one on the end of the leg.take this single eye and touch to pos of battery,take the neg of voltmeter and hook to neg of battery.touch pos lead of meter to one of the 3 eyes if it shows voltage go to next and the next.(it should have the same voltage on all 3)
now take the trio off the battery and turn it around,hook the pos lead to the single eye and touch the 3 eyes to the pos of battery,one at a time and check for voltage.they should all be zero if any of them show voltage whatsoever it is bad. one other thing to note.it has been 20 yrs since i have done this so my volt readings could be backwards so test it both ways but in the end you just want voltage going thru the diode in one direction.
also to test rectifier is the same but you need to pull the separate tangs away from the 3 studs to test those 6 diode,for this i usually left rectifier in the case and touch the pos bat post on back of alternator to the battery terminals to test one direction and then touch to the case to test other direction.it's kindof a pain so if trio is good i would just replace the rectifier and be done with it.
also while you have the brushholder out press the brushes in and hold them in with a toothpick before putting back in,there are 2 holes for this,then after you get the alt back together just pull the toothpick out the back.

cherokeepeople

also if you have an older alternator with the separate diodes they test the same way,BUT they are marked + or - on the back,so you test all the positive ones then reverse your connections and test all neg ones.GM made the easiest alternators to work on and replace parts.so please if it's the original fix it DO NOT replace it.all the reman unit's they take and grind off the amp rating so you have no idea what the heck your getting.

plonka

That is what I meant. Thanks for clarifying.

plonka

Here is a good tutorial on Delco Remy 10SI alternators or ones similar. I ordered some new parts for my alt today, brushes, diode trio, resistor etc. The cost was low and I'll get the satisfaction of repairing my alternator. I'll let you know how it went. As always, thanks to all for chiming in on the topic. Cheers!

cherokeepeople

that is the newer style alternator totally different animal.also i hope you got bearings too.the front is a 203ff the rear is an mnj 471 i think.

plonka

That alternator in the video is the same as mine. A Delco Remy 10SI. They became standard on GM vehicles by 72, and were used up to 88. I've already torn mine apart, cleaned it, and diagnosed the problem. It was the diode trio. The one on the far right was bad.

cherokeepeople

cool.thought the trio controlled the light been so long since i had one apart.didn't check out video,just saw the pic there and that is the newer alternator,clicked on video and yup thats the unit

plonka

Got the parts, alternator is back in and no issues at all. Satisfaction!