Alternator charging issue - The 1947 - Present Chevrolet & GMC Truck Message Board Network

muncher · 06-17-2016, 12:25 AM

Hi All. I have been enjoying many trouble free miles until last week.
I noticed the battery/charge gauge reading on the dash went from 13ish down to 9ish. So when I got home I removed the alternator opened her up and saw the brushes were almost worn down. I ordered a new set and a new voltage regulator at the same time. Put it all back together and still not charging, same as before.
I don't have a voltage reader to check the charge rate across the battery, buying one tomorrow. I have just been using the gauge as a guide.
Anyone out there ideas on what I can check next? Stumped me a bit this one.
Its a 1987 GMC Sierra 305 tbi engine.
Cheers

mac · 06-17-2016, 07:58 AM

Might be a brush not seated on the ring or an open rotor winding. You'll need an ohmmeter to ck tho.

muncher · 06-19-2016, 03:05 AM

Thanks for the reply. I bought a multi meter yesterday. Should have bought one ages ago, good bit of kit. Well with the multi meter I could confirm there was no charge coming from the alternator. was also able to confirm the wiring from alt to battery was good as was the earth. So I opened up the alternator and performed a diode test on the diodes. I found that on the diode trio two of the three contacts didn't work. Also one of the diodes in the rectifier was faulty. Hopefully I can order these parts here in NZ and get my alternator fixed. I bet some of you are saying just chuck it and get a new one haha. But I like to fix rather than bin it.

homemade87 · 06-19-2016, 07:33 AM

Quote:

Originally Posted by muncher

Thanks for the reply. I bought a multi meter yesterday. Should have bought one ages ago, good bit of kit. Well with the multi meter I could confirm there was no charge coming from the alternator. was also able to confirm the wiring from alt to battery was good as was the earth. So I opened up the alternator and performed a diode test on the diodes. I found that on the diode trio two of the three contacts didn't work. Also one of the diodes in the rectifier was faulty. Hopefully I can order these parts here in NZ and get my alternator fixed. I bet some of you are saying just chuck it and get a new one haha. But I like to fix rather than bin it.

Sounds like your on the right track with the diodes . It has been so many years since I bought a alt . I am like you , I always pulled it apart and fixed it . You can get bearings for them also .

**hatzie** · 06-20-2016, 10:10 AM

I'd refresh it with bearings and brushes while you're in there and give it a good clean-up.
New Diode Trio packs and Rectifiers are available for the SI series alternators. There's a lot of garbage out there. Be real careful of the Asian stuff.

Look in the 1973-1991 manuals thread linked below. The earlier GM service manuals have decent sections on the SI alternators. If you page down you'll find you can also download the 16xxxx Charging and Starting textbook and the 19xxxx truck charging systems manual/textbook.

muncher · 06-25-2016, 01:01 AM

Guys thanks for the info. I received the rectifier and diode trio the other day. Put it all back together and result. All works as it should. Total cost was way below a replacement alternator.
If anyone else has charging issues and the bushes and the voltage regulator don't solve your problem then do a diode test, I bet one or more of your diodes will be stuffed.

06-17-2016, 12:25 AM	#1
muncher Registered User Join Date: Nov 2014 Location: Tauranga New Zealand Posts: 47	Alternator charging issue Hi All. I have been enjoying many trouble free miles until last week. I noticed the battery/charge gauge reading on the dash went from 13ish down to 9ish. So when I got home I removed the alternator opened her up and saw the brushes were almost worn down. I ordered a new set and a new voltage regulator at the same time. Put it all back together and still not charging, same as before. I don't have a voltage reader to check the charge rate across the battery, buying one tomorrow. I have just been using the gauge as a guide. Anyone out there ideas on what I can check next? Stumped me a bit this one. Its a 1987 GMC Sierra 305 tbi engine. Cheers

06-17-2016, 07:58 AM	#2
mac Active Member Join Date: Jan 2005 Location: Spokane WA Posts: 166	Re: Alternator charging issue Might be a brush not seated on the ring or an open rotor winding. You'll need an ohmmeter to ck tho.

06-19-2016, 03:05 AM	#3
muncher Registered User Join Date: Nov 2014 Location: Tauranga New Zealand Posts: 47	Re: Alternator charging issue Thanks for the reply. I bought a multi meter yesterday. Should have bought one ages ago, good bit of kit. Well with the multi meter I could confirm there was no charge coming from the alternator. was also able to confirm the wiring from alt to battery was good as was the earth. So I opened up the alternator and performed a diode test on the diodes. I found that on the diode trio two of the three contacts didn't work. Also one of the diodes in the rectifier was faulty. Hopefully I can order these parts here in NZ and get my alternator fixed. I bet some of you are saying just chuck it and get a new one haha. But I like to fix rather than bin it.

06-20-2016, 10:10 AM	#5
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Alternator charging issue I'd refresh it with bearings and brushes while you're in there and give it a good clean-up. New Diode Trio packs and Rectifiers are available for the SI series alternators. There's a lot of garbage out there. Be real careful of the Asian stuff. Look in the 1973-1991 manuals thread linked below. The earlier GM service manuals have decent sections on the SI alternators. If you page down you'll find you can also download the 16xxxx Charging and Starting textbook and the 19xxxx truck charging systems manual/textbook. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 06-20-2016 at 10:17 AM.

06-25-2016, 01:01 AM	#6
muncher Registered User Join Date: Nov 2014 Location: Tauranga New Zealand Posts: 47	Re: Alternator charging issue Guys thanks for the info. I received the rectifier and diode trio the other day. Put it all back together and result. All works as it should. Total cost was way below a replacement alternator. If anyone else has charging issues and the bushes and the voltage regulator don't solve your problem then do a diode test, I bet one or more of your diodes will be stuffed.



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