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voltage drops

Posted by joe_mn (My Page) on
Wed, Mar 31, 10 at 11:04

96 seville. thought i had fixed the erratic voltage issue on car. put on new ground strap for battery. cleaned bolt for engine ground. cleaned chassis ground point. voltage seemed steady for 2 months. now it is dropping again. will stay at 14.3 at start. drive a mile and it goes to 13. than drops to 11.2 and slowly will go to 11.1. usually will jump back to 13 and repeat. checked voltage drop thru alternator case. ok there. this is 2nd alternator. acts the same as prior unit. what was the opinion before? lost thread info so i don't recall.


Follow-Up Postings:

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RE: voltage drops

is alt. belt tight enough.how old is the battery?


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RE: voltage drops

Most GM alternators that I am familar with have a voltage regulator built into the alternator, and uses a "voltage sense" wire for regulator control. This sense lead is a small wire running from the positive battery terminal to the alternator, thus, the regulator can "read" the voltage at the battery terminal and ignore voltage drops in the high current leads. This circuit must be in good condition. The exciter (rotor) current may come from this wire and not the big current wire.

One place where the sense wire circuit can become intermittent is at the lead connector to the battery terminal. If the postive battery cable is old and there seems to be some corrosion where the sense lead enters the connector, it may be suspect.

On rare occasion, copper wires cast into the lead connector may loose circuit, or be intermittent, but seem secure in the connector.

It is possible that a failing regulator becomes intermittent as it heats up. A typical behavior is for it to work from a cold start of the engine, and then becomes 'flaky' as it warms up. However, you have experienced the same behavior with two different alternators which leads me to look at other causes. (You did say that both alternators were known good ones?)


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RE: voltage drops

alt #2 just died. dash reads 11.2v, its a digital dash. battery reads a few tenths higher. took it apart, one brush was broke and wore a 1/8" deep groove in commutator. got another used unit for $20 at the local pull it yard. had it tested. dead. no returns. took it apart. lots of crud on commutator. put in new brushes and cleaned commutator. still will not work. alt #2 sorta worked and than died. #3 is doa so i am going to have it looked at by a shop that rebuilds alts. guy said they can do an inspection and than replace what is worn. at least the brushes/holder is new


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RE: voltage drops

i get no idiot lights even though displays says 11.1V seems to me this sophisticated caddy would tell me something is wrong. when i finally got down to 10.8 the message center said "service charging system". no kidding. you think something is wrong? i can run the car now and dash says 11.1, battery reads 11.4 with vom and yet i get no warning lights.


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RE: voltage drops

**alt #2 just died. dash reads 11.2v, its a digital dash. battery reads a few tenths higher. took it apart, one brush was broke and wore a 1/8" deep groove in commutator. got another used unit for $20 at the local pull it yard. had it tested. dead. no returns. took it apart. lots of crud on commutator. put in new brushes and cleaned commutator. still will not work. alt #2 sorta worked and than died. #3 is doa so i am going to have it looked at by a shop that rebuilds alts. guy said they can do an inspection and than replace what is worn. at least the brushes/holder is new**

For pete's sake why are you scr*wing around with this so much? Take it in for a proper diagnosis and repair. If you're going to guess and throw parts at it, at least spend the $100 for a good rebuilt or $200 for brand new from NAPA or some place. jmo


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RE: voltage drops

trying to save money. yes its a good idea to not waste time also. i think of this site as an ongoing blog. i just like to tinker with stuff. anyone else would take it to a garage and spend 500 for them to put on a new alternator. i am not like that. i have always worked on cars and will probably never stop.


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RE: voltage drops

Me too. But I wouldn't expect anything electrical to work out of a 14 year old wreck in a junk yard. Throwing money away there.


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RE: voltage drops

Wht not go to your favorite auto parts store and buy a re-manufactured alternator, and be done with it. I suspect those salvage yard units you've been getting have problems other than worn or hung-up slip ring brushes. Blown diodes are common failures. If the yard can't test the alternator for fuctionality, pass on it.

If you buy a rebuilt alternator, check the pulley (sheave) size before you turn in your core. One time, I refused 2 alternators out of the box at the store because of wrong pulley sizes.


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RE: voltage drops

put on good alternator and still no voltage. i checked 2 pin connector on alt input and got no reading with ignition on. i took apart connector and 1 of the wires snapped off with a gentle tug. got new connector and checked voltage at pins and got something. plugged into alt and now it works. 14.4v at idle. i checked a deville in boneyard and the connector is identical but only uses 1 wire. this is a 4 pin connector but i only use 2 middle pins on the seville.


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RE: voltage drops

Glad you got it figured out.


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