desertmike1

The Alternator produce's A/C power, this is converted through the Silicone/Diode rectifier assembly to D/C power, and then regulated to the desirable output voltage..

So it is possible the OP has had three bad regulator's! but still..

drive it

A friend of mine just went thru your same problem.
Turned out it was a bad Positive connection with high resistance intermittently at the terminal block-follow the positive from the alt back to it.
It kept burning out alt and batteries and showed the same type of voltage fluctuations.
So a logical place to start looking.....

volkswagens-for-life

Ohhh I see! So what other options do manufacturers have other than diodes in order to produce dc?

desertmike1

the other option would be an DC generator, they are nearly bullit proof, and widely used on commercial aircraft. the down side is they produce very little power at idle engine speed, and were abandoned in the 50's on most every automobile.

volkswagens-for-life

Interesting. So why is it that only c4 corvettes have issues w/ the diodes blowing then? Dont really hear much about that in other makes/models of cars.

sgtskid

I'll check that and see what I come up with. The holidays, travel, and work have kept me from looking at the vette anymore. Poor thing has been sitting in the garage all alone.

AGENT 86

Have you tested the field and sensing wiring to alternator ?
If the field wire or sensing wire had an intermittent open condition, the alternator will stop producing power.

Do you have a service manual ?

aDigitalPhantom

The number of diodes, and other parts will depend on the quality of DC power that is output. It will also depend on how things are wired.

These links require JAVA
Half-Wave Rectifier
Full-Wave Rectifier

desertmike1

very cool link..

A Quality Tester will look at the Wave Form, and determine if Diodes are failing..

Once again, they don't work one day and not the next, they are either good or bad.. I personally would be taking a nice hard look at the "field" wiring, this could be a simple spot of corrosion under a large crimp connector, or a bad chassis ground..

sgtskid

Unfortunately I do not have a fsm. I have a Haynes "service" manual. Is this the one? http://www.amazon.com/gp/aw/d/B000ELOWSQ/ref=mp_s_a_1_1?qid=1418092020&sr=8-1&pi=AC_SY200_QL40

89dd

I also got 3 bad alternators from the Twilight Zone for my 77 Jimmy.
I try not to buy remans anymore.alts, starters,pumps ect. There a crap shoot and just not worth it.
Bite the bullit and order a new Powermaster from Jegs or Summit.

Tom400CFI

^These. I would be very carefully inspecting the wiring. 0.5v drop from the alt to the battery is too much, for starters.

aDigitalPhantom

What AMP Alternator do you have?


I have a 1 volt drop from the alternator to the battery, and this is with a new battery cable from the alternator to the battery. It was cheaper to buy a 29" 4ga battery cable then it was to buy 29" of 10ga, and ends to connect it. I think the 100AMP alternator I have is too small, but the car always starts.

sgtskid

Finally got some time to work on it again. Bought a fsm abs went through the diagnostics of battery over charging/under charging. Connected volt meter to terminal "l" and ground and got no voltage. All other wires have battery voltage.
Next step is to jumper between "l" and ground and watch the "charge indicator".
so dumb question, what is the charge indicator? Volt gauge or battery light? My message center hasn't worked in who knows how long so if it's on it. ...

Ripe93

I went to O'Rielly's and ordered the HiPo one. $125. When it came in it was dirtier than the one in the car. I went down to my local rebuilder and for $85 he replaced all the guts, extended test, and has been working perfectly for 6 months..

1984Z51auto

I agree completely, if there is a good local electrical rebuilder having a custom rebuild done is the way to go. A good shop can use heavy duty parts and make up an almost fool proof alternator.

8a8mfh

I don't think it's the alternator. Maybe its not doing it job because its not being 'told' to.
It has to reference battery voltage to charge.

sgtskid

I'm Still not convinced it's not my car. Three alternators still seems very suspicious.

gerardvg

Yes for sure some new "SI" 86-91 alternators are garbage, i have come across bad tolerances. Bearings not gripping the shaft, the shaft can move back and forward which means the brushes will not contact the communtator and charging stops. You will notice tapping the alternator with a screwdriver handle will make it charge again.

I got sick of replacing new reconditioned alternators, fitted a powermaster and have not had any issues since that is many years ago the voltage at idle is also so much better.

http://www.ebay.com/itm/251811352420?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Cliff Harris

Because alternators generate 3 phase power. You need 3 positive diodes and 3 negative diodes to get the full output.