Alternator Diode Question
Thread Starter
Intermediate
Joined: Jan 2007
Posts: 36
Likes: 0
Alternator Diode Question
I have an ,80 L48, the battery light started flickering then went solid on. I checked connections and belt then took it to the auto parts store to have the alt checked. They said a diode was bad. After the testing, on the drive home, the light was off and the voltmeter was at ~14V( before it was at 13V and would only rise slightly with ^rpm,s).
Question; Was their test wrong or can the diode work intermittantly or is it something else altogether?
Question; Was their test wrong or can the diode work intermittantly or is it something else altogether?
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
When the diode trio goes bad they are bad they are basically a one way check valve only flowing current one way. It will cause a dimmly lit light What it sounds like to me is your internal voltage regulater is going bad. All are readily available at the local parts stores. You can overhaul our alternators for around $20 worth of parts. Very simple to repair.
Thread Starter
Intermediate
Joined: Jan 2007
Posts: 36
Likes: 0
Ihave an article from Corvette Enthusiest on how to replace diodes/reg/brushes. $20 sounds better than $80. The alt looks old, are the windings still good or am i better off w/ a quality reman? I plan to drive this car alot and out-of-state cruises.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
IMO, find yourself a quality reman unit....or a SI17 unit which is 108 amps off a '79 seville, slightly larger diameter but almost twice the available output....The repair kits are almost useless, as they do not cover the main power diodes in most of them that I have seen....the diode 'trio' runs the regulator but not the main power output to the 12 volt stud....
another upgrade is a CS144 unit, but some wiring needs changing...no biggie....
another upgrade is a CS144 unit, but some wiring needs changing...no biggie....
Le Mans Master
Joined: Mar 2004
Posts: 7,449
Likes: 1,477
From: Little Rock AR
I have fixed a few of those myself
The diode trio looks like a computer chip and is there for the warning light circuit. If the alternator output is good but the dash warning light is "on" then you need to take the alternator apart and test the diode trio. This is easily done with a common multi-meter. Set the meter to Ohms and check for continuity on all 3 of the leads, one at a time. Measure for resistance in one direction on all 3 then reverse the red and black probes and do that test again with the test leads reversed. You are looking for continuity in one direction only on all 3 leads. If any one of the 3 leads has continuity in both directions, you need a new diode trio. This problem will show up as a warning light even if the output is still good.
You can also check the main diode rectifier bridge the same way. It looks like an aluminum heat sink with 3 threaded studs on it. Check the same way as the diode trio looking for continuity between the individual studs and the aluminum heat sink. Then check again in the other direction.
Hope this helps,
-Mark.
You can also check the main diode rectifier bridge the same way. It looks like an aluminum heat sink with 3 threaded studs on it. Check the same way as the diode trio looking for continuity between the individual studs and the aluminum heat sink. Then check again in the other direction.
Hope this helps,
-Mark.
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Intermediate
Joined: Aug 2007
Posts: 38
Likes: 0
From: dallas ga
Diodes are either bad or not bad, never intermittent. I have seen rectifier bridges, and diode trios have one bad diode and they will drain your battery. If the dash light is coming on it usually is the regulator, or the brushes.
