I'm banging my head trying to figure this one out ..,.

I purchased my 1991 Convertible this spring. It's a great driver but needs a little TLC. The car was running and driving great till recently. I pulled the passenger door panel off to replace the outer window seal.

After this (probably just a coincidence) the battery seemed to drain very quickly. I took the battery out and had it tested. They said it was too low to test social left it overnight to slow charge. When I picked it up the next evening it tested perfect. I took the battery home and installed and drove it around a bit.

The following morning the car would not start. I have to now jump it each time I drive. When running voltage is 14.3. I drove over to two different auto stores today and just out of curiosity to have the alternator tested. Both said bad diodes.

The alternator was replaced August 2020 with a Duralast Gold from the dealership. Could it simply be a bad alternator or sound like I have a drain somewhere?

 

With bad diodes, the alternator itself will drain the battery when not running/charging. The purpose of the diodes is to prevent current flow from battery voltage to ground.

On a related note, I just replaced my own alternator yesterday with a new Remy 91313 from Rock Auto: https://www.rockauto.com/en/moreinfo...pt=2412&jsn=14

 

You can fairly easily test for a current drain. There are numerous threads on the CF or search with Google, and you can test for bad diodes too, just to confirm that.

 

-Correct, alternator with bad diodes will drain the battery.

 

Duralast gold should be a lifetime warranty.

I bought a brand new one (DL Gold) from autozone back in 2011 and it lasted a week. Got a warranty replacement and it’s been fine ever since.

 

eh

The diodes purpose is actually to convert AC power into DC power. Also, AC power looks like a ground to any DC circuit- that is what used to light the indicator lamp on the dash of vehicles before computer control alternator -> Diodes go bad, DC can 'see' the AC, indicator light is wired via dash circuit this way, thus the AC acts as a ground for the indicator bulb in those vehicles

 

On another note- why would a GM dealership put in an autozone alternator?

or did you mean a non GM dealership where you bought the car?

Regardless. They’re cheap and easiest thing to change on a c4.

 

Thumbs down for the new Remy alternator that I posted above. It seized up on me after about 5 miles of normal driving and snapped my two-sided Gates belt. It didn't take much effort to get it to spin freely again, but it is not charging now with a new belt. I was happy with the belt alignment and tension, but it failed nonetheless. My installation is unique, but it has been fairly reliable for 12 plus years with the same setup. Never had one fail so quickly before. Battery was fully charged beforehand and the new alt was not subjected to extreme heat, high rpm or high load conditions.

 

If you have a DVOM, you can test the diodes in your vehicles alternator without removing the alternator from the car.


Set up DVOM for 0-20vDC (preferred) or auto range feature--connect black lead to ground, red lead to the large "red" wire at the back of the alternator (best) or battery positive.
Start engine (run about 1500 RPM) and monitor charging voltage-- should be in 13.5-15.5 vDC range --turn the head lights on (load on charging system)--voltage should drop quickly and then return very, very close to the voltage before placing the load on the charging system. (verifies proper operation of voltage regulator in charging system/alternator)

To test the diodes---run engine at 1500 RPM, no load on engine (lead set up as stated above)
Turn function dial on DVOM to AC volts-the DVOM should read in the 0.03 to 0.10 vAC range. (load off) (See photo as an example). It may take 30 seconds to a couple of minutes for the AC reading to “settle down”, especially if using the ‘auto ranging’ feature of your DVOM.

The reading displayed is the alternator “ripple” voltage. Look for stable/steady readings.

Test with a “load on” as described above. Allow another 30 seconds to couple of minutes for reading/s to settle down. The DVOM reading should increase as higher “load” on alternator.

Naturally, if the battery is low on charge the alternator ‘ripple’ voltage will increase as the alternator is under load.

If the alternator has defective diode/s—AC voltage will be much higher than this reading and never “settle down” (jumping very low to very high—load or no load)

For “practice” would suggest testing on a known “good vehicle” to verify settings and results on your DVOM as each operates slightly different.

 

I'm having a battery drain problem in my '84. I found, after much troubleshooting, the starter solenoid is stuck in the 'closed' position but with the battery drained, the starter could not turn. I measured the resistance between the battery cables (yes, disconnected from battery) at zero ohms, a dead short. Whapping the solenoid with a hammer broke the contacts free and resistance jumped up as the contacts opened. This was not a total solution, it just proved in my situation, the solenoid has an issue; the contacts are stuck in closed position. I'll remove the starter and replace or maybe just replace the solenoid.

Note: The "dead short" is actually the coil resistance within the starter motor, a very low value but with the battery discharged, the motor could not turn. The closed contacts of the solenoid are because (1) the contacts are 'dirty' due to arching or (2) a failure in the solenoid coil(s).

In the troubleshooting process, I pulled the fuses one at a time noting if the short was removed. That would isolate which fused circuit was shorted. The short was not removed by pulling the fuses so I then yanked the fusible links from the + battery cable lug. Their connectors are easily accessed and separated (with the battery removed). Again, the short was not removed. I isolated the alternator by removing the B+ cable and the connector from the internal diode pack. The short remained so that left the solenoid as the only remaining circuit directly connected to the battery (through the ignition switch, antitheft relay and fusible link at the solenoid). When I tapped the solenoid with a hammer, the short opened.