Good afternoon everyone, I am new to this website, but not new to forums, I've had a forum before for the exact truck I am working on, but I cannot for the life of me remember the login for that account. On a different note, I am on here because the blue truck that I am working on is a 1993 Chevrolet C1500, 5.7L, 4L60E, extended cab. Recently my buddy drove the truck and severely overheated the engine causing it to send coolant into the oil and damaging almost everything. I wasn't about to take it apart and machine it and then have the vehicle down for an extended period of time, so I found one on the Facebook Marketplace, completely went through it, disassembled all the way down to bare block, rebearinged everything, reringed the pistons, cleaned up the mating surfaces, and then got it running. The engine was out of a 1986 Chevrolet something, according to the block stamp. My buddy and I got it working with great oil pressure, cooling just fine, charging just fine, but then I was driving it to work, 25 miles away, and I was watching the charge gauge fluctuate back and forth back and forth. The truck would charge then drop to 9 volts or so, then return back to charge. At that time, I thought the voltage regulator was bad, so I took the alternator out and had it tested, sure enough it was not charging, luckily it was under warranty so they exchanged it out for free. I brought it back home put it in and the truck still was not charging. I checked all the grounds and found good continuity between all points; I ended up just replacing all the ground straps with brand new ground wires of thicker gauge wire. I started the truck up after replacing them and still nothing. I drove the truck to the auto parts store where they tested the battery, starting and charging system. The results were as follows: Battery needs charging, Starter failed, Alternator voltage regulator failed. The auto part store I was at didn't have the parts so they sent me to the closest one that did, but they had to test the parts before they could exchange for the warranty. I pulled both the alternator and starter; they tested each one outside the vehicle with good results on both tests. The parts store wouldn't replace either one because they tested just fine. I put both back into the vehicle and drove it home. On the way home the voltage was continuously fluctuating on the dash throwing the battery light on over and over and over again. This thing has really stumped me. I went as far as disconnecting the main harness at each point that the battery sends voltage to so I could make sure it's passing through correctly and it is, I printed off the wiring diagram from the Chevrolet tech service manual and ran down each place that gets battery voltage and everything is good. Our thought was maybe the instrument cluster diode was bad, so we got a used one from a pull your own part salvage yard, plugged it in, and still no charging. I need some help because both my buddy and I are at a loss for figuring it out. Here is a list of parts that are all brand new: 1) battery 2) negative and positive battery cables 3) all ground straps 4) dash cluster 5) alternator. Any help would be greatly appreciated.

 

I'm not sure how to link other forums that I've started but my other username is Locorotc, but I haven't got access to that email anymore so here I am with the new content.

 

I forgot to put in the original post, I took the entire alternator bracket off and cleaned up the backside to ensure that it had good continuity to the engine block, as well as cleaned up all the connections on the block itself. We tried everything and still have no success in finding why the alternator will not charge. I also forgot to put in here hat when the engine is cold, the gauge shows that its putting out 14 volts, but then as soon as the engine begins to warm up the charging system decides to take a crap and quits charging, voltage drops down to 10 volts and the charge light comes on but will not charge regardless of what we do.

 

It's very possible your new alternator is faulty - it may test ok on the bench but when warm and under load in your truck it fails.

 

Thats what I had thought, when I test it in the vehicle with it running and with a voltmeter its fluctuating like what's on the dash, but the auto parts store still wont exchange it because when they bench test it, everything reads out just fine. I do know that we bought a remanufactured one when we put it in, but again, it still tests just fine on their bench tester. Would you recommend getting a brand new one and putting it in to see if it fixes the issue, or no? I also know that they ask if we have the OEM voltage alternator or the higher amperage one, could it be that the engine from the 1986 requires the higher amperage alternator?

 

See if they will test in in the vehicle when it's having the problem.
This is an 86 block; I assume you are using the '93 computer (ecu)?

 

Yes sir it is the 93ecu, but from the wiring diagram, the ecu has no connection to the charging system. Not unless I am mistaken.

 

Yea, Ihave a 93 Suburban 5.7, I would agree the alternator charging circuit does not seem to rely on the ECU. That's why I think you have a bad alternator. Did you get one for a 93 oran 86?

 

I have the alternator for the 93, the 86 is a different amperage/ less, as well as a total different bracketing due to being carbureted.

 

I think your alternator is bad