Voltage drop as engine warms up

I'm 99% sure it's the alternator, but before I drop the cash on a replacement, I wanted to see if there are any other logical explanations. 

LQ4(LS) swap in a S10.  When I start up, voltage is about 13.5-13.7.  As the truck warms up, it starts dropping.  The lowest I have seen it go is around 12.2, I think.  Does go up a bit with revs, but not a ton.  I am assuming I have an alternator that is already marginal, but the excess heat due to the small engine bay is making it worse.  I've looked around, and as far as I can tell, I don't have any bad wiring.  Anything else I should look at before I give in and buy the alternator?

Is the ecu controlling the alternator?  I notice on my 2015 silverado that it will drop voltage sometimes.  Always has done that and doesn't effect anything (so far).

Other thought is that if you have a huge fan / fans and they are kicking on, you don't have enough oomph to keep up.

In reply to wvumtnbkr :

Thank you.  You sent me down a rabbit hole with the ecu controlled alternator.  As far as I can tell, mine probably isn't(it's a 2001), but I am going to keep digging, just in case it has a more primitive method of output control than the later GM ECMs/BCMs.

I am using an electric fan, but this is happening even when it doesn't run.  The donor truck was optioned with a heavy duty alternator, too, so I really hope any accessories would not bring it down.

I had the stator winding coatings dying on a VFR800 once that act like this.  Good power output when cold, dropped slowly as the engine heated up.  That said, I doubt it's exactly the same issue, and probably more like a dying regulator/rectifier section of the alternator being affected by heat in a similar way.  

Your rpm also drop as it warms up, so maybe that's all there is to it? Maybe 12V is all she's got at hot idle, but the higher cold idle generates more volts

In reply to cyow5 :

It happens whole driving, too, not just idling.  When the RPMs go up, voltage is going up, but usually only a few tenths.

Does the battery stay charged? Sounds like it could just be typical smart alternator behavior. 

I had the exact same thing happen in Midlana. A Chevy truck alternator was used so that it was easy to source, and it did what you're finding. I concluded that it's most likely due to a poorly designed (cheap to make) voltage regulator. It could also be that it's that way on purpose, but I don't know what the benefit would be. I changed things around so that it referenced the voltage right at the battery instead of its output terminal and that got rid of some of the voltage drop. As an experiment, you might try running a small cool air duct to the backside of it, where the air gets sucked in, to see if that cures it. 

This is often intentional. Batteries can take more charging voltage when cold than when hot.

At least take it off and have it tested. If it tests out ok, triple check your battery ground for excessive resistance. Electrical is 99% ground related for problems IME.

If it's a stock PCM, it's not controlling the alternator and it's not smart. I had a similar thing happen on my LQ9 swapped trans am. My issue was just a poorly chosen reference voltage for the alt. Moved it to the battery terminal and it jumped up to 13v+ even at idle.

I've only driven it a bit, so I don't know if it would eventually drain the battery.  Since two people have mentioned reference voltage, I'll have to take a look into that.  As far as I know, I've done a pretty good job with grounds, but it could be an issue, too.

It is the original 2001 PCM.

Took it for a drive this morning, so a few new data points.  This drive was much longer than the last one, about half an hour, but similar conditions, other than the ambient temperature being cooler.  Voltage did not drop anywhere near as fast as before, and even then, it only dropped to 12.4-12.5 at stoplights during the last few minutes of the drive.  Oddly, it started a bit lower, around 13.2-13.3.  In my driveway, voltage dropped to around 12.3 when the radiator fan kicked on, but went back up to 12.4-12.5 fairly quickly.

Once I got home, I pulled out the multimeter.  With the engine running, measured voltage between the positive terminal of the battery and the alternator case was within 0.1 V of the OBD2 reading.  After I shut the engine off, I remembered to check the resistance.  Between the negative battery terminal and the alternator case, it was 1.2 Ohms. 

Not sure what this means for diagnosis yet, other than underhood temps are likely affecting it.  Just had a thought, and checked the temperature of the alternator with an infrared thermometer - 153F, about 10 minutes after shutting down the truck.  Hood is up right now, I'll recheck the values in a bit when I pull it in the garage to start some work on it.

Between the negative battery terminal and the alternator case, it was 1.2 Ohms. 

I don't like that number but I don't know what it should be.  Anyone?  My guess-ish number would be 0.5 ohm as an ideal upper limit.  Maybe this isn't so critical so input from others who have checked this on different vehicles would be nice.

I found this as a nice little walk-through with some general specs for some checks to do with a DVOM and the engine running; Some on-engine voltage checks for alt. performance.

If that's not a particularly high end multimeter, check the resistance when you tap the probes together. I've seen that measure more than 1.2 ohms on low end meters.

Went back, and rechecked with the truck cool and in the garage.

Resistances

• Multimeter lead to multimeter lead:  0.2-0.3 Ohms (odd, since the next reading is lower)
• Battery negative cable from one end (at the battery) to the other (bolted to the passenger cylinder head):  0.1-0.2 Ohms
• Battery negative to alternator casing:  0.4-0.5 Ohms.  Don't know if I was not holding it"right" before or what, to get a different reading.

I'm guessing this means grounding is probably okay?  It is the thick cable off the battery that connects to the cylinder head, and a smaller one that connects to the chassis.

I'll have to go through that PDF once I have the truck running again, I have a few things torn apart working on it.

When the vehicle is warmed up and the charging voltage drops try tapping on the alternator case with a hammer.  If the voltage jumps up, the brushes in the alternator are probably bad.

Probable resolution, and I may be an idiot.  Started pulling the alternator to have it tested, and when I put the wrench on the positive post, it turned way too easily.  I had checked by hand when I was trying to diagnose it, and it was tighter than I could turn, so I assumed it was good to go.  Snugged it up, fired up the truck (breaking my rule about not starting a vehicle on jackstands), and was presented with this:

It's still no guarantee there is not a drop when it heats up, so I'll still need to do some test driving, but I guess as long as it stays above 13V on the drive, it should be okay for now.

You had an ID-10-T error.

I had a loose terminal wire-EDM its way through most of the alternator output bolt on my AE92...

Glad you found your source.

To nitpick though, you can't check electrical faults with an ohmmeter.  A single strand of wire will read 0 ohms but won't pass any meaningful current.  One needs to check anything that passes current with a voltage drop test on the circuit while it is loaded.  In this case you'd put one voltmeter lead on, say, the alternator case, and the other on the negative terminal of the battery.  Should be under a tenth of a volt with the engine running.  If it's more then you divide and conquer, narrowing down where the voltage drop lies.  If you see, say, half a volt between the negative terminal of the cable and the negative battery post, that's where your resistance is and you need to clean and tighten the terminal, for example.