Alternator undercharging?

[riderdan]

Be sure to check the alternator after you get the new batteries in. 14.5 volts is not what you should be getting out of it... it should be right at the upper edge of 28 volts if you have a good meter.

I suspect if you kill a second set of batteries they won't give you a replacement set free, so be sure to verify your alternator voltage again.

 

[Action]

Be sure to check the alternator after you get the new batteries in. 14.5 volts is not what you should be getting out of it... it should be right at the upper edge of 28 volts if you have a good meter.

I suspect if you kill a second set of batteries they won't give you a replacement set free, so be sure to verify your alternator voltage again.

If it is a 12v alternator, and he test the output with new batteries in, wouldn't it show around the 24 v - 28v ? and then slowly drop after use until the battery pair got down to the 14v?

 

[orgnal]

If it is a 12v alternator, and he test the output with new batteries in, wouldn't it show around the 24 v - 28v ? and then slowly drop after use until the battery pair got down to the 14v?

Yes

McReddy

 

[kc5mzd]

Once both batteries are charged and "topped off" install them. Measure the voltage before turning anything on. It should be around 24-25 volts. Then start the engine turn the headlights and heater fan on. If the voltage is less than the first reading you still have a problem. If it is higher you are probably going to be fine. If you still have a problem it sounds like the first thing to check would be the alternator. I think you will be ok.

 

[donquijote]

So the last couple days i have been able to get the batteries back to 12v+ each and here are the numbers after letting the engine idle for 10 min.

ENGINE RUNNING (NO HEAT)
Alternator - 25.02v
Batteries - 25.04v

ENGINE RUNNING (HEAT ON)
Alternator 24.73v
Batteries - 24.80v

Weird how the alternator is back up to 25v after reading 14v the other day? The battery gauge stayed toward the lower yellow mark.

 

[juanprado]

So the last couple days i have been able to get the batteries back to 12v+ each and here are the numbers after letting the engine idle for 10 min.

ENGINE RUNNING (NO HEAT)
Alternator - 25.02v
Batteries - 25.04v

ENGINE RUNNING (HEAT ON)
Alternator 24.73v
Batteries - 24.80v

Weird how the alternator is back up to 25v after reading 14v the other day? The battery gauge stayed toward the lower yellow mark.

looks like batteries are ok. Did you load test?

25v too low for alt, so assuming batts are good, alt is suspect or the internal regulator.

 

[donquijote]

load tested each one and they look fine.

 

[Action]

......

Weird how the alternator is back up to 25v after reading 14v the other day? The battery gauge stayed toward the lower yellow mark.

This is exactly what i was stating. Even if it is a 12v alternator, you will get a reading of closer to 24v. Because you charged thd batteries.
Drive it every day for a couple weeks and see what the new reading are.
.

 

[Bulldogger]

Some auto parts stores will test an alternator for you. Yes, it's a heavy beast to pull out, but I agree with those who point at the alternator as the culprit. Maybe it just needs adjusted (I cannot find online references to whether the Delco 21SI you've got has an adjustable regulator, the MIL alternator does).
Your posted results in running/non for voltage look like an alternator problem. 27-28V is recommended by the manual.
Bulldogger

 

[kc5mzd]

Battery voltage will drop under load and then go back up after the load is removed. There is also a surface charge that will build over a short time ( over night) when a battery sits with no load. It is a sign that your batteries are probably in great condition. The only 2 other things I can think of is a bad alternator and or a bad ground. I would try running a temporary wire from a good ground on the alternator to the - of the batteries. It probably won't make a difference but will help rule out a bad ground and is easy to do.
At this point I am guessing it is something in the alternator. Remember to never run a vehicle with no battery or the battery disconnected. It can fry the diode pack - sometimes instantly.

 

[Action]

Have you checked voltage after driving some during the past week?

 

[donquijote]

Have you checked voltage after driving some during the past week?

I picked up the same alternator which oddly had a different model number than the 1st one. Everything was the same except the positive cable stud on the back was bigger. The new Delco reads 27.32v and is running normal. I also bought a new 24v regulator for the 1st delco that was showing 24v, and will be replacing it tomorrow and putting that alternator back in the truck to see what it runs at. Having the alternator mod for the delco makes it an easy 5 min swap. I'll report back on what I find when I replace the regulator in the 1st Delco.

In the picture above, the top alternator with pulley is the one that was reading 14v.



The 1st delco that seems bad does in fact say 24v on the side. So it def. was a 24v alternator....