Is it really as simple as just getting a smaller pulley for alternator?

[Oldworldtenacity]

So, I purchased an 100amp Delco alternator from 74M35A2 almost two years ago and only got it put on about a year ago. Of course it has to be idled at 1200rpm to produce what it should at 750rpm and I havent used the truck hardly at all this year and now is the time to confront this issue before Winter.

Is it really as simple as just getting a smaller pulley for the alternator to increase voltage to what it should be on 8.3CTA? I have been told it is, but am skeptical.

Can anyone suggest a part number for NAPA/Advanced Auto/or Cummins dealer?

 

[simp5782]

So, I purchased an 100amp Delco alternator from 74M35A2 almost two years ago and only got it put on about a year ago. Of course it has to be idled at 1200rpm to produce what it should at 750rpm and I havent used the truck hardly at all this year and now is the time to confront this issue before Winter.

Is it really as simple as just getting a smaller pulley for the alternator to increase voltage to what it should be on 8.3CTA? I have been told it is, but am skeptical.

Can anyone suggest a part number for NAPA/Advanced Auto/or Cummins dealer?

Smaller pulley has less travel to make a full circle so it spins faster. Much like smaller tires vs big tires on a trailer. Smaller tires have more revolutions per mile than a larger tire.

Measure the outside diameter of your pulley in inches and choose a smaller pulley with the offset and I believe it is a 7/8" keyway.

 

[74M35A2]

It works fine for most, a few see this issue. There are a couple of variables here. Depends on the alternator load. Of course the alternator does not make 100 amps at idle. How old are your batteries? I usually ship them with the smallest pulley available, but the 100 amp Delco units are brushless, and do have lower idle output than a conventional brush type alternator.

Do some battery checks first, but I would be happy to exchange it for a brush type alternator if you’d like. They are 70 amp, but still plenty of output.

Yes on 7/8” shaft diameter, but no keyway. You can use a keyed pulley, just as slip fit, tighten nut with impact. Pulley size can be tough. They don’t make them tiny, because it decreases belt wrap, and it will then slip under load or when wet.

Aged batteries pose a continuous draw on the alternator, and can exceed the output of a brushless unit at idle.

 

[Oldworldtenacity]

hey Clint, The batteries were newly installed the same time as the alternator from NAPA..so no issue there. Everyone is telling me its simply a pulley issue with regards to the voltage output readings.
Voltage readings at my 24 to 12V converter terminals were......

23.6V@low idle/600RPM

24.6V@950RPM

Peak voltage of 26.6V is achieved @1250RPM an doesn't get any higher regardless of RPM.

I'll respond to your pm too.

 

[74M35A2]

Your batteries are severely discharged (equivalent to 11.8V on a 12v battery). Recharge them fully, they need to read close to 25.6v with the truck off. 12.8v per 12v battery represents 100% fully charged, healthy battery. 12.0v represents 80% discharged (20% charge). You are even below that. 26.6v is charging, so it is working, but the state of battery charge is putting a big load on the alternator, and it can not keep up at idle.

Fortunately, these alternators can handle a lot of output continuously, unlike the stock military one. Set the engine RPM to your 1250rpm or so and go to a long lunch. Or just put a regular charger on the batteries overnight, each. See if you can get their voltage up.

A lot of people think as long as batteries start the engine, they are good. That is not the situation at all. Even if the batteries are new, if they sat discharged for some time, it alters their chemistry and accelerates their aging drastically.

I can still send you a brush type alternator to try if you want instead. They do have more idle output, and can bring up discharged batteries at idle better than a brush-less alternator can, if the batteries are able to be brought up. As they deteriorate, they will be unable to reach 25.6v at engine off, and will instead be pulling on the alternator in a never-ending attempt to reach it. This is what kills the stock military alternator, a never-ending load from old batteries at idle when alternator cooling is the least.

 

[Oldworldtenacity]

Your batteries are severely discharged (equivalent to 11.8V on a 12v battery). Recharge them fully, they need to read close to 25.6v with the truck off. 12.8v per 12v battery represents 100% fully charged, healthy battery. 12.0v represents 80% discharged (20% charge). You are even below that. 26.6v is charging, so it is working, but the state of battery charge is putting a big load on the alternator, and it can not keep up at idle.

Fortunately, these alternators can handle a lot of output continuously, unlike the stock military one. Set the engine RPM to your 1250rpm or so and go to a long lunch. Or just put a regular charger on the batteries overnight, each. See if you can get their voltage up.

A lot of people think as long as batteries start the engine, they are good. That is not the situation at all. Even if the batteries are new, if they sat discharged for some time, it alters their chemistry and accelerates their aging drastically.

I can still send you a brush type alternator to try if you want instead. They do have more idle output, and can bring up discharged batteries at idle better than a brush-less alternator can, if the batteries are able to be brought up. As they deteriorate, they will be unable to reach 25.6v at engine off, and will instead be pulling on the alternator in a never-ending attempt to reach it. This is what kills the stock military alternator, a never-ending load from old batteries at idle when alternator cooling is the least.

I wasnt aware that the batteries could be that low, so Im going to check the readings at the individual battery terminals with truck not running tonight. The readings I got were inside of the truck at the 24 to 12V Converter output terminals...so will get back with you tomorrow. Also, I have let the truck run numerous times for at least 30 mins at 1250rpm every couple of months so it will be interesting to see what the readings on the batteries are.

 

[Oldworldtenacity]

Your batteries are severely discharged (equivalent to 11.8V on a 12v battery). Recharge them fully, they need to read close to 25.6v with the truck off. 12.8v per 12v battery represents 100% fully charged, healthy battery. 12.0v represents 80% discharged (20% charge). You are even below that. 26.6v is charging, so it is working, but the state of battery charge is putting a big load on the alternator, and it can not keep up at idle.

Fortunately, these alternators can handle a lot of output continuously, unlike the stock military one. Set the engine RPM to your 1250rpm or so and go to a long lunch. Or just put a regular charger on the batteries overnight, each. See if you can get their voltage up.

A lot of people think as long as batteries start the engine, they are good. That is not the situation at all. Even if the batteries are new, if they sat discharged for some time, it alters their chemistry and accelerates their aging drastically.

I can still send you a brush type alternator to try if you want instead. They do have more idle output, and can bring up discharged batteries at idle better than a brush-less alternator can, if the batteries are able to be brought up. As they deteriorate, they will be unable to reach 25.6v at engine off, and will instead be pulling on the alternator in a never-ending attempt to reach it. This is what kills the stock military alternator, a never-ending load from old batteries at idle when alternator cooling is the least.

Ok, so here were the readings at the battery terminals(not converter in truck).

24.6V with truck off.
24.7V @650rpm(idle)
25.4V@950rpm
27.2V@1250rpm

Interesting that it's not really any different running at idle then turned off I thought.

What do these readings tell you overall though?

 

[Russ Knight]

I ordered one of these and after several uses, it has noticeably perked up the starting of my truck. My batteries had gotten so weak, the fuel shutoff solenoid would not stay energized when starting the truck. Now, the truck starts instantly.

https://smile.amazon.com/gp/product/B004LWTHP2/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

 

[74M35A2]

It shows your batteries are at about 60% state of charge, and the alternator is working. The load into the batteries is more than the alternator makes at idle.

Put a battery charger on each battery overnight to see if you can bring the charge level up.

 

[Oldworldtenacity]

It shows your batteries are at about 60% state of charge, and the alternator is working. The load into the batteries is more than the alternator makes at idle.

Put a battery charger on each battery overnight to see if you can bring the charge level up.

Ok will do. It may not be until the weekend that i can get back to the truck, but I will report back and thanks.

 

[Oldworldtenacity]

It shows your batteries are at about 60% state of charge, and the alternator is working. The load into the batteries is more than the alternator makes at idle.

Put a battery charger on each battery overnight to see if you can bring the charge level up.

So, I finally got a chance to go run the truck on two different days this weekfor about 20 mins each time.

First day I was reading 24.8V resting and on the second I went back they were reading 25.6V and I ran it another 20 mins and when I turned the truck off they were resting at 26.2V?? I was surprised they charged that high, but there you go.

 

[74M35A2]

Excellent news, thank you for following up on this. It is why I stress the age and/or condition of batteries is directly proportional to the energy balance of an alternator's output at idle, volt gauge needle position, as well as the fuel shut off solenoid extending during cranking on the 8.3L. Just because batteries crank a truck, does not mean they are healthy, and they will exhibit other anomalies before finally not being able to crank the engine well.

Looks like your batteries and alternator are well. Congrats. A lot of users have noted better battery health after switching to a modern civilian alternator.

Thanks again for the reach back on this.

 

[Oldworldtenacity]

Excellent news, thank you for following up on this. It is why I stress the age and/or condition of batteries is directly proportional to the energy balance of an alternator's output at idle, volt gauge needle position, as well as the fuel shut off solenoid extending during cranking on the 8.3L. Just because batteries crank a truck, does not mean they are healthy, and they will exhibit other anomalies before finally not being able to crank the engine well.

Looks like your batteries and alternator are well. Congrats. A lot of users have noted better battery health after switching to a modern civilian alternator.

Thanks again for the reach back on this.

Hey, no problem man..I was just glad that they are still functioning properly per your advice. I see what you mean about the batteries being directly proportional to the energy balance of the alternators output because the alternator was reading a higher output with each time I ran the truck and the batteries reached a higher resting output. i thought output would be the same regardless of what the charge the batteries are actually holding, but now know thats not the case.

 

[Russ Knight]

I found a smaller pulley at my local NAPA store. This isn't a NAPA part, said it might be an Arrowhead part number, ADR5055.

 

[Russ Knight]

https://www.ombwarehouse.com/pulley-8-groove-2.html

 

[74M35A2]

That pulley looks same as the one I use and include. I don't think they make one smaller than that (?). Even that one, being as small as it is, may slip or at least squeak if the alternator is under full load, and the drive belt is wet. The smaller the diameter, the less belt wrap you have.


Mostly this post is to say I think that is the same pulley I use (different source), which is the smallest diameter I could find. I don't want a rush of people to order them, only to find out it is the same thing. Measure yours and compare first.

 

[Russ Knight]

That pulley looks same as the one I use and include. I don't think they make one smaller than that (?). Even that one, being as small as it is, may slip or at least squeak if the alternator is under full load, and the drive belt is wet. The smaller the diameter, the less belt wrap you have.


Mostly this post is to say I think that is the same pulley I use (different source), which is the smallest diameter I could find. I don't want a rush of people to order them, only to find out it is the same thing. Measure yours and compare first.

Yes, it is smaller.

 

[74M35A2]

? No, it is not. Both specs says exactly same diameter. 2.37" OD (60mm).

 

[simp5782]

? No, it is not. Both specs says exactly same diameter. 2.37" OD (60mm).

That's not what she said.

 

[Russ Knight]

? No, it is not. Both specs says exactly same diameter. 2.37" OD (60mm).

You may be right. I haven't measured it.