M923 alternator

[clews1975]

I just drove an m923 home from Maine. It was a five hour drive and had no issues except batteries were dead. Gage on dash showed needle in green so i assumed batteries would charge. Upon arriving home i shut it off and it wouldnt start without a jump. I took it to a job today and left it running for a half hour when the horn sounded for about 30 seconds and the truck died. When i jumped it again the alternator guage was way in the red and after a few seconds the horn started sounding again and the guages started blowing out of the dash and the resistors in the guages were on fire. Was this a battery problem that damaged the alternator.... has anyone else seen this, and where do i start with repairs? Thanks in advance for advice.

 

[juanprado]

start by first disconnecting all batteries, check individually for proper fluid level, specific gravity, load test. charge individually ones not up to full charge.

I suspect regulator in alternator is fried (should have charged batteries not uesd alt. Alt not designed to recharge fully dead batteries, especially 4 of them) but start with the basics- batteries and go from there.

 

[Vintage iron]

yup! what he said! I hope you didn't fry anything else too.

 

[Ferroequinologist]

The M900 series trucks have a habit of this it seems. If the batteries are dead or low, DON'T drive the truck without disconnecting the alternator. I have picked up many lately where almost everything electrical is fried, and it is due to a blown alt. One was pumping 63volts at 110amps. That truck also had gauges that had blown out and caught fire. The ABS box is fried too, along with most of the lights and blinker.

Bottom line, guys need to be aware of this when picking up trucks with dead or low batteries before driving them home. Sorry that it happened to you. You're going to have to check everything electrical on the truck for proper operation. Sending units, gauges (obviously) the ABS system, the CTIS if it has it, etc.

 

[clews1975]

I have read that the alternator could be switched to a civilian type (delco 10si) is this correct?

 

[slapshot61]

The M900 series trucks have a habit of this it seems. If the batteries are dead or low, DON'T drive the truck without disconnecting the alternator. I have picked up many lately where almost everything electrical is fried, and it is due to a blown alt. One was pumping 63volts at 110amps. That truck also had gauges that had blown out and caught fire. The ABS box is fried too, along with most of the lights and blinker.

Bottom line, guys need to be aware of this when picking up trucks with dead or low batteries before driving them home. Sorry that it happened to you. You're going to have to check everything electrical on the truck for proper operation. Sending units, gauges (obviously) the ABS system, the CTIS if it has it, etc.

I am trying to learn about these trucks and all the quirks. This is a super dumb question, how can you drive the truck with the alternator disconnected. Won't the low batteries completely die? Can you give me more details?

 

[5tonman1971]

I am trying to learn about these trucks and all the quirks. This is a super dumb question, how can you drive the truck with the alternator disconnected. Won't the low batteries completely die? Can you give me more details?

All the battery's do on a diesel is start the truck. You don't need them nessacarily when the truck is running. My m813 you could rip every wire out of the truck and pull start it and it would run the same. It is also mechanical I'm not 100 on the m900 series truck

 

[ichudov]

All the battery's do on a diesel is start the truck. You don't need them nessacarily when the truck is running. My m813 you could rip every wire out of the truck and pull start it and it would run the same. It is also mechanical I'm not 100 on the m900 series truck

You need batteries to operate the fuel shutoff solenoid.

Also a few more things, such as gauges and ABS.

i

 

[Jeepsinker]

They do have a kill cable. The shutdown solenoids fail all the time.

 

[slapshot61]

You need batteries to operate the fuel shutoff solenoid.

Also a few more things, such as gauges and ABS.

i

So I do have to have the batteries connected, based on the last three replies.

 

[doghead]

You can't pull start an M939 series truck, so the answer is yes, you need them connected.

If you read the -10TM, you may learn more.

Why would you want to operate the truck with no batteries connected?

 

[DJones]

So I do have to have the batteries connected, based on the last three replies.

Batteries yes, alternator no.

 

[Farmallmilitary]

I just had my regulator go out on my m925 a few days ago. I tested the output voltage and it was 35v, luckly I didn't fry anything. Now all I need is a new regulator.