how to test m35 alternator system?

[flatheadvff]

I'm sure this has been covered but I am in kind of a hurry and am not finding a thread. Our vfw fire tanker is a m35 and noticed it is no longer charging the batteries. I don't know how to test and see which part I need other than just throwing parts at it. I've charged the batteries back and checked all the connections. Any help for a quick diagnosis would be appreciated. New tanker is being built at this time but old bettsy is what we depend on still at this point. (besides I'll be sad to see her curbed.)

 

[Guyfang]

M35A2? M35A3?

 

[G744]

The stock 60A alternator uses an 'on' command 24-volt wire from the ignition switch.

That would be the first place to test. If it is hot with the engine running, and no charging occurs, the alt is probably shot. Most failures are with the internal regulator. If it is the usual model, the back plate can be removed with a phillips screwdriver. Pull it off and inspect the guts for burned stuff.

DG

 

[ToddJK]

Is there a battery disconnect on the truck, a separate shut off from the accessory switch? Possibility there's a short somewhere if the alternator proves to be working.
I'm curious, does the voltage gage stay in yellow or green while the truck is running? Usually the gage will dip into the yellow if the alternator quits and keeps dropping as the batteries die. This also happens if you end up having a bad battery. When my alternator went out, I tested it by pulling the posi cable off the battery with the lights on. The lights died out but the truck will run due to the IP doing all the work.

 

[flatheadvff]

It has the all fuel engine , I believe that makes it an a2. We also have an a3 with the cat and allison, but that's beside the piont. Yes the gauge is in the yellow, first indication of problem. Pulley is worn out so belts wear and get loose quickly. they've been tightened. I will check the voltage coming from ignition seitch and battery terminal at alternator this evening after work. The main question I had was answered in that it is internally regulated. So as long as I have battery voltage and ignition voltage the problem has to be internal.

 

[swbradley1]

The first thing I would do is remove the battery cables and clean them to shiny metal then move to the connectors on the alternator and do the same.

With truck shut down (not running):
Put it back together and check the voltage across the batteries at the batteries themselves. Should be 13.6v for each one and 27.2v across both. Then check the main terminal on the alternator and it should be 27.2 (maybe a slight drop).

Make sure all that is good before starting the truck. The cables and batteries can cause a bunch of problems.

With the truck running:
The main terminal at the alternator should have 28.8v if the batteries are fully charged and in good shape. If not make sure the start terminal on the alternator has 24v.

The voltage may be lower if the batteries need charged but will come up as they charge.

I've had alternator problems with a Deuce and an 813 and although extremely simple they can be a real pain to fix. Start with the basics and build from there.

If you have charged batteries and the alternator main terminal reads 27.2v or so with the truck running then the alternator is bad.