M923 with No alternator output

[LiqTenEXp]

Hey,

I did my homework and am really looking for suggestions on what path to take from people who have had similar/same issue.

Volt meter shows in top of yellow range
Batteries individually measure 12.6+
Battery voltage across the whole bank = 25.6

I hear the safety relay disengage when disconnecting it from the box

Battery voltage at wire 566 with battery switch on = 25.6
Battery at wire 568 disconnected = 25.6

Engine cranks/starts fine, volt meter shows in yellow range and measures a little lower = 25.3

I removed the regulator back off of the alternator while the alternator was still on the truck. The potting material doesn't look burned/bubbled/damaged. There was no smell, just some dust from the brushes.

Should I be replacing the whole alternator per TM or regulator. My gut says whole alternator. I would probably go Delco 21s. It looks like you just need these items:

http://www.amazon.com/ALTERNATOR-19...+SERIES+1991-1992+3675174RX+10459026+10461235

http://store.alternatorparts.com/partno241507.aspx

and 1/2" of some pipe as a spacer to make it fit, anyone know what diamter the pipe is?

 

[Mike929]

I changed mine over to a commercially available alternator and it is purring like a kitten.

You will probably have to notch the stock bracket where the back of the alternator bolts up. No idea the spacer diameter. I actually had to drill out my lower strap to accept bigger bolts then the military alternator for my replacement.

 

[tractors0130]

http://www.steelsoldiers.com/showthread.php?85317-M925A2-Delco-Style-Alternator&highlight=

 

[LiqTenEXp]

I was looking for pictures of an install on a NHC-250 and didn't come across any. The alternator is 180 clocked from those pictures for an NHC-250 and wasn't sure what mods had to be made to make it fit.

 

[LiqTenEXp]

Looks like I might have found a bad component and would make sense as to why the alternator has no output. I will let you guys know if it works after replacement.

 

[74M35A2]

Likely the voltage regulator. It is simply a FET (transistor), which is just a fast switch to send current to the rotor depending upon measured system voltage. The change to a Delco or Delco type (Chinese clone) would likely yield a higher quality replacement with a longer life all at a lower cost, unless you are trying to preserve the way it looked when it was made. Many alternator shafts are the same diameter at the pulley end, you may be able to simply swap yours over rather than buying new.

 

[LiqTenEXp]

Well I found a bad zener diode that is connected to one of the brushes and the voltage regulator on one side of it. I replaced it and no luck, still no output. I thought I nailed it but I could still have a bad voltage rectifier. I am probably going to go civilian...

 

[74M35A2]

Another option would be to install an externally mounted voltage regulator onto the alternator you have now. Such is available as old OEM stuff, in addition to newer aftermarket styles.

 

[gem1410]

Thought that diode would do it--Try John for the used alt--I will also check around for a used one --talk to you later Brad--George

 

[74M35A2]

If you really want to keep the original alternator, I'd purchase a Delco 24V regulator ($15) and patch it into the original alternator. Another low cost and more reliable option to consider.

 

[LiqTenEXp]

Well the Delco 21si type alternator doesn't really go in as easily as one would think, at least if you read the forums on a M923 w/NHC-250.

I am almost done but not quite. I bought the parts you see above. So far I needed to fab up the following:
3/8" spacer from 3/8" pipe for FWD lower bracket to line up pullies
2 1/2" spacer from 3/8" pipe for AFT lower bracket to line up to the location of the FWD tab
3/8" spacer from 1/2" pipe for the top tension bracket

still to do:
widen adjustment opening in tensioner bracket to accept 1/2" bolts (the size the Delco 21si has for its threaded tensioner bolt)
get shorter belts (stock ones are too long for this configuration)
wire up and test

I needed to get:
(1) 2 1/2" long 1/2-13 bolt for the top bracket
(1) 2 1/2" long 7/16-14 bolt, washer, lock washer, nut for the FWD bracket
(1) 5" long 7/16-14 bolt, washer, locker washer, nut for the AFT bracket
(1) piece of 3/8" pipe to cut to length
(1) piece of 1/2" pipe to cut to length
(2) shorter belts - length to be determined tomorrow

Pictures to come tomorrow. This is not a tough task just not as easy as I had hoped. Hopefully this info will help others who have thought about ordering a Delco 21si equivalent alternator.

My trucks info related this topic:
1985 AM General M923
Last depot 2011
NHC-250 (new as of 2011)

 

[98G]

I've got a truck to retrieve with a bad alternator. I'm contemplating doing this instead of installing a military alternator. Mine is the 8.3. Are the brackets the same?

Or maybe I should just do this to my M925 and take the OEM alternator to switch out for recovery. Is it safe to assume the nh250 and the 8.3 use the same alternator?

Thanks

 

[tractors0130]

I've got a truck to retrieve with a bad alternator. I'm contemplating doing this instead of installing a military alternator. Mine is the 8.3. Are the brackets the same?

Or maybe I should just do this to my M925 and take the OEM alternator to switch out for recovery. Is it safe to assume the nh250 and the 8.3 use the same alternator?

Thanks

You should be able to just swap pulleys, put the serpentine pulley on the alternator that you're using on the 8.3. Brackets are different between nhc250 and 8.3, alternators (other than pulley) should be same.

 

[LiqTenEXp]

well bad news. it went in fine, got the belts, etc. It had no ouput, looks like I got a bad unit.

To double-check my wiring I did the following.

Measure battery voltage @ Pin C of electronic control box = checked good 25.8 volts
Measure resistance from Pin D of electronic control box connector to positive connection on alternator wiring = less than 1 ohm, good
Relay for batt power clicks when engaging power switch in cabin. Batt voltage reaches alternator connections and all accessories work. Starter works fine. I think that eliminates the electronic control box.
Chassis ground to ground connection @ alternator = 0 ohms, good

Anyone else want to throw some rocks at my testing?

 

[gem1410]

Brad--how much did the new alt cost--also, when you measure the ohms at pin D it is sometimes necessary to consume large amounts of beer before the alt kicks in--

 

[LiqTenEXp]

120, they are taking care of me and will test it, it has a 1 year warranty.

 

[gem1410]

That is cheap--if it all works out I will start doing the same change outs--thanks Brad

 

[LiqTenEXp]

Ok, so I got some time and started to look at the old alternator again. I picked up a cheap (<70 dollar) regulator and popped it on. I am back in business. I had a bad zener diode and regulator.

The other good news is I have a ready to go kit to drop in a Delco 21si J180 style mount alternator. Not sure if anyone would want it to do theirs but PM if you do.

 

[74M35A2]

If you replaced the regulator with the same type, you may need the Delco sooner than you think...

 

[gem1410]

Good you got it fixed--past summer I had an alt. putting out 30 volts--bought the same regulator for $60-- back in business--I did not know the regulator could 0 out the voltage when bad