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Alternator just died

S

scottmh

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Pretty sure my alternator decided to stop alternating on my last outing. Went to crank it up this morning and didn't have enough juice to turn it over fast enough to get her rolling. I put a battery charger on the front battery and VROOM she cranked up and ran. Across the batteries I am reading 20.1Volts and the volt gauge is in the red, while running. Rear battery is roughly 11 volts and front is around 9 volts. I am thinking its the alternator but I want to see if there is anyway to verify before I either pay an arm and a leg to buy a new one of pay an arm and a leg to have this on rebuilt. So any advice on how to proceed?

M109A4
 
Last edited:
gimpyrobb

gimpyrobb

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Have you tried adjusting the output on the alt? There should be a brass plug near where the wires connect. Take that out and there should be an adjustable pot under the plug. I usually plug a DMM in the slave port to watch readings while "tuning".

Edit, just saw its a 109a4, not certain they use the same alt as an A2.
 
Warthog

Warthog

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Or it could be bad batteries.

Never attempt to diagnose electrical problems without fully charged and load tested batteries. You will just be chasing your tail unless you truly know the condition of the batteries.
 
doghead

doghead

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You guys are just repeating what was already posted in the similar threads.
 
welldigger

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My alternater took a dump on me a while back. First thing I did was have the batteries checked. Both were good. I then checked for voltage at the exciter wire. Had 24.5 volts there. Cranked the truck and checked the output of the alternater and I was only getting 24.5 volts. Basically I was only detecting battery voltage. Mine was bad. Didn't even bother attempting to adjust the pot since it went from working to not working in a day.

I'm using tmamerica's conversion kit to convert to a Delco 27si. Had a local starter and gen shop build me a 24v 70 amp alt. With brackets, a new pulley, and rebuilt alternater I'm still going to be at less than half the cost of a new or rebuilt presolite.

My truck is an a2 but m35a3's and m109a4's use the same basic alternater. Tmamerica has brackets for the cat powered trucks also.
 
S

scottmh

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Front battery at 100% and 1400cca we will see if it holds. The rear on the charger at 34% 1500cca. Once the Batts are at 100% I will hook them up and crank her up and see if the alternator does anything. If not it's getting removed and sent to the alternator shop to be rebuilt. Hopefully cheaper than the price of a new one.
 
goodguyzy

goodguyzy

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I lost one on an A3, it was cheaper to buy a new one. This is a real common problem on these trucks but if you didn't smell it burning then I bet its the batteries.
 
swbradley1

swbradley1

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Get a couple of blemished Group 31s from Interstate or two new Commercial Group 31s from NAPA for the price of one 6TL and go on.

If your batteries are very low or almost completely dead charge wise the current draw when you start the truck will be too high for the output of the alternator and therefore kill the alternator. I've done two alternators like that now and put a civi version in the Deuce and now how the bracket to put a civi version in my M813. $107 plus shipping for a brand new Delco 1-wire versus whatever you pay for a rebuild or new military Leece Neville. My newest waiting to go into the 813 tested out at just over 80 amps. With almost all the lights getting changed to LED it should be more than enough.
 
Dipstick

Dipstick

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My alternator suddenly refused to get out of the yellow on the gauge. Sometimes it would work and go green for a few minutes. I bought a replacement alternator only to discover that the ground connection from my alternator to the frame was done by a moron. I installed a new ring terminal. Re-grounded it to the frame and presto my.....failing alternator sprang back to life. That was months ago. I'm not saying that's your problem, but checking simple stuff can sometimes save you big bucks.
 
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