My 120 Amp alternator is going bad. I ordered a rebuild kit (first time so I'm excited ) but won't be here for 2 weeks. I'm going to pull one from the junkyard and use it and rebuild mine and then I'll have a spare.
My question is: How do I tell which one is the 120AMP? If it has the tow package does that mean it's the 120AMP?
Thanks, Austin

 

if the alternator has never been rebuilt there should be a factory sticker that tells you the output. plus if you are only it temporarily does it really matter i you get a 90 amp alt instead of a 120 amp alternator? a 90 amp alt should power most thinks anyways

 

I appreciate the response wolf! I was just about to post I answered my question. Found the part#s in the FSM at the back of the charging section (how did I miss them). I would like to get a 120AMP so I do have a spare. I carry spare parts (axles, u joints, seals, calipers, brakes and hardware, etc with tools) because my job I'm on the road a lot and can't have downtime. They pay me for the mechanic work though

 

there is really not that much of a difference between a 120 and 90 amp alternator tho. most stock setups come with the 90 amp which is more than powerful enough to run everything needed. plus used alternators don't like sitting around. the 2nd picture is of a 75 amp alternator that i got sitting around that worked just fine when it came off my 91 and has kind of locked up from sitting in my nice dry basement. alternators especially the denso ones don't like sitting around. the older large dodge alternators seem like they are less likely to lock up from sitting but i have a feeling the tolerances on the older ones are nowhere near as tight as the new ones which is why the new alts are much smaller than the old ones

 

Good to know! I'll try and lubricate it once and a while if it locks up so be it. I need the 120 (actually might go dual alternators, because I have so much accessories. I have on board air, train horns, lockers, LED light bars, cb, ham radio, gauges, hid headlights, rock lights and underglow. It puts strain on the alternator, so I think bigger would mean it wouldn't have to work to hard. I'm probably wrong though

 

After adding all my accessories up plus the truck, and a quick Google search, I'm going to upgrade to a 340AMP alternator. It's suppose to be a direct fitment

 

hahahaha ya that is not how things really work. your alternator is there to to keep the battery charged and your battery powers everything. it is not like you have all the equipment you listed running at the same time. it is really not hard to have more draw than alt output. a 90 should be able to keep up with your needs. a 120 or larger would just be a bonus. i just know anything over 120 starts getting really expensive. even the factory 160 or so alts are pretty expensive.

 

I was always told whatever is running all the time your Alternator should match it or be above it.

 

but you are not running 120amps of lights and equipment all the time. most of the time running down the road you are likely using 60 amps or less.

 

I only have a 120 or 150 amp in my semi with 4 dual purpose batteries. It'll run the engine electronics, lots of lights, wipers, 2 blower motors, lap top computer, refrigerator, stereo, and CB while recharging the 4 batteries from the overnight hotel load. Make sure there is a big enough cable to actually get all that amperage to the battery. Most vehicles seem to have an undersized cable from alternator to battery.