Hey guys, im a noobie here and really need your help. First off i have a 1987 f-250 with the 7.5 and a 4speed manual tranny, now i bought it about a week ago and realized the alt was bad when i bought it. I have since replaced the alt 3 times with brand new units from ford. All 3 have died in less than 30 seconds after starting the truck. Battery is new also. I am so confused, any ideas why it is destroying altenators? Also it is not killing the battery like if there were a wire grounding out, checked the voltage at battery both runnung and off always reads around 12.2v off and 11.6 to 12.3 running.

 

Check the connector to the Alt for any damage, hot spots, cracks etc on it.


Common problem on these old 2G alts in these Fords, and IS a fire hazzard!

 

is the new battery fully charged? if not thats supposed to cause alt failure, i have never had this cause i just throw it on a charger when i change an alt

 

While it's not the best thing to use an alternator to charge a battery, it shouldn't kill it, especially a new/rebuilt one and repeatedly in only 30 minutes, and even if it is the trouble-prone 2G.

Something else is wrong. I would strongly suspect a wiring issue - perhaps the initial failure caused a wire to overheat, melt the insulation, and now it's shorted out.

 

people said when they did the alt swap to make sure the battery is fully charged before you start the truck with the new alt because if it is dead it will kill the alt. i have never had any experience with this i was just relaying the info

 

The 2G has a known issue with the charging plug overheating and shorting internally.
Most of us get rid of the fire hazard and go with a later model 130A 3G alternator that has a lug and cable instead of a plug and undersized harness.

I am surprised the auto parts store didn't tell you must change the plug and pigtail. Most rebuilders won't warranty a 2G alternator without it.

Buy the pigtail plug for around $10 from any autoparts store.
Read Ford Technical Service Bulletin #96214
Any of these are what you want:

Borg-Warner PT-780
Standard Motor Products S-542
Power Plug 4628011
KEM Parts 350-220
Pico 5711
Dorman/Motormite #85123 / 08622

 

Thanks for all the info guys, i am going to check the harness and plugs when i get home to see if thats the problem, only reason i didn't suspect a shorted wire was the fact that my battery does not discharge it keeps a good enough charge to start and run the truck even after several days. How hard is it to change to a 3g alt. I would much rather have the heavy cable up to the solenoid than those cheesy 2 wires

 

Welcome to the forum n4twz!
The plug can short out internally and short open to the battery.
So, you'd get no big sparks when you hook up the cable but the alternator would try to charge itself as soon as you started the truck.
Your showing that the battery is not charging at all is an indication.

You can probably just change the plug (like Ford says to do at every charging service) and everything will be fine.

Changing to a 3G is not difficult.
There are a zillion threads on this (and I seem to be the resident know-it-all)
....Not to mention we have the same truck.
If you go over to the '80-'86 forum there are a number of threads stickied at the top.
Your best bet would be to fine a Taurus/Sable at the junkyard with a recently replaced alternator.
That way you can get the charge cable and fuse you need from it as well, all for around $50.

I went with new everything after the third time my 2G left me stranded.
(mostly because I didn't know any better then)

Bought a 175A megafuse, regulator harness and 2 Ga, charge cable from RJM.
A heavier ground cable to match from NAPA.
A pulley shim washer at HomeDepot and a reman Motorcraft alternator from Ebay for $75 delivered.
I used the 8.25" alternator from a 3.8l V6 and needed to straighten out my adjuster bracket with a hammer. (that spacing seems to work well with my stock belts)
I also Helicoiled the adjuster ear of the alternator 3/8"-16 so I could use one hand to tighten it.

 

Here's one from a couple of weeks ago.
I posted pictures and explained the differences.

https://www.ford-trucks.com/forums/1...r-upgrade.html