Problem installing 130 amp alternator

Last night I was putting a new 130 amp alternator in my '87 F-250 and found I couldn't get the two belts over the pulley on the new alternator. The new alternator is slightly larger and will not swing toward the engine far enough to even stretch the belts over. The truck has a 460 engine and used to have a stock 60 amp alternator. I am planning on just buying longer belts. I am wondering if anyone has had a similar problem? Is it possible I have the wrong alternator or will new belts be a simple fix? Thanks for any advice.

 

I have an '88 F-250. The altenator recently died and I told my mechanic to put the highest Amp one he could. I believe 90 was the largest that could be used without making major modifications (read expensive) under the hood. Since that truck has an under the hoods belt drive pump for the plow, I settled for the 90 amp instead of spending the extra $. I can ask the mechanic for the specific details tomorrow if you want, I didn't needwant to know at the time.

 

Thanks BRL, if you can talk to your mechanic I would appreciate it. The employee at my local Auto Electric repair and service shop had me bring in the old one when I picked up the new one to verify that it will fit. I can fit it in just fine, getting the belts on is another story.

 

I had this exact same problem - only with a '92 f250. Finally ended up that the 90 amp was the biggest that would fit. To go to the larger alternator you have to install a different mounting bracket ($$). The easier solution is to take your old alternator to a rebuild shop and tell them that you want to have it rebuilt to the higher 130 amp. It's much easier and done all the time. They'll just use your old housing (so you know it'll fit) and it'll be less $$$ and much less hassle. Good Luck.

 

Hi Jason - just a quick question with regard to getting the alternator rebuilt to the higher rating. I was wondering how the long term durability will be with the smaller case and internals putting out the higher amp rating, especially with the severe demands that plowing places on the electrical system. (I run two 850 cca batteries on my GMC (350/T400) with a 100 amp Delco unit (big case)) Not saying there will be any problems, just wondering what your experience has been with durability.


1975 GMC C-35

 

75-you are thinking like me-the high output small case alternators do not have the heat sink available to dissipate heat,so the burn up quickly,both the rectifier,and the rear bearing.Another thing is that a 130 amp conversion will only put out the additional output at high rpm's,at low RPM's they put out same as stock,since they arent able to get any more windings in there.I put a large case alt on my 89 GMC,I had to fabricate brakets for it,and a 2" longer belt,but i feel its worth it.With the 140 amp alt,and dual batterys,this truck can plow without ever dimming the lights.

 

Thanks, John DiMartino - I was wondering about durability but you bring up another very good point concerning output at low RPM, where a plow truck spends a lot of its time. On my own truck, I found that the big case alternator (admittedly a different (older) style than what would be on your '89) fit quite easily when I used an aftermarket chrome set of brackets, not sure why but it worked! One other question comes up, and I've seen this mentioned somewhere in an old thread: Is it practical to mount a larger pulley to the alternator to increase it's RPM at lower engine speeds, or is this more trouble than it is worth?


1975 GMC C-35

 

If you want to speed up the alternator-you need to put a smaller pulley on it,or a larger one on the crank.The problem with putting a smaller pulley on the alternator is with V-belts,it is much more likely to slip,and if you wind the motor to redline or close,the alternator speed could get to high-resulting in it literally blowing apart.I blew one apart from adding a smaller pulley,It wasnt even rebuildable after that.I wouldnt go any more than a 10% increase in speed,that is what im at now on my trucks,I did that on my Iroc,and blew it to pieces,but the truck never goes above 4500RPM's,the Iroc hit 6400 on occasion.

 

John - you're right, I had that backwards. That's what a day of welding will do to you! Based on the fact that my system is (and has been) working fine, I think I will leave things the way they are. Thank you for the information, that's a couple more things I just learned today!


1975 GMC C-35

 

Well I just got longer belts and am going to see if they will solve the problem. I'll let you guys know how it works. We have a winter storm warning for 15" tonight up here in the northwest so one way one another I am getting that thing back on the road tonight.

 

Hope it works OK for you, if longer belts are all it takes to solve the problem then that's a bonus. Hey, mind sending some of that snow our way here in the East? I haven't cranked up the plow truck since last Thursday!


1975 GMC-35

 

Well you can fit a 130 amp alternator in a '87 Ford. I had to build a bracket to drop the bigger alternator down about 1.5 inches and actually get a shorter belt between the alternator and engine. The stock belt between the alternator and the emission control pumps still fits. $5 worth of steel from the hardware store was all it took to make the bracket. Hour or two of cutting, bending, and grinding, and I was moving snow again.

Remember #3

 

Ryanw: Glad to hear that it is working for you, and that it wasn't a nightmare job either!