Hey guys i have a 1990 f150 xlt lariat that i recently decided to take on as a project. Lately ive been just working on the mechanical aspect of the truck but ive hit a speed bukp with the electrical part. I recently put in a new battery due to the old one being absolutely dead and i thought it would have fixed the power problem but when shifting into any gear the truck seems to lose power still on the battery gauge. I noticed while shopping around for the truck that the alternators available vary based on ac or not etc and i noticed when looking at them that the alternator in my truck resembled the none ac option. My truck was retrofitted for ac so i was wondering would this cause the significant power loss? It seems while in gear when not accelerating it drops too. Please help!

 

The difference is the size of the pulley to compensate for the extended length of a serp belt with the AC.

Check connections/terminals and clean everything up (body grounds, battery terminals, etc..). Pull the belt and check all of your pulley to ensure that they spin freely with no resistance on the non-powering ones. Take the truck to auto parts store and they can test the alt for you, if you do need to replace it I recommend going with the TYC 130 amp 3g alternator from rockauto, cost me $110 shipped. You will need to replace the main wire with at least 4awg (I used a battery cable wire from AZ) as well as a 150amp fuse or breaker (ebay), also would be a good time to replace the battery terminals since you are tearing into things. Doing the upgrade will cost about the same as buying a normal stock replacement from an auto parts store so it's worth the bump in juice output for basically the same amount of work as just replacing with stock.

If the alt is not your problem then I would suspect that one of your components is bogging down the engine with the clutch in or you do have the wrong size pulley on the alt and it's just not putting out enough juice at engine idle.