1990 F250 4x4

[John Pannebaker]

Hi all

I'm thinking of buying a 1990 F250 4X4 truck, 70k miles, 5 speed manual trans.
Is there a way to tell is it has a limited slip diff?

Thanks in advance john

[BMFer1]

Read the tag on the differential cover and write everything on it, then post it back here.

[John Pannebaker]

thanks for the reply, would the VIN number help?

[BMFer1]

I doubt it. The differential tag will tell you all you need to know. Not much to decoding it either.

[alpurl]

Originally Posted by John Pannebaker

Hi all

I'm thinking of buying a 1990 F250 4X4 truck, 70k miles, 5 speed manual trans.
Is there a way to tell is it has a limited slip diff?

Thanks in advance john

Actually, there is.

Chock the front wheels, jack up one of the rear tires so it clears the ground.

Put it in neutral, and then try spinning the tire that's off the ground. If it spins, it's a limited slip differential. If it doesn't, then it's a posi-trac, meaning positive traction.

Here's the difference:
The planetary gear in the differential in a limited slip diff will transfer power from one wheel to the other. In a turn, the inside wheel will travel a shorter distance, and spin slower, the outside wheel will travel further, and spin faster. Limited slip diffs will accomodate this difference by transferring power from the inside wheel to the outside wheel. This eliminates slipping and wheel hop when cornering. It also is of no help when one wheel has no traction and the other does. The wheel with no traction will spin while the other doesn't.

Posi-trac rear ends are locked from one side to the other. In cornering, the rear tires will hop and shudder a bit as they are unable to compensate for the difference in the cornering track. The advantage is that in compromised traction situations, both tires will spin regardless of whether one has traction or not. You'll be more likely to get yourself out of a sticky situation, and less likely to get into one in the first place.