looking at a 91 2 wheel drive with a 518 and overdrive only works when the trucks cold. any ideals

 

Bad coolant temp switch or bad trans fluid temp switch - or the OD clutches are toast and don't work when they get warm.

 

were are those switches located? how much are they? i havent bought the truck yet i dont want to buy a truck that needs a trans. you dont thank its the tps that seems to be a common problem. thanks for your help. hopefully its a cheap fix.

 

Coolant temp switch is in the head, trans temp switch is in one of the cooler lines. You don't really need either one, they can be bypassed and the sytem will work fine.

To find out if it's electrical or mechanical, go under the truck and find the OD solenoid plug. Driver's side, has only 2 wires and one is dark blue. Rig up a temporary connection so you can ground the wire that ISN'T blue. Hop in the truck, get up to 40 or so and ground the solenoid. You should get OD. If hotwiring it won't work when the truck is warm, the OD unit needs a rebuild.

 

maybe i should try this tip to see if my o/d is any good

 

Simple and easy to do. just hook a toggle switch to the wire that wanna described and the other side of the toggle to ground.

 

were is the o/d solenoid plug is it the one that plugs into the vss