1986 C1500 w/ 700R4

[Evrett Janzen]

my torque converter lock up only works when its cool out side, during the day when its hot, doesnt take lock up, though in the late evening it cools off and it starts to work, im wondering if its the switch on the firewall. thoughts?

 

[Charlie]

 

[1987 GMC Jimmy]

What are your symptoms, or what are you using to gauge inoperative lockup? Are you experiencing any torque converter chatter or any other problems? My ‘86 700R4 has a failed torque converter clutch, and it’s way more problematic when the ambient temperature is hotter. There are other parts of the circuit that can are known to fail such as the vacuum switch, 4th gear pressure switch, and the actual solenoid. I don’t know of a specific test for the vacuum switch besides the generic one of seeing if it holds vacuum, which I would try if you have a Mityvac or equivalent.