1988 Toyota Pickup Overdive Problems

Me too. A few years ago on my '88 4cyl. There are apparently 3 devices controlling overdrive. One is a temp sensor. Then there's a relay under the dash. The 3rd and the culprit for me was the overdrive solenoid at the tranny. The wire was broken where it enters the solenoid and was making only occasional contact. Beyond those 3 items there are of course many other possible answers ( a whole wiring harness and an on/off switch). Replacing the solenoid fixed mine. I negotiated about a 3rd off the price of the part at the dealer on the basis of some price I found on the 'net. You will of course want to try to verify the fault before spending for the part. You probably already know that the engine has to warm up before the temp sensor will activate the overdrive circuit. This of course assumes the 6cly and 4 cyl systems are similar.

Regards Patrick