I have a 90 toyota with an automatic. I have a problem keeping her in OD on the highway. The motor has to be wamed up to a certain temp to shift and it does but aas soon as you get on the highway, the temp drops and the truck downshifts and stays in low gear. I have tried everything, radiator, waterpump, temp sensor and switch, had the tranny checked out and truck, handfull of thermostats, fan clutch. Im on my lst leg with this thing anyone know whats up?

 

Possible cause...

1) Failed sensor on top of transfer case (this one was on a A340F in a Mitsubishi low probability)

2) Corroded splice in coolant temp switch circuit.

3) Temperature switch shorted inside of the cruise control module.

4) Bad connection in wires behind power steering pump (1990 Toyo Pick up)

5) Worn boost valve in valve body (fix was replace boost valve and drill a hole for pressure relief)

I would opine that # 5 or # 4 or # 3 would be your problem. Anyway this is a start, good luck. Keep us posted.

Just so you know, NONE of these problems set a code.

 

I believe the sensor requires a temp of 157 before it allows overdrive. I'd look at your connections; they might be fooling the computer into thinking the temp dips below 157.

 

I had the same issue last year, there was nothing mechanical wrong with my truck at all, even though the stealership wanted to replace selenoids and sensors... The problem for my truck was after the dealer instaled a new water pump a peice of gasket material got sucked up into the water inlet that allows warm water to flow over the bank of sensors, so when my truck would get hot on the back roads it would drop right into OD but then as soon as I would get out on the highway it would jump right out of OD and stay out till it got good and hot under the hood again. The fix... I had to drain the cooling system, pull out the thermostat which I replaced 2 times, I bought one at Advanced auto and the POS stuck open so I ended up getting a Toyota which I reccomend to anyone.. Then there is a U shaped rubber tube on the back of the engine by the fire wall, this is not easy to get to, I used 10" long needle nose pliers to get to it.. take the hood off its a lot easier trust me. Once I got this off I hooked I tried to blow through the metal tube, nothing, this tube goes all the way to the thermostat houseing. I hooked my air compressor up and shot about 70psi though the tube and what looked like a black rubber fishing work flew out of the thermostat housing! I put the system back together after a thourough flush which caused more black crap to come out, filled it up with the 50/50 miz Prestone and have not had annother issue with it coming out of OD since! Hope this helps you and anyone else with this problem! If you need a hand shoot me an email or give me a call, Mbernazzoli@hotmail.com or 304-582-1855 Matt