Hey everyone. I'm new here, but not new to the older Toyota pickups. Here was my problem and here is how I fixed it... for free.

1986 Toyota pickup. AT. Overdrive would always cut out at high speeds after about 20 minutes and then the O/D relay would start clicking on and off really fast.. and it was a mess. Most likely a temp sensor ECM thing?, but my truck was definitely not overheating. Here's the O/D control circuit: https://web.archive.org/web/20140816.../6overdriv.pdf

Here's what I figured out. You can safely put a bypass switch across pins 1 and 2 on the O/D relay. (refer to circuit). The relay is way up and back above your left foot. Mine is labeled "Transmission Relay" and is green... for some reason.?

This is safe (at least for this exact configuration). I did a prior freeway test with a multimeter attached to those pins and checked out the behavior. Basically your O/D solenoid should ALWAYS be energized when overdrive SWITCH is ON, and NEVER energized otherwise. It's just those pesky engine temp protection circuits.. and the ECM.. and who knows what else... that disable it sometimes when there is "problem", and they aren't very smart, or maybe just broken. Or both...

Just bridge pins 1 and 2 on the O/D relay with a toggle switch and your problem will be solved (If that's your problem ).

I hope this helps you out there because it took me way way too long to get it...

 

Welcome to YotaTech.

Wow! All of those engineers in Tokyo worked for years with relays, diodes, computers ... But you figured out that all you needed was a switch!

Uh, no. You've got it backwards. The temperature sensor is to prevent the transmission from shifting to O/D (or lock-up) until the engine is warmed up. http://www.cygnusx1.net/Media/Supra/...taTech/h16.pdf You're lugging the transmission, and if done with a cold engine drivability suffers. So the relay doesn't close until the engine is warmed up, regardless of the state of the O/D switch.

Since you had a clattering relay, my money is on a bad connection somewhere. The Temperature sensor is just an NTC resistor, so isn't likely to fail "intermittent." But the wires sure can.

But a bypass is a bypass. As long as you remember to never throw your switch until the engine reaches operating temperature, you should be okay. It will just be inconvenient.

 

Interesting. I assumed it was to prevent people from say.. driving up a mountain in O/D and overheating the engine. Anyway.. I'm happy to be done with whatever protections that circuit was offering, or not offering in my case.

 

If you are going up a hill, the transmission will simply shift from overdrive to 3rd when needed. As Scope103 said, the temperature circuit for keeping the transmission from shifting into overdrive until the coolant warms up to a certain temperature.

 

Thanks. Yep. I get it. The funny thing is mine would do the exact opposite. Overdrive always worked when cold and would quit when warmed up. And also I have to manually shift to make it up the hills. It only downshifts when I practically floor it. Maybe the extra weight? (Weighed in at 4600lbs). Or maybe something out of adjustment? Anyway, this is my fourth 80's pickup over the years but the first A/T. I'm not liking the A/T much at all...