'89 Pickup Automatic Trany Overdrive Issue

[19holekc]

Recently bought an '89 4x4 pickup that sat idle for many years, so it has very low miles (38k). I'm having an issue with the automatic transmission. When driving at highway speed it keeps slipping in and out of overdrive. The trany oil is original, so I'm planning on changing it. Any other suggestions to alleviate the problem?

[Jayota]

You might want to check the coolant temp sensor aswell. As it only lets the trans go into o/d when the motor is warm. It might be telling the ECU that the motor is too cold..

Changing the fluid and putting in a new filter sounds like a good idea. Don't bother if the fluid is really bad.

[fourwd1]

When it does it, are you going uphill? have weight in the bed?

[chrismalm]

I'm going through a similar situation in a 2wd pickup. I have the A43D transmission and you have the A340 so I can't advise but I can guide you.
Look up the '93 Tacoma Factory Service Manual on the internet by searching: utulsa buchanan 93 FSM
Mr Buchanan was kind enough to upload it.
There's a section on the overdrive control for your transmission.
Mine is controlled by a thermo switch / relay / solenoid. Yours may be also.
My relay is on the upper firewall under the dash on the far left. It's blue.
I replaced my dead solenoid with a junkyard special and it snaps on and off like it's working but still no 4th gear If I take it back out and it bench tests OK, I'll have to conclude that my 4th gear has failed (debris clogging valve body or failed plates). Good luck with yours.
Chris

[19holekc]

There does not seem to be any pattern of when the transmission slips out of overdrive; flat surface, uphill, downhill, it just doesn't matter. When I change the trany fluid and filter this weekend I will check the sensor. Thank you for the suggestion.

[19holekc]

I will also look up that '93 Owners manual. Should be real helpful for futures.