91 SR5 V-6 doesn't warm up

[Big Wave]

My 1991 SR5 4WD has started running cool, which unfortunately prevents the automatic transmission from shifting into overdrive.

It is acting like the thermostat is stuck open. It takes a long time for the temp gauge to come out of full cold, when it used to take just minutes to fully warm up. Even then, it almost never reaches the middle of the gauge which must equate to about 170 degrees, because that's when it should shift into overdrive.

I've replaced the thermostat twice. The last replacement, I tested the old one at home, and it appeared to function normally.

Is there any reason besides a stuck thermostat that would cause my truck to take so long to heat up? I drive around in highway conditions, and doing so without overdrive is killing me, not to mention what it's doing to my gas mileage.

This seems to be stumping the local Toyota dealership as well.

Thanks for any help.

 

[mikes4runner]

IIRC there is a solinoid that tells the tranny when it is warm to not run in O/D maybe this has something to do with it.



Mike

 

[Big Wave]

Mike,

Thanks for the response. My Toyota mechanic has mentioned troubleshooting the solenoid, but I wouldn't let him based on how the truck is acting. By all appearances, the solenoid is doing its job. When the coolant reaches a particular operating temperature (I've been told approximately 170 degrees), the solenoid is performing correctly.

Having owned the truck since new, I can tell that it is not warming up as quickly nor as completely as it used to. When it finally reaches the normal temperature range, the truck will shift into overdrive.

As I mentioned, I it is behaving exactly as though the thermostat is stuck in the open position, yet three thermostats have all resulted in the same condition.

I may eventually replace the solenoid anyways, just as a last resort.

Thanks again for the information.

-Big Wave

 

[JP4RNR]

When the coolant reaches a particular operating temperature (I've been told approximately 170 degrees), the solenoid is performing correctly.


-Big Wave

Actually once the coolant temp reaches 120 degrees the Trans will shift into Overdrive. The 120 degrees is documented in most owners manuals. This being said I would say the sensor is acting up.

BTW if you disconnect the temperature sensor for the solenoid the trans will always shift into overdrive regardless of the coolant temperature. I disconnected mine with no worries about 8 years ago