Bought an '88 Trooper yesterday...question

Just bought a 4 cyl automatic with 140k on it. Love these trucks...I drove an 88 5 speed from 2001-2005.

I was driving home on the freeway the RPM's jumped up like the truck downshifted...then the RPM's lowered back down again...then they went back up and it hasn't shifted up since.

This may sound ignorant...but are are overdrive and 4th gear the same on these? I've been listening as the car shifts through gears and am thinking it's not getting into 4th gear. Driving it on the freeway is pushing the RPM's up over 3000 by 55mph or so.

I read a comment on carsurvey.org where a guy said "the overdrive does not engage until the car is warmed up. I had a temperature sensor out that caused the transmission not to shift into overdrive." I don't see anything about this sensor in the Haynes Manual.

Can someone tell me where to find this sensor or if they have any other ideas?

One final thing that may have nothing to do with this issue, but the heater does not work- it blows, but only cold air.

Thank you.

Searched around planetisuzoo.com and itocuk.co.uk and found a picture of the engine with the sensor highlighted. Pulled the wire off the sensor last night and overdrive worked! Of course it dropped the RPM's significantly on the freeway.

Now the question is where to find a replacement sensor...or as one guy mentioned, is it wise to just leave it unplugged and manually turn the OD on when the engine is warm?

As you discovered, the OD does not engage unless the engine (and thus the trans) are warmed up at least. I don't have your model, but that is common for most automatics. I am concerned about the no heat problem....if related could indicate a blockage or restriction that may be responsible for the problem, ie: sensor not seeing real coolant temp. In any case, you need more detailed info on what else the sensor might have an influence on. On mine the ecm adjusts fuel mixture by temp too.
I don't think you will hurt converter by manually locking it up, but don't lock up too early.....fluid circulation slows down and cooling also slows down, that is why older trans with OD often burned up while towing. Good luck.

Thanks aftdmike. I plan on having it looked at as soon as I find the right mechanic.

I could feel that the hoses behind the engine going into and out the dash are warm, but I'm not sure that means the coolant is flowing. The thermostat connected to the dashboard gauge does show the engine warming up slowly as I drive, and not getting too hot. What would you think is enough warm up driving time before it's a good idea to manually shift on the OD?

Now why the heater is blowing cold I do not know. The lever that controls the AC, heat and defroster is a little off. It's hard to push it into the heat setting, and when you push it through then it goes all the way past heat to defrost...but no air to come up on the window vents.

The heat issue is one that I solved over a few months. For a long time, I thought it was a lingering problem from when I had a leak in my radiator from off-road debris and replaced the whole unit. I ruled that out and decided on the A/C system (as the heat-exchange is still necessary to provide heat in your A/C system) Radiator full, and A/C system running fine, there was only one part that was in the way without having to look into the heater core: The Thermostat.
If your engine temp gauge requires you to go on a road trip to get it past C, then your thermostat is stuck or dead.
Mine had been stuck open, causing the engine to have to run for long periods of time in order to build up enough heat to run efficiently.

I don't have any help for the OD issue, though.