Torque converter/transmission problem, 2004 Sport

My wife has a 2004 Montero Sport that's experiencing a problem with the torque converter and/or transmission. When driven at steady throttle from about 25-45 miles per hour, it feels like it's constantly shifting in between gears. I *think* (NOT 100% sure) it may be a failed torque converter locking and unlocking. The transmission fluid is the correct fluid (from the dealer, Diamondstar (??), IIRC) and it's clean. (I've changed it about three times in the last two years.) Aside from having a failing torque converter, the only other issues I can think of would be: A) bad input/output speed sensors on the trans.; B) bad transmission control module; C) restricted trans filter.

Has anyone experienced a similar problem? I think Mitsubishi had TC problems around that timeframe, both in SUVs and cars. I'm leaning toward replacing the converter, but I'd like to do a little research before I go down that road.

Any/all help is much-appreciated!

-Jim

Add to your list of possibles is ECT (engine coolant temp) sensor. It won't allow trans to shift to OD until engine warms up. Your heat gauge will look normal but the sensor may be off spec. Have you been Having any drivability symptoms other than trans?

You can test the ECT on the engine. Test resistance cold then with hot engine and see if within spec. My 92 specs are 2.37 +- .24 K ohms at 68F and 290 +- 32 K ohms at 176F. Yours will probably be the same as these are usually standard thermistors.

No, just the converter and/or transmission acting weird. At steady throttle the problem is the worst. Getting on the gas seems to temporarily alleviate the problem.

The other possible is the TPS. It is part of the computer input for shift points. I'd inspect it's adjustment. On your 2004 there may also be a vacuum hose that transmits engine load info to computer.

Also I guess you've checked your trans fluid level.

Yep, the fluid level is correct, it's clean, and the proper Mitsubishi brand fluid. (Purchased at the Mitsubishi dealer).

Can you elaborate on the vacuum hose? (If you could let me know where it's located so I can have a look at it, that would be great.)

I don't know if your 04 has one - just putting that out as a possible to research. Maybe someone else here can help.

Do you have any manuals?

Well, I think I've inadvertently fixed the problem. It had also started to misfire at stop lights when the air conditioning was running. I was pretty sure the plugs and wires had never been changed, so I went ahead and swapped in some new ones. It not only fixed the misfire when stopped at a light, but it fixed the weird shifting, as well. I'm at kind of a loss to explain why it even fixed the problem. The only (admittedly non-technical) explanation I can come up with is that the misfiring plug was confusing the electronic controls. (One of the plug wire boots was bad and the electricity was traveling down the outside of the ceramic of the spark plug.) In any event, it's been several days now and it does appear that changing the plugs and wires fixed the problem.

Thanks for the responses, pcc!

That makes sense. The computer is monitoring load. If the engine's missing then the it will require a lower gear to handle perceived load. My suggestions, were a stab at getting to the parts that measured load - which would allow shift to OD or not OD.

Glad you got it fixed without an expensive looky-see at the torque converter.