P0710 Transmission Temperature Sensor circuit malfunction
 

I couldn't find this information on the forum, so I wanted to add it here for other people's reference:

I recently bought a rather neglected 2001 Montero Sport with a P0710 code. I found the cause for the code and I wanted to describe the physical symptoms and what I did to correct it.

The code and what it means: First, my Montero had a P0710 code, which is that the computer could not detect the transmission temperature sensor -- the circuit was open, meaning one of four things in declining likelihood:
1. An unplugged connector (most likely),
2. A bad sensor (second most likely),
3. Damaged wiring, or
4. A bad computer (least likely).

I reset the code and it came back after less than an hour of driving.

The physical symptoms were that the transmission would jerk the entire car as it shifted, whether downshifting or upshifting. It was not slow in shifting, just slamming into each speed. My transmission shifted through all gears even with the error. One web link is referenced below.

I found several web references that the connection on the wiring harness inside the pan of the transmission often would come unplugged from the temperature sensor because the little tab would break off that clipped it into place.

So I drained the transmission pan, undid the 18 10-mm bolts holding on the transmission pan, and pried it loose. I took the pan down and sure enough, when I touched the connector, it fell off of the sensor, but stayed attached to the plastic ribbon wiring harness. I put the pan on temporarily, waited several days for the transmission to finish dripping, and went back in for a closer look. I ended up wiring the connector onto the sensor, using thin copper wire, looping the wire around the sensor's mounting bolt. I installed a new transmission filter and a new gasket, used some RTV sealant and put the pan back on. After filling it with 5 quarts of fresh Mitsubishi _-III transmission fluid, I took it out for a drive, and Bingo, the problem was gone. I cleared the code and it has not come back.

A few suggestion-type notes:

Note one: Any time you are servicing the innards of an automatic transmission, it pays to wait and let it drip if you can afford the time. But almost anything you remove will have more fluid behind it so get the oily part out of the way first by taking off what you are going to take off in one go, if possible. In this case, I pulled the pan, the old filter, and the temperature sensor. Each time, I got fluid on my hand and down my arm that didn't drain when I pulled the drain plug earlier. I had lots of time so I put the pan back on, tested the sensor and ordered parts and fluid.

Note two: I bent the lip of the transmission pan about 5 or 6 mm out of flatness when I pried it off of the transmission. To remedy this, I laid the pan on a flat surface on a piece of cardboard with the sealing surface down. Using a narrow piece of oak and a hammer, I put the wood onto the backside of the sealing surface (not the metal edge but just inside it) where I had bent the lip. I tapped end of the stick of wood rather firmly ONE time with the hammer. I checked the pan for flatness with a ruler, and stopped. All it took was one sharp blow.

Note three: Get all, I mean all, of the old sealant off of the transmission body and the pan. Then use brake cleaner to clean the surface before applying new sealant. It is a lot of work, but worth the good seal that you will obtain.

Note four: The temperature sensor can be tested in a pot of water for resistance. I was able to find the resistance figures here. They are quoted below. The same link gives excellent pictures of the same procedure:

Quote: At a [TRANSMISSION] oil temperature of 0 degrees Celsius, the resistance of the sensor should be 16.7-20.5 kilohms, and at a temperature of 100 degrees it should be 0.57-0.69 kilohms. If the measured resistance does not match the above values, the sensor is for replacement.

How easy is the job? if you can change a transmission filter you can do this job. It requires 1 piece of copper wire about 8 inches long, 5 quarts of Mitsubishi transmission fluid, an after-market filter, a pan gasket, some RTV sealant, and a lot of patience with spilled and dripping transmission fluid. Total cost was less than US$100 for me. You may find the fluid more expensive than I did, so you might budget US$150.