Chrysler Repair: 2000 Dodge Dakota Transmission: code 0731, speed sensors, oem code

Question
QUESTION: I recently had a engine light come on on my 2000 Dodge Dakota. All my fluids levels are good and clear. Problem: it shifts from 1st gear to second gear (automatic trans)and will shift between the two. However, it will not shift to 3rd or 4th. I had a OEM code check and the code # is P0731- "Detected a incorrect gear ratio condition in 1st gear." Please Help!

ANSWER: Hi Vic,
About the only possible non-expensive expanations for this code, in the absence of any other code, is that one of the two transmission speed sensors is starting to get flaky. Those sensor readings are what is used to calculate the speed sensor ratio for each gear, so if one is inaccurate that will cause a ratio error. Another approach is to test the 'low' clutch by means of a diagnostic readout box controlling the operation of the transmission. That would be something that a transmission shop can do. If you pass that test then focus on the sensors. If not, then it means opening up the trans and doing some clutch part replacements.
The speed sensors are located on the left side of the transmission and are easy to replace if they are accessible. I don't know the layout of a Dakota so can't tell you about that. They cost about $80 each. So you could try replacing one or the other (if the speedo looks accurate then try the turbine sensor first) and seeing if that corrects the problem rather than going to an independent trans shop for the clutch evaluation.
Roland


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QUESTION: There are two sensor that I have identified on the AT. One towards the front of the AT and on towards the rear (meaning the rear of vehicle). Can you tell me which is which and what one would you start with. 1st gear seems to be ok, but second seems strange.

Answer
The front one is the turbine sensor, the rear one is the output speed sensor. If the speedometer seems to be functioning as normal and similarly accurate as in the past, then that would help me to believe that if the issue is a sensor inaccuracy it would be the turbine sensor that is suspect. However, that is just a guess on my part; I am uncertain what the % error in the ratio has to be to set the code so it may not be noticeable as a speedo error if it is the output speed sensor.
The manual suggests doing the first gear clutch test first, then if it passes that test, make sure the gear shift lever is appropriately 'set' relative to the shifter on the transmission, and finally check the speed sensors. The clutch test basically involves monitoring the turbine sensor while with the engine running and the road wheels lcoked and the trans in low you momentarily increase the throttle angle and note whether the turbine speed responds (e.g. the underdrive clutch or l/r clutch slips), But I would let a trans shop do that test with the aid of a diagnostic readout box that will measure the turbine speed, and do it safely.
Roland