Question 1994 New Yorker. Took to mechanic for bad PRNDL switch showing R when in P, and car in Low gear limp mode. Car speedometer also went crazy showing 70mph when in P. When I shifted the car into D the speedometer went back to zero but car would not shift out of low gear. Speedometer did not work correctly as when the actual speed got to around 30mph it would show around 100 mph before the car would not go any faster and nosedive until the speed fell below about 30mph.
The mechanic replaced The PRNDL switch as well as both speed sensors. The car now shows PRNDLE lit up in park as well every other gear selector. All stay lit when when shifted into all other gear selections. The car now stays in 3 with the speedometer and tach working properly. I can drive the car up to at least 60 mph(maybe more) and it never shifts into D. I assume I have a bad TCM. Key codes are 12 and 55 so are of no help. Any advice on Isolating the TCM.
Answer Hi Bob,
I believe that you driving in 2nd gear only which is the strategy the TCM adopts when there is some fault code stored in the memory which is mitigated as much as possible by going into that default "limp-in mode"*. The TCM memory is accessed only by a plug-in code reader (OBD I type) via a socket under the dash by the steering column. The codes are also 2 digits so you do have to be sure to have a reader that will communicate with the TCM. A shop with a Sbap-on reader of that vintage should be able to get the code(s). Ask for the number, what it means, and what they recommend. Then write back and we'll compare that to the manual.
*The way it goes 'limp' is to open the trans relay #9 in the power box. You could test that situation out by removing the relay and jump a wire from the front to the rear pin of the socket which should restore the multiple gears, but be aware that there is something wrong which needs attention to avoid further damage.
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