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P0710 (DTC 37), changeover switch, heater valve question


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2000 Boxster 2.7 Tiptronic, 115k mi.  I am chasing a P0710 (DTC 37) error code (using Durametric).  Consistent with the repair manual, I checked continuity and resistance on the appropriate pins at the transmission socket and at the ECU connector, all good.  I checked ATF level, also good.  As the repair manual states, the heater valve is always open (allowing coolant to flow to the ATF cooler).  I confirmed mechanical operation of the heater valve by pulling the SAI vacuum hose off of the changeover switch (99660512301, now 7PP906270) and noting that the lever on the heater valve is operating.  The changeover switch is pulling vacuum at completely cold start, confirmed at the nozzle on the changeover switch.

 

I understand that the ECU should not be telling the changeover switch to allow vacuum to the heater valve because the temperature is ~25C at startup (with matching resistance measured at the pins ~1.05kohms).

 

My question is...should I conclude that the changeover switch is malfunctioning or is there something else I should be looking for as a cause?  I have read that these switches go bad on other vehicles, but have not seen anything specifically about 986's having this problem.

 

Thanks for your help!

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DTC 0710 indicates an open circuit or short to ground in the transmission temperature sensor   Normal faults are wiring on the sensor harness, the temp sensor itself, or the Tiptronic control module.

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Thanks JFP.  Wouldn't the continuity checks and resistance measured indicate that the harness and temperature sensor are ok?  I was expecting the harness / temperature sensor to be the culprit when I started down this path based on the other posts that I have read about these issues, but the electrical checks appear to indicate that they are ok.

 

I forgot to mention that driving/shifting have not been any different than normal...in case that is any indication of the control module itself.

 

 

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