odd gearbox fault
 

Got an odd fault with the wife's 2007 Chrysler Grand Voyager fitted with the 41te auto transmission.

Gearbox appeared to be in limp home mode, 2nd gear only but no DTC's were stored. A bit more investigation showed that the TCM thought the gearbox fluid temperature was -19 degrees C which is why it was giving me 2nd gear only.

The odd part is that both my X-431 diagun and taking a direct reading with a volt meter both showed that the voltage from the gearbox temperature sensor was giving the correct reading, about 3.6V when cold and then dropping down to 0.6V after being driven round a while an warming up.

I've bypassed the problem by cutting the wire from the temp sensor, this fools the TCM into thinking the temp sensor has failed and it then uses the engine coolant sensor and others to estimate the gearbox fluid temperature and it seems to work fine with the gearbox changing up and down as it should.

What I'm confused about is why would the TCM report -19 degrees when it can also see the correct voltage? is it a faulty TCM? if so it's a bit odd for just that to fail???

any ideas?

Regards

TFB