A Case of the DTCs

Or to give them their full name, “Diagnostic Trouble Codes”. I have a Jaguar XK8 in the workshop at the moment which drives everywhere in “limp mode” and displays “Gearbox Fault” on the dashboard.  Plugging in the laptop reveals a DTC of P0706 which translates as,  “Transmission Range Sensor Circuit Range/Performance”

Now having a case of the DTCs is perhaps a bit like having the DTs, only a little more alarming when you discover the uncontrolable tremors in the wallet region upon finding out how much the manufacturer charges for that little widget which has failed. It can also, like the DTs,result in hallucinations which leave people believing (or at least wishing to believe) that if they clear the code it will just go away….

The truth is that although technology is advancing steadily, basic concepts remain ever the same: Fougasse and McCullough, in their motoring handbook of the 1930s described the temperature gauge like this: “A clever little device which indicates how hot your engine is. If it points to hot you need more water or a new fanbelt, a new radiator or a new engine, or else a new little device.”  DTCs are a little arbitrary in the same way as the old fashioned type of warning device.  If it says “P0706” you have faulty cable adjustment, a faulty earth, a faulty J-Gate micro switch, a faulty connector block, a faulty rotary switch, a faulty input speed sensor, or a faulty Transmission Control Module.  New rotary switches are in the region of 250 pounds; no one usually dares to ask the price of a new Transmission control module. Therefore it’s going to make good sense to  test the circuit thoroughly before condemning anything.

The cable adjustment was indeed a little out, but correcting it and clearing the fault code hasn’t helped. I’m suspicious that it will turn out to be the rotary switch – particularly since this one shows evidence of having been disturbed before.  It works by generating a 4 bit binary coded signal over a parallel interface, and Jaguar are thoughtful enough to document the signal sequence.  When I’ve removed sufficient bits of underbonnet trim to excavate as far as the multiplug this will be the next thing to check.

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