After dealing with ever increasing limp mode events and associated DTC's with my 2004 XK8, I'm convinced that the TPS is the culprit and needs to be replaced. Fortunately I was able to find a NEW 198500-3300 TPS on eBay from a guy in Taiwan.

Looking at the existing TPS on the throttle body, it clearly has some adjustment range and even has a glob of white “calibrated” paint on one of the mounting screws. My supposition is that the TPS is somehow calibrated to the throttle body; likely explaining why the TPS and TB are always sold as an assembly. I've searched the forum and the Internet but can't find any procedure to calibrate a new TPS to an existing TB. The rebuilders clearly know how to do this but I guess they consider that their secret sauce 😀.

So what I've done is carefully measure the voltages on both the TPS track wipers at idle. (see attached drawing) Then, after installing the new TPS, I'll adjust it to the same voltages. I'll probably adjust to the 3.48 V across pins 2 and 4 and verify that the others track.

So, QUESTION: does this sound like the right approach? Has anybody done this?

 

Hi.

Checking in WDS/IDS for my -97 (AJ26 engine) gives idle range 0,68 - 0,98V, and throttle fully open minimum 4,87V.

As you have a 2004 (AJ34 engine) the throttle body is completely different so the values may not be valid.

Now checking again in the WDS/IDS for a 2004 using a random VIN only gives a value of 0 - 5V.

The best solution would probably be to calibrate the TPS with a WDS/IDS connected to the car.

 

Found this technique, but was hoping for something more scientific

 

Spent this morning on the internet researching this. Didn't find anything specific to a XK8, but found lots of YouTube videos on TPS adjustment on othe cars. With few exceptions they all recommend setting the TP1 voltage to between 0.75 and 0.95 V with the throttle closed. This corresponds reasonably well to the 0.71 V I measured on the TP1 wiper.

I think I'm over analyzing this and should just do it! ������

 

I'm surprised no one has come back with a definitive answer on this, but for what it is worth your approach of setting the new tps to the old measurements sounds like the right way to do it.

Please report back as to how you get on.

 

OK, I replaced the TPS today. I set the voltage to about 0.75 V. Everything seems to be fine - idle, full power, etc. My intent was to set it to about 0.85 V but that set the idle way too low. There may be some other interactive adjustment here with the throttle servo or something else to get the TPS to 0.85 and the idle at about 650, but since everything was working well I didn't spend any more time on it.

Now, of course..............a few months of driving without limp mode will determine if the TPS was _really_ the culprit. ;-)