Hi Rog, it is curious that the three cars having the same issue are 1992 models. I had a quick look at the parts site to see if there was a component unique to the 1992 MY but if there is I didn’t spot it.
I’m sorry your c. c. is still not working properly. I revisited this thread in which your cruise control issues were first discussed:
Preparing for restoration of my XJ40 Daimler ISSUES - #6 by Rog?
The possibility of a sticky ‘Set’ button was raised but I don’t know where that ended up, other than it didn’t seem a likely suspect for the reason you cited. Now, to add to the mystery, here is a snip from the circuit diagram:
Note that both the Accel/Set button (switch) and Resume switch communicate to the cruise control ECU using the grey/blue wire. I had not noticed or paid attention to this before now. Assuming these are simple off/on or open/closed type switches (1) why are there two separate buttons/switches, and (2) how does the ECU know which button is depressed? The answer to (1) is obvious - because the ECU needs to ‘respond’ differently to each case. The answer to (2) is likely found in the fact that in addition to the switches there are also resistors in the circuit. My assumption - and it is purely an assumption, because I have not been inside the switches panel, nor metered the resistance of each switch - that those resistors have different values. The ECU could determine which button is depressed based on the resistance seen on that grey/blue wire.
Is it possible the ECU is sensing resistance on that wire (being interpreted as a depression of the Accel button) even though the button is not depressed?