Cruise control slowly accelerating


I can’t seem to fix a rather annoying issue with my cruise control. Once set, it will continue to slowly accelerate. When I am doing about 50km/s the revs will ever so slightly creep up and it won’t stop until I disengage.

I’ve read similar topics where changing the ABS accumulator fixed the issue. I however do not have any issues with ABS. No lights to report and the brake lights also function properly.

Any suggestions are greatly appreciated.
Matthijs van Zetten

Welcome to the forum. I too have this same issue. I posted info on someone else’s post on the same issue, but have not been able to fix the problem. I have not replaced the accumulator like the other poster had done. In that case, the CC issue went away after the accumulator change out, though no one weighed in on how that makes sense. I just noticed another curiosity - I know of only three posters who have had this issue, and all three cars are model year 1992. I confess to have no idea what this means other than coincidence.

Atleast I am not alone! Yes it really is strange. It seems to function better at lower speed.

When I am doing about 50 (km/h) the creep is really slow, where as I am doing near 120 it’s quite fast.
I can feel the pedal being pulled further by the cruise control system.

But I am hesitant to change a part out when I have no clue whether it will fix anything. Especially since since the part is around 150-200 euros.

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?

Wow! Great write up. Thanks for your investigation.
This could very well be what is happening. Althought it’s a mystery to me why the cruise accelerates slower when you are actually moving slower aswel.

I could remove the cruise control panel and see what button is sending what current maybe?

After further studying the wiring diagram and the input/output information (below) my previous statement (above) is off the mark:

The inputs from the cruise control panel Accel/Set and Resume buttons to the ECU are voltages as shown for input LB66-17.

I see. So could it then still be a sticky set switch? I don’t think somebody ever ruled it out?

Just to add I had an example of this on my ‘92 ‘40 as well but that was a fleeting instance and it cleared up by itself, so no help with the diagnosis.

Well, now there are 4 posters of the same, or similar issue. And, all with 1992 model year xj40s. Fifty percent of these had their issue clear up without any targeted fix. So, I believe if I wait long enough, I have a 50/50 chance of it fixing itself! Lol

Seriously though, in reading non Jag specific info on this issue, one article (and only one as far as I saw) blamed the control unit. So i did some digging into xj40 specific control unit space. The part number is DBC1169 and is the same for all years from at least 88 to 92 (didn’t look at 93 and 94). However, in looking on ebay for used units, I noticed the many sellers varied in the years they called out, and in their pics there was an “issue 4” or an “issue 5” printed on the unit’s enclosure. This indicates to me that there were at least 2 different major control unit versions by the German manufacturer. Possibly there were numerous EPROM updates for each version as well. I write all this because it’s an interesting topic to me, and maybe to others.

Of course, electrical connectivty could be to blame, say, for example, the control unit not getting a reliable speed feedback signal.

All of this is merely food for thought because I have no definitive solutions.

Gentlemen, any time there is supposed electrical-based weirdness going on with these XJ40s sound advice is to try cleaning the connections, especially the ground connections. Larry (abercanadian) is chasing an issue with his climate control (a/c) system - here is a link to that thread with info about the logic grounds that should be checked / cleaned:

Probably worth doing as a matter of preventive maintenance, even if it doesn’t cure this c. c. issue.

I’ll look into it. I don’t think I can use that diagram sadly, my car is a battery in the front type. Ill have a go at cleaning the CC connections anyway and while im in there I can measure what the set button is putting out.