Preparing for restoration of my XJ40 Daimler ISSUES

I have recently bought my first Jag… finally after being a JAG lover for years, i managed to buy a 1992 XJ40 Daimler 4.0 LHD . The car was a daily driver in recent years, so it has 200.000 kmrs on the dash but the engine and automatic gearbox are in good working conditions (i hope so :slight_smile: )
i have done a list of what does not work on the car, as i want so repair it before some internal and bodywork restoration:

    The abs light turn on when i switch the key and turns off when the engine start so this is a normal behaviour. While driving if i push the brake sometimes the pad light turns on and seldomly also the ABS light , i have tried a panic stop anche the ABS does not works as the wheels lock.
    Reading on forums the problem can be a wheel abs sensor not working or the abs accumulator… any suggestion?

    Heating fan works only at maximum speed or defrosting , the heating panel seems to respond to commands.
    Here i think is the Darlington transistor failure in the fan assembly. But how can i check if both fans are running or not ?


if i turn the cruise on and set speed, the car start to accelerare very slowly and does not stabilize the speed , as it continues to speed up until i disengage the cruise. Here i have no idea of the cause and solution.

Maybe someone can help me for those issue troubleshooting?

Welcome to Jag-Lovers!
Unfortunately, you are on the wrong list. Your XJ40 has very little in common with the Series I, II, and III XJ6s and XJ12s discussed on this list. The XJ40 was a completely new design and unlike our older models.
I recommend that you post on the Jag-Lovers XJ40 list where members with your model are likely to help.


Hi Andrea, and welcome! You will find LOTS of information, support, and encouragement on this forum. Great bunch of folks that share their collective knowledge, wisdom, and occasional mis-information :smile: here.

As for your questions:

  1. Now is a good learning opportunity - Jag-lovers, prior to re-inventing itself as, was, and there is stil a treasure-trove of info and photos archived there, including an e-book of helpful technical info including this:

  2. Re your cruise control issue, I learned through my own stupidity that (a) the module that controls the vacuum to the c.c. bellows is in the engine bay, left side, forward of the wheel arch and (b) there is an open port on the bottom of that module, and (c) that port must remain open [uncapped] so the vacuum can be relieved, and (d) if that port is capped the c.c. behaves exactly as you describe.

Hope this helps.

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thanks for the suggeestions… i will do some tests!

Andrea - circling back on my comment about the cruise (speed) control:

Based on this diagram of the control system:

From “

I believe I mis-spoke about the effect of capping the open port on the pump module (item 1 in the diagram). There is a separate dump valve (item 2) that would dump, or relieve, the vacuum. Nevertheless, as I originally stated, capping that open port on the bottom of the pump module (item 1) caused the system to malfunction as you describe.

Mike, i remember from another post that you saw the empty port and plugged it. When you unplugged it your cruise went back to normal. My 92 cruise exhibits the same malfunction so I took a look at that open port. There is nothing stuck in there intentionally, but I’m wondering if you can envision something else blocking it, like insect nest or the like. Do you think it would be safe to use canned air on it?

Roger - as I stated in my previous post (the one with the diagram of the components) there is a separate dump valve that could be the issue if it can’t open up to break the vacuum. You might locate that and check it out.

The dump valve works fine when triggered by either pressing the brake pedal or by pulling the speed control switch toward me and then releasing. Both of these actions cause the cruise to disengage immediately, even in the failure mode when the speed is increasing steadily. I assume this indicates the dump valve is working.

With numerous times trying to set the speed, it sometimes works properly, about 1 in 5 tries. Similar results when using the “resume” feature.

In thinking about it, I can see where a sticky “set” switch might cause this, since the dump valve can be ruled out. It doesn’t seem like a pump failure would cause this, nor an actuator failure.


Roger - perhaps this will help with your troubleshooting:

Cruise Control.pdf (45.3 KB)

Note the grey/blue wire that goes from the c.c. accelerate / set / resume switches to pin 17 of the c.c. ECM. If those switches are ‘sticky’ you should see continuity to ground from that grey/blue wire even when the switch is released. Of course, continuity to ground would be normal when the switch is activated.

I haven’t had to deal with any cruise control issues and TBH I never think of using it (!) - however, as this is a vacuum operated system, seems to me that any “creep” would point to a small vacuum leak in the system?

If the “creep” was a reduction in speed, maybe. But in that case, there is a dedicated vacuum pump for the system that would kick in when the speed decreased. The issue here is a continual increase beyond the set speed.

Mike, thanks for the circuit diagram. If I understand the diagram and your explanation correctly, I wouldn’t need to have the system on to test for set switch stickiness. Now, just need to access the wire…

Larry, thank you as well.

I agree. Good luck and keep us posted (pun intended).

In at least one way, the theory of the set switch sticking makes little sense. For one example, if the set switch sticks causing the constant increasing speed, and one turns off the CC causing the dump valve to do its thing, upon turning the CC back on, wouldn’t the stuck set switch cause the whole mess to start up again? In the many times I did this very thing, that result never happened.

I’m now wondering about the “vaccuum pump & control valve”. The circuit diagram shows a symbol in its box that’s identical to that which is in the dump valve box. Could it be that the vaccuum adds and subtracts during speed maintaining are all done by the pump assembly based on ECM signals, and the dump valve is only engaged for the immediate complete disengage as in brakes applied? In this scenario, if the pump can’t vent a little vaccuum to reduce speed, the ever increasing speed would be seen. Might this explain why the CC goes in runaway mode when that mystery port on the pump is plugged?

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Could be. I have not been into the vacuum pump and do not know how seviceable it is, but it may well be worth removing it to check its operation and see if the one port is open. If it is plugged, or partially so, I wonder if applying vacuum with another vacuum pump (like a Mityvac) would clear it?

Think I’ll whack it with a hammer or something like that.

Well I tried the hammer to no avail. Next step will be to examine the pump and see what I can see.

I’ll post any findings or lack thereof. It might be a little while before I get to it, however.

Roger - if you haven’t already done so you might carefully and gently probe that open port with something like a straightened paper clip to see if you detect any blockage.

I did stick the small tube that came with my can of canned air up there and it hit a hard stop after about 1.5 inches. I assume this is expected. I did this by reaching from under the unit with the air cleaner assembly still in place. I was going to shoot some air up there but couldn’t find room for the can.

My next steps are to remove the air cleaner and remove the pump and dump valve. Inspection and replacing all the old vacuum hoses will be done while it’s out. Depending on what I see, I’ll move on to bench testing the pump assembly, once I figure out how to do that.

I appreciate your interest in and suggestions on this issue. It’s not the typical failure mode for CC and every idea is welcomed.

Rog - simple thing to check - on my c.c. vacuum pump / control valve unit the inboard port (the one nearest the engine) on the bottom of the unit is connected to a vacuum line, and the outboard (nearest the fender well) port is open. I am guessing if this was reversed the c.c. would not work at all.

With the ignition off, I used a Mityvac hand vacuum pump to apply vacuum to both ports and both were open (would not hold vacuum).