So, after my last shop visit, the cruise control stopped working.

I am getting speed readings, as the trip computer is registering.

The fuse is good for the pump, which is located on the drivers wing, close to the forward radiator bulkhead.

Surprisingly, the vacuum output of the pump is Tee’d. One line runs to the cruise control bellows (which is actuating and holding vacuum). The other line is marked with an orange stripe, and disappears down somewhere the front of the engine. Is that to a dump valve?

So, I have cut the orange striped line and plugged it an allen wrench. Headed out on the highway in a bit to see if it works. I figure the worst is that I get to test the top speed of the beast.

 

Well, that didnt work...

 

Are you certain that the CC is connected to a 'pump'?
Most of the CC systems I've worked on operate on engine vacuum.
On my 94 v12 that was from the port at the back of the right hand side cylinder head,
That port also served the heater AC controls and the ECU.
If you cut a vacuum line, did you plug both ends?

 

6 cylinder cars have a dedicated cruise control vacuum pump. It’s separate from the engine vacuum

 

He has a 92 V12.

 

This definitely has the pump. It is on the drivers wing, just behind the radiator

 

If its a '92 facelift v12 5.3 then look here at pat 4. this melts and gets all soft and pulls out of the holder part 2


https://parts.jaguarlandroverclassic.../brand/jaguar/

 

I wish it was that, but I have already checked it.

I think I have to test the motor, both for running with a direct 12v, and checking the harness for voltage.

 

Replace part #4 with Nylon or better Phenolic.

 

Knowing you just got it back from the shop is like a picture worth a thousand words.
It doesn't matter if your car is a v12 or a 4.0 or anything else.
Put a piece of known good vacuum line on the bellows and suck.
If the bellows collapses then work away from there until you find a cracked hose.
Once you get to the pump, and you know it's a working unit then you have other problems.
Make sure the brake pedal switch is working correctly.
With the slightest pressure the switch should show a change in conductivity.
I don't recall if the switch is normally on or off.
These things are the most likely failures for the symptoms.

 

The bellows are working. I want to check the pump. Where is a good source of 12v under the bonnet? Is there a battery post up front?

 

There's a post on the firewall on each side of the engine under a rubber boot. They are rather hidden at the back of the engine. Be careful as it's easy to short to adjacent metal items. The posts are directly connected to the battery thru a large cable.

 

There are two B+ voltage junction posts on the firewall. Hmmm.....how to describe location?

Look at the firewall immediately aft of the cam covers, then down a bit, then outboard a bit. You'll see heavy cables. The posts will have rubber boots...if they haven't turned to dust

Cheers
DD

 

I have the same Issues with my 92 XJS 5.3 Watching with interest. Good Luck!

 

I'm not sure about V12s but late 4 litre cars have both +ve & -ve bulkhead terminals so be careful

Paul