To All

I discussed in a prior thread that my cruise control stopped working. I have checked a number of things in the cruise control system. One thing that I have not investigated is the speed interface.

I have it on the bench and would like to know if anyone can give some advice on testing.

If I understand the CC system it works like this.

Assumption: Cruise control switch is engaged and everything works

Speed sensor send signal to speed interface
Speed interface sends signal to vacuum pump which draws vacuum on bellows
This holds the throttle in position to maintain speed

Very basic but I think this is correct?

The one thing I noticed when I pulled the speed interface apart. It looks that there are 3 wireS on the backside of board that are soldered together. These three pads were dirty so I cleaned with some contact cleaner and brass wire toothbrush....Cleaner but looks intentional. On the top side they are separate. One connector has three wires and the other has 4. Of the 7 Separate solder points on the top side the black, light blue and 1 bare wire (looks to be part of a fuse able link) from the three wire connector. So on the bottom side you have 4 solder points.


SORRY about the rambling but this whole has me puzzled.

Thanks for your help

Softball60/Paul

 

Some pics might help here.

 

Vee,

Thanks and I will try and figure out how to upload photos.

Softball60/Paul

 

Hopefully attached are a couple photos
First attempt at attaching photos

NOTICE ON THE BOTTOM THAT 3 OF THE PADS ARE SOLDERED TOGETHER?????

Softball60/Paul

 

That's odd. I will have to check mine as well. My cruise doesn't work either. I have swapped out the obvious parts, the dump valve, the cruise ecu in the trunk and one other part, all without any luck.

From the picture, the blue wire solder looks bad.

Also, I know that the brake switch is also somehow tied into this as well, isn't it?

 

Vee,

Thanks For pushing me to figure out how to attach photos.....ONE SMALL STEP FOR PAUL.....ONE GIANT LEAP FOR THE JAGUAR FORUM!!!!

I will check out the solder point on the blue wire.

What’s early strange to me is that one (1) of the three (3) solder joints together is the black wire.....Normally a ground? I would think, Not.

Ordered a speed interface and a CC unit both used but the total was under $100

Thanks

Paul

 

So far no luck with the repair of the cruise control.
Here is what I have tried.

1. Replaced the on/off console switch (Console)
2. Replaced the Speed Interface Module (Trunk)
3. Replaced the cruise control computer (Trunk)
4. While still in vehicle put 12V power to the vacuum pump and it worked.


Did notice one (1) thing when powering up the vacuum pump and that was that the blow off valve was blowing air while pump was working. The vacuum pump pump did not seem to pressure up the bladder on the throttle linkage. I believe this is what sets the cruise control????

Don’t know if the valve was supposed to be bleeding off air while the pump was running???????? Or if it releases air when a signal is received to turn off CC.

If anyone can give me more input I would certainly appreciate.

Vee,

For your info if you haven’t checked yet, the three(3) points on the solder board on the speed interface are in fact soldered together.

Thanks

Softball60/Paul

 

Thank you for following back up. I have also:
1. Replaced the on/off console switch
3. Replaced cruise control computer
4. Put 12v power to the vacuum pump, and also replaced it!
5. Tested (put 12v on it and heard a click) and replaced dump valve. (LNA1950AA)

The last item to check would be the brake switch?

I haven't gotten around to crossing that one off my list.

 

What does the brake switch have to do with the CC working?



Thanks

Softball60/Paul

 

I come across posts that mention it is involved. See last post on bottom:

https://forums.jag-lovers.com/t/xj-s...rking/331783/3

 

You don't mention if you checked the integrity of the vacuum system.
I just had a long standing problem on my current daily driver.
The first thing I checked was the vacuum.
Guess what, one of the hoses was cracked.
Job done

 

Equiprx,

Did give the vacuum system a once over check. Did not see anything that looks bad. All the rubber looks solid.

I did mention the 12V test on the v a umm pump.

Any ideas?

ThanKs

Softball60/Paul

 

My vacuum system is good too.

 

I do have brake lights!!!, so it is not the brake light switch.

But I understand the brake switch is different
Where is it located under the dash, drivers side or passenger. According to the test procedure. (Engine Not running) 1. I disconnect the plug at the CC Module located in the trunk located on drivers side wheel well, 2. Turn ignition on, 3. Drop into Drive.

Should have 12V at brake switch.

Thanks

Softball60/Paul

 

I'm out of ideas.

 

I went back over the vacuum pump.

1. Put 12V power to the pump and pump runs.......I placed my fingertip over the open port located at the bottom of the pump ( one of the ports is open the other runs to the rubber chamber on the throttle side) and it looked to suck the rubber chamber in.

I tried it several times and for some reason it would not suck a vacuum.....Is it possible the pump is losing its ability to suck a vacuum.

2. When I put 12V to the other contacts for the dump valve I can hear the clicking sound.


This has me puzzled as it seems that while I am driving the car and doing these these various things it seems like when I set the CC master switch it feels like the CC is trying to works but doesn’t have enough suction to fully set the system.................IS THIS POSSIBLE!!!!!!!!!


Than ks

Softball60/Paul

 

It's highly likely that your issues are all vacuum related.
Pump needs to be rebuilt or replaced.
The smallest bit of debris will completely destroy a vacuum.
Rubber was never meant o last for 30-40 years.
I would completely remove all the hoses, pump and solenoids.
Check everything on the bench and retest.

 

Equiprx,

When the dump valve was n out energized with 12V I got n o resistance when blowing or sucking. When it was energized I could not blow or suck.

Does this mean that the dump valve is OKAY?

Thanks

Softball60/Paul

 

I will check my spare when I get home today!

 

Isn't the source for vacuum generated from the intake manifold?
One aspect of a vacuum check is how well it holds a vacuum.
You should be able to get at least 10-15 inch/ Hg and hold it for an hour or so.
Get yourself a "MightyVac" test kit or a suitable knockoff..
It's probably less than $25 and a good diagnostic tool.