Hoping to get my cruise control working on my 1994 4.0L. Checked the 'easy' things: fuse looked good, bellows seems intact. Where should I look now?

 

Next would be the vacuum pump motor and dump valve on the inner wing.

If you download the wiring diagrams from the sticky you can get a good idea of the signals going into the cruise control ECU also, and you can start to check all of that. On my 92 V12 the cruise ECU sits infront of the passenger seat underneath the carpeted cover.

I did all of that and the conclusion was my pump is dead :-( looking for a second hand one now....

 

I would almost put money on it being a fractured bellows. Remove it and check it for leaks don't just inspect in the car.

cheers

 

Ok, when I take it out, besides visual inspection, any other way to check it out. I was imagining compressing it and covering over the vacuum tube hole and seeing it it 'held'.

Also, someone suggested in a thread I read on one of the forums to check the vacuum to the bellows. Dumb question probably - how does one do that ?

 

That will work just make sure it does not leak from the vacuum port.

Best way is to use a vacuum gauge, you can also use your finger, put it over the vacuum line and it should be sucked onto the hose.

If I remember there is 2 vacuum solenoids that control the diaphragm check these while you are at it.

 

Haven't had a chance to pull out the bellows but I pulled the hose, collapsed the bellows and it stayed stuck with my finger. I started the car, turned on the cruise, and put my finger on the vacuum hose - nada, nothing, no suction. Should I still pull the bellows or concentrate on the vacuum side?

One thing I'm confused about, when does the vacuum take effect - whenever the car is on? when the control is turned on? or only when it is set?

 

Looks like the bellows is ok. The system consists of a vacuum pump and dump valve. The book is vague on how the system works but is looks like it does not use manifold vacuum but the pump to control vacuum to the bellows, and the dump valve that dumps vacuum when the system is disengaged.

Check operation of the pump you can use a 12v battery and make sure the dump valve is not leaking.

cheers

 

The Vacuum actuators from a 4.0 and the earlier V12 are 2 completely different systems. The 1994 system is the same as the XJ40 sedan system (and a VW Jetta system for that matter). There is a motorized vacuum pump and vacuum actuator for the 6 cylinder system and a dump valve that needs to close for the system to work. The pump and actuator use a vacuum for operation and the dump valve is the safety to keep the actuator from uncontrolled WOT (Wide Open Throttle). You can 'PINCH' the dump valve hose to eliminate the valve from the system to see if the cruise works, but it is a safety item and eliminating it from the system is a danger. When it fails, the valve will not close to let the actuator use the vacuum from the pump/manifold.

You can test the valve with a 12 volt power source to see if it operates normally. The last one I bought from the dealer was several years ago at less than a hundred dollars.

bob gauff

 

Thank you both for the responses. Are there any pictures of this system in place or if I post a picture of the system can someone show me were the actuator and dump valve are. The bellows is pretty easy to spot as is the throttle linkage. The vacuum hose seems to disappear down into the depths of the motor compartment.

 

The vacuum actuator IS the 'bellows'.
The dump valve should be under the wing strut bar mount point area. It has a stainless steel mesh on one end.

bob gauff

 

Ahh, thanks. Tried to follow the hose and I did see the part you are talking about. Dumb question probably, how do I use 12v to test it?

BTW, I'm from Denton originally.

 

You can 'pinch' the dump valve hose to see if the valve has failed. When the valve will not close, there is no vacuum available to the actuator (it is 'dumped').

If the system works with the dump hose blocked, then you need to replace the valve as it is a safety item.

You can put power to the 2 wires with it unplugged to see if it operates.

bob gauff

 

Finally was able to get to the Cruise problem again. I pinched off the dump valve (actually put an end over the t-out to it) and the cruise still didn't work. so what do I try next. Only three parts left right? ==> control module, switch, and vacuum pump?

BTW, system seems totally unresponsive. Listening carefully with all else off and hitting the button I don't here anything at all - ie listening for that vacuum to engage.

 

Reviving an old thread...

My cruise control didn't work when I got my '91 coupe (5.3 V12) in January. After cleaning all fuse bays it started working perfectly (along with some other things). Last week it just as suddenly stopped working.

The fuse is good (even tried another known good), and there is power there (13.8 volts, does that sound right?). The bellows looked good, but upon removal, it would not stay compressed while I kept my finger on the vacuum tube nozzle. Applying 12 volts to the wires leading into bellows yielded a click each time but nothing more.

There was no vacuum felt on the hose I pulled from the bellows. Tried with car ignition on Accessory, car running, gas applied, etc. Could not locate the dump valve to try pinching off its hose. Anyone have a photo of what it looks like and where it resides?

My best guess is that the bellows and dump valve are shot. Any other ideas?

 

Does anyone kknow if the dump valve is suppose to be closed when the cruise control is off ? I did a smoke test and smoke came from my dump valve when the engine was turned off .