Hello,
My cruise control has weak vacuum (it tries to engage but doeesn't stay engaged). I have traced it to the brake/cruise switch area.

IF I disconnect the line at the brake/cruise switch and seal the hose with my finger, the cruise control engages and works well. But, when I reconnect it to the brake/cruise switch, it engages but does not pull strongly enough to keep the car at speed.

What do I need to do here? Does the switch need to be replaced? If so, what is it called, and where can I find one?

Thanks a lot!

Howdy xluke,

What do I need to do here? Does the switch need to be replaced? If so, what is it called, and where can I find one?

You should always indicate the model and year of the car you are working on.

Volvo calls it a valve. I would call it a vacuum switch. Whatever you call it, it is not something that would be in the aftermarket. The junkyard would probably be the place to get it.

Before you run out to find one, it may just need to be readjusted. Get a length of vacuum hose and connect to the switch so you can blow into the hose. Push the brake pedal down so you can use your finger to actuate the switch. Does it close off completely when you push up on the actuator? If so, it is working. Adjust it until the brake pedal closes it completely and reattach the system vacuum line.
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 52 year old fat man. ;-)

'94 940 wagon.

Parts/tools needed: vasoline, small straight screwdriver. Optional: 3/4 or 15/16" socket.

The switch was making good contact. I took the switch out (simply turn it counterclockwise like a bolt. As a tip, you can use a 3/4 or 15/16" socket to save some hand strength but be careful not to over-torque and crack plastic). I then popped the 4 clips (small straight screwdriver) that hold the unit together to inspect the inside. The mechanism is simple. There is a plug that goes into a small ring gasket to seal the vacuum tube. There was some dirt in there so I cleaned it out. This alone may have solved the problem but to be sure I put some vasoline on the end of the plug/plunger so that it would make a better seal. I reassembled it and it seems to work great. Road test to follow.

General diagnostic strategy for cruise control vaccum leaks:
1) test cruise control diaphram by removing hosethen press down on the diagphram so it collapses and hold the vaccum tip closed with your finger. The diagrpham should stay collapses. This indicates that component is OK.
2) test brake switch by attaching a piece of vacuum line to it and blow. You should not be able to blow if the switch is working correctly. Depress the brake and you should be able to blow again.
3) test individual lines by removing them and attach them to the diagrpham and see if you can create and hold a vacuum by sucking as indicated by the diagphram collapsing and staying collapsed. This is a good way to check for leaks in the lines.

Thanks for the tip on blowing into the unit, that was the magic bullet!