Cruise Control problems and diagnostics
Several problems can arise with the cruise control system. If your cruise control will not work at all, the first thing to check is the cable attachment to the throttle under the hood. It can pop loose, it can easily be popped back on, and a small piece of wire to hold the cable's attachment in place usually solves the problem. Another problem with the cruise control can be a defect with the vacuum reservior - a SHOtimes member describes the problem and a fix below :
After changing my starter last night to a new 12 toother, I noticed that my cruise control vacuum reservoir by the starter was in the same vicinity so I pulled it out to inspect it (since it's been inoperative since I bought the car). I figured I would give it the once-over in addition to making sure that it didn't have any major, obvious defects as I know my cable is attached/not binding/etc. Well the reservoir did have a defect. Just before I bought my car, the clutch was replaced. Also, the preivous owner stated that the cruise control worked despite my questions to him a few days after I took delivery. It appears that when the clutch was replaced that the dealer must have broke one of the vacuum nipples off (closest to the engine/transaxle), causing there to be a vacuum leak to the reservoir which was effectively rendering the cruise control inoperative. Also, I noticed that these vacuum lines don't fit on real tight and could easily become detached if you are wrestling with something down in this area of your car (starter/oil filter/etc.).
With some liberal application of Loctite 495, the nipple was as good as new and my cruise control (happily) is working for the first time (for me).
If neither of the above problems is what is causing your cruise control to fail, try the following cruise control diagnostic test (Thanks to Matthew Hepner) : You will need a voltmeter/ohm meter.
First if you have an MTX SHO you will need to find the light blue wire with a black stripe going to you engine processor it should be on pin 28.
If you have a ATX SHO you need to find the same color wire at the cruise module under the hood it should be pin 5.
Other ford products either use the ATX style or have there own computer under the dash but look for the light blue black striped wire.
This test takes two people. With the volt meter check voltage to the wire with some one pushing the on button. There should be 12v. this tells the cruise to " wake up" ( it always has power).
Now switch to the ohm setting on the meter while having some one push the buttons look for these resistance values.
OFF 0 ohms Resume 2200 Ohms Set/Accel 680 Ohms Coast 120 Ohms
all readings are plus or minus 5%
If you do not have these readings test the switches at the steering wheel.