The cruise control on my 1991 Jeep Cherokee have never worked since I've got it about 5 years ago. I've replaced the hardlines I've checked the vacuum Reservoir behind the bumper. I've replaced the cruise control stalk with a new one. replaced the module under the hood for the cruise control with another used one. that I disassemble befor install and checked all individual servo to make sure they completely seal and checked the vacuum diaphragm. the cruise control does engage but only at a high rate of speed say above 55 miles an hour and you must hold in the set button on the end of the stock once you release it it shuts off. even then it doesn't do it every time I push the button at that speed. I tried swapping out the PCM for a another 91 pcm that was a auto ( mine is a manual ) I could not locate a manual PCM locally but with that PCM it worked in the exact same fashion no change. I tried that as I was told 91 - 92 were controlled using the PCM for the cruise control. as 91 did not have the little yellow box under the dash like older years that operated the cruise control functions. I cannot locate any manuals for that specific year online from Chrysler that tell how to diagnosed problems in the cruise. everything I am running across is either older with the yellow box or newer where it has the buttons in the steering wheel. I am also including a attached picture of how much vacuum I'm getting at idle at the cruise control module. any idea on anything I missed would be great as I would like to fix this problem?

 

After all these years I finally fixed it when I originally tried an new ECM for my 91 it was a automatic ECM which doesn't allow the cruise control to function in a manual car. you can tell it's an automatic ECM because the shift up light doesn't work anymore. The ECM the car originally came with when I purchased it was a manual ECM but It was from a 96. Car still function completely fine including the shift light that's how I knew it was an actual manual ECM. but cause the cruise control to only functional while the button was held down. Acquired an original service manual to discover that if you have a 91 or a 92 the ECM must be from a 91 or a 92 and it must be automatic or manual according to whatever your car is. Anything else the car will still function just fine just will not have a ability to work the cruise control properly. Took me a while to find the proper ECM and I had to buy it from the complete opposite side of the country but it fixed my problem completely and my cruise control works as it should!

 

Good to hear that you got it sorted and thanks for posting back, even if no one was able to chime in back when you first posted. It's useful as it may help someone else with the same situation in the future.