7th July
written by Todd Harrison

I was working on my 1994 Jeep Grand Cherokee today. Just changing oil and replacing an upper radiator hose. I put my hand on my cruise control module and leaned a bit too hard on it – “SNAP”. I broke off the vacuum nipple at the base of the plastic molded housing. That’s the end of that! Well, my cruise hasn’t work for over 10 years so I wasn’t too sad.   I thought I might as well do an autopsy to see how it worked and if it didn’t work then what went wrong so long ago.

Here is the full module with vacuum diaphragm canister control and throttle cable.

Below are the control solenoids that dial in just the right vacuum to pull the diaphragm and throttle cable back to a selected cursing speed.




The inside of the vacuum chamber which has a diaphragm return spring.

The diaphragm with the throttle cable connector in the middle.

The three solenoids each control the bleed vacuum through three different sized holes.

The control solenoids simply pull out of the plastic case.  They have rubber stoppers on the tips and o-ring seals.

The PCB with only one fly back diode used for the solenoid that controls the largest bleed hole.  The engineers must not have been concerned with fly back on the other two solenoids. Just guessing, maybe because the other two only engage ever so shortly. This is wired as a common anode control.

In the video I setup experiments to test each solenoid using my bench supply. Each worked just fine so my problem with my cruise for the past 10 years is under the dash. I really don’t care to fix it because I never use cruise.

Thanks for joining.


  1. Brad

    Very interesting! Thanks for sharing! :)

  2. […] [Todd Harrison] took a look inside the business end of the cruise control system from his 1994 Jeep Grand Cherokee. We were a bit surprised at how the system operates. The parts seen in the image above make up the throttle control, using a trio of solenoids to vary the level of vacuum inside the device. […]

  3. Mark Smith

    Do you have the same problem I had with my ’93? My intermittent wipers had stopped working. Turns out, under a kick-panel in front of the driver’s side door, there’s a controller board that is notorious for burning out a trace. Supposedly bridging the burned-out trace (if that’s the issue) solves the problem, but the mechanic I had working on it pitched the unit before I found this out.

  4. 10/07/2012

    @Mark Smith, My wippers work fine but I will check inside that control box if I find it and post back later. Thanks!

  5. Daniel

    While testing the solenoids did you or could you figure out the resistance of the coils? That would be perfect for a project that needs 3 coils depending on the resistance. Thanks!

  6. 10/07/2012

    @Daniel, two measured 45.8 ohms and one 45.4 ohms using my Fluke 87-V.

  7. Trevor

    With three solenoids there are only eight different combinations. So does this mean only eight speed settings, or is it more sophisticated than that ?

  8. 11/07/2012

    @Trevor, No it is not digital like that. The ECM “electronic control module” a.k.a “computer” can turn the solenoids on/off fast enough to simulate an analog signal control. It will have some base algorithm that it uses to flutter the different solenoids to get the exact set speed. Computers can doing such control so fast it feels analog to the driver.

  9. Trevor

    That’s very interesting, although I’d would’ve thought that solenoids don’t have a very fast switching time. Obviously fast enough. Thanks.

  10. 31/08/2012

    Cool site you have here! I like all of your reviews and teardowns, I am a Manufacturing Engineer at Sturdy Corporation in Wilmington, NC and just thought this article in particular was cool because the founder of the company I work for, David Sturdy Sr. was the inventor of cruise control, he has since passed and his son David Sturdy, big surprise has taken up his mantle and since the auto industry has died down we are now designing and manufacturing a lot of Engine Control Modules for CAT and other industrial products. We have also been manufacturing the new Coca Cola Freestyle touch screen coke machines! Keep up the good work with your site, congrats you got a new reader!!


  11. 31/08/2012

    Well thank you Jordan. You may like to know I picked up a used servo module from the junk yard for $10 and my cruise now works again. I used it on a 3k trip this summer which really made it nice.

    I have a lot of family in NC. My father grew up in Burlington, NC and I have been to the beach at Wilmington a couple of times. Nice place.

  12. […] Cruise control autopsy and experiments […]

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