A look inside what makes cruise control work

[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.

We categorized this as a repair hack, but [Todd] is just rubbernecking and doesn’t have any real plan to fix the system. It’s been on the fritz for ten years and this piece may not even be the culprit. But we’re still satisfied because he gives us a look at the system which uses the amber-colored stoppers on the three solenoids to plug three different sizes of weep holes. The unit is a vacuum enclosure where a throttle wire connects to a rubber diaphragm and adjust engine speed as the diaphragm moves. The vehicle’s computer actuates the three solenoids, allowing leakage to vary the level of vacuum, thereby keeping the throttle at just the right level. Neat!


  1. Nomad says:

    Well, at least that’s how old cruise control works. Modern vehicles have electronic throttle, so it’s not a huge act to move the airflap (gasoline engines) or increase fuel injection (diesel engines)

  2. Will in Austin, Texas says:

    Vacuum based cruise control. Used for 10-20 years, 80s and 90s. Was first electronic cruise control.

    1994 may have NTSA OBDII on the computer control, which includes cruise control. I have the factory manual for Jeep cherokee which explains all electronic and mechanical and goes deep into the computing and transmission shift control, etc. Wish i had it with me now and post some relevant stuff. cruise control knows engine RPM, throttle amount, gear engaged, ground speed, and logically makes every step to increase/decrease speed, and shift gears with all necessary steps like disengage clutch, etc. My manual even has the baud rates for the OBD, the serial codes and all that.

    I thought about hacking my own GPS based cruise control instead of speed based but never got around to fixing my broken factory cruise control to begin with; kinda like this guy! =)

    • codenamegamma says:

      The 94 Jeep Grand Cherokee was still running on the Chrysler SBEC III ecu which is OBD I. The JTEC (Chrysler’s OBD II ECU for the late 90’s) ecu didn’t make its way onto that vehicle until 1996.

  3. skitchin says:

    Usually not on the nay-sayer’s team, but I’d really prefer you all not go hacking up cruise control systems on your cars.

    • Pilotgeek says:

      Why not? It didn’t work anyway, and I’m sure he won’t actually run it unless it’s working right. Even professional cruise control setups aren’t a guarantee to safety. Ford had a few cruise recalls…

      Things in this world are dangerous. Get over it.

  4. friction57 says:

    In my experience, the main control units for the Cruise Control were never really the culprits. The usual suspects for a failing or intermittently working cruise system were the vacuum hoses. They would dry out and develop small leaks at the ends, and they would fail.

  5. Galane says:

    Now hack in range finding to add automatic adjustments for the driver ahead of you who can’t hold a steady speed.

    • karlexceed says:

      I’ve wanted to do this for a while! My current vehicle has no factory-installed cruise; it was only a dealer-added option, so it’s tempting to buy the kit and see what I could manage.

      I want to be able to hit the “follow at a safe distance” button and only have to keep it between the lines… Luxury.

  6. markey1979 says:


    I concur! It is not usually the main control module. Being a professional mechanic, I would venture to say that ~70% of the time it is really, really simple, like the brake switch is worn or out of adjustment. Some GM cars today will disable the cruise control if the check engine light is on also…..

  7. markey1979 says:


    Google “Bosch Automotive Handbook”. Its a little blue book, that is absolutely packed with fantastic information. I have the seventh edition, and I use it all the time. It has sections on adhesives, comfort systems, math, physics, control systems, communication systems, ….. etc.

    There were even sections on hybrids, and “adaptive cruise control” (automatic adjustments for the driver ahead of you……)

  8. IJ says:

    Hook the solenoid rig up to a microcontroller and create a break dancing (well, hopping) robot!

  9. ImNotASissy says:

    Really? Nobody else noticed that the cruise hasn’t worked for 10 years…on his 1994 car…in 2012?

    • Tech Joker says:

      What is there to notice?

      1994 –
      18 Year old

      So evidently the Cruise Control worked correctly for around 8 years

      >Reply is easy, math is hard. ;^)

  10. KillerBug says:

    Those old vac units really sucked, glad we have progressed to electronic throttles that can malfunction, go wide open, and kill anyone too stupid to figure out that turning the key will make the engine stop.

    • newt says:

      Yes, there were never, ever, in the history of the internal combustion engine, any mechanical throttle assemblies that have stuck open. Very valid point.

      There are also no other safety measures on modern engine management systems, like ignition timing reduction, spark and fuel cuts, etc. when the throttle control is outside of parameters.

      The news reports you’ve likely seen have definitely got all the facts spot on.

    • Whatnot says:

      Assuming you have a car that has one of those old style turning keys.

  11. DrAvIn says:

    i have had alot of those stupid vacume connectors break lol .. that part needed PC7 epoxy paste

  12. cellurl says:

    My car won’t cruise below 35mph. Why don’t you “Hack” a cruise to run slower than that. That would be cool.

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