Q: I have a ’94 Jeep Cherokee. The cruise control has gone out. I was wondering what your thoughts were for troubleshooting. And if I can’t fix it, is it possible to add an aftermarket cruise control to the car? — Al

A: Al, I feel your pain! I’m a cruise control junkie as well. Let’s look at how your cruise control works, how a pro would diagnose it, and a couple of easy checks you can make yourself.

Your Cherokee uses an old-school vacuum controlled cruise control system that is fairly easy to diagnose, and the commonly offending parts are reasonable in price. While an aftermarket cruise control system could be purchased and installed, this option is more cost and trouble than its worth.

The Cherokee’s turn signal lever/cruise control switch sends two needed signals to the PCM (Powertrain Control Module), On and Set, to request cruise (speed) control operation. The PCM takes a look at gearshift and brake pedal position, engine speed, and vehicle speed inputs, and if correct issues commands to the cruise control actuator to maintain proper throttle control. The actuator is under-hood, about the size of a softball, black and silver, with a vacuum hose, wires, and a throttle control cable attached to it.

Engine vacuum is used by the actuator, which employs a diaphragm and three vacuum metering control solenoids to precisely tug on the throttle cable. For safety reasons, the vacuum, vent, and dump solenoids will all deny or release vacuum in the event of an electrical problem.

The most efficient test method is to use a pro-grade scan tool to check for applicable trouble codes, successful switch request signals, and consistent/correct engine and vehicle speed values. Then a tech would follow published procedures as appropriate to isolate the fault. Let’s now look at some things you can try at home that may be productive.

A loss of engine vacuum to the cruise actuator is a fairly common fault. Find the vacuum hose connection to the cruise control actuator. Idle the engine, shut it off, and as quickly as you can, run around and unplug the hose. Feel and listen for vacuum (air rushing into the disconnected hose). If no or weak vacuum exists, follow the hose back to the vacuum reservoir (a plastic tank), and another hose that connects between the tank and the engine’s intake manifold. A kinked or leaking hose, or cracked plastic tank is possible. Also check the throttle cable for possible disconnection.

If the vacuum supply and throttle cable are OK, check fuse F-6 in the power distribution center. If it’s good, my hunch would be a faulty control or brake light switch, or cruise actuator. If bad, a short exists in the cruise circuitry. A replacement control switch can be purchased online for about $30, and the brake switch for $20 (both could be temporarily plugged in as a test before installation). A $30 wrecking yard actuator could also be worth a try, or $100 for a new one. If you’re handy and armed with wiring diagrams and a volt-ohm meter, an old-fashioned test of all cruise control circuits and functions would be better than throwing parts at it.

— Bergholdt teaches automotive technology. Email questions to under-the-hood@earthlink.net.