Troubleshooting the Cruise Control by Oliver Scholz
I don't know about you, but every second owner Fiero owner I come across has cruise control problems. Even with my three Fieros, two had a non-functioning and an intermittent cruise control, respectively. Reason enough to give the Fiero cruise control system the treatment it deserves and hopefully get you ‘cruising’ again. Even if your cruise is working fine now, it can start working intermittently or not at all in the near future; so it may be useful to remember where you store this article for future reference.
First, the Fiero used two fundamentally different approaches to the cruise control. The 1984-1986 4 cylinder and all V6 Fieros use a separate cruise control module. It is located behind the carpet on the driver side center console. To access it you need to remove the radio console and flip the carpet aside. Fortunately these units do not go bad very often. I have never seen a defective unit. But you may need to access the module connector for troubleshooting. The second system involves the 1987/88 4 cylinder Fieros with DIS. On these cars, the ECM takes care of the cruise control functions. The advantage is that you can troubleshoot this system with a Scantool. The Scantool can tell you whether or not the brake and/or other switches are working correctly. The downside is that you need access to this Scantool and if the problem is with the ECM, replacement is expensive. But again, the ECM hardly ever goes bad. The far more prevalent reasons for cruise control problems lie in the mechanical parts of the system. Before we get into these, let's have a very general look at how the cruise control system works.
The cruise control module (or ECM) is fed an electrical signal from the speedometer, telling it the current speed. The Cruise control stalk has a couple of switches telling the module whether it should keep the current speed, accelerate or decelerate, or switch off. The cruise module then controls the vacuum actuator ("servo") located near the trunk. This actuator pulls on the throttle and can accelerate or decelerate the car. To see how much it pulls, the position of the actuator is fed back to the control module. The brake and clutch pedal (if equipped) also have a switch connected to the cruise module to turn it off as soon as you depress either pedal. The system is not too complicated, but it is complicated enough to render it inoperative if only one of these subsystems fail. And since it is not a vital system, it often stays unrepaired and that is why so many Fieros have a non-functioning cruise control. Let's assume that the radio fuse is not blown (this is the fuse for the cruise control system):
The prime suspect if your cruise control is non-operative is the cruise control stalk. The switches in the head module go into the one wire that runs down the steering column. You don't need a special tool to check it. Just disconnect the connector at the steering column and plug in a known good switch. This one can be from almost any GM car from the eighties; I use a switch from a Fiero with the wiper-washer portion broken. Plug it into the harness and go for a test drive. If the cruise works now, you need a new stalk. Since the connector is already disconnected, tie some wire to it and yank the stalk straight out of the steering column. Pull it all the way out and use the cable you tied to the connector to feed the new stalk's wire through the column. The new stalk easily snaps into place. Hook it up and you're all set.
The second suspect are the brake and clutch (if equipped) switches. This is especially true for intermittent cruise operation. Usually you only need to readjust the cruise control switch. Push it forward and let the brake (or clutch) pedal push it back by itself. Don't push it back yourself, because if you push it too far, you will get the same problem you are trying to fix. Both switches should be closed and open only when you depress the pedals.
Another problem area is the speedometer. If your speedometer doesn't work, chances are that your cruise is broken too. This is because the speedometer provides the speed signal to the cruise module. If the speedometer does work and you have a Scantool, check if the Scantool displays the proper speed. If it does, you can be certain that the speedometer speed output works.
Our final focus will be the vacuum servo. Make sure that there is vacuum present at the servo. If not you have a vacuum leak somewhere. Many times the vacuum leak is caused by rust forming on the edge of the vacuum canister due to the harsh environment it is in. Check to make sure the canister can hold a vacuum. If the canister is not the problem, check for voltage at the vacuum release solenoid. There should be battery voltage present between the two solenoid contacts. If not, recheck the brake switch adjustment and wiring. If there is no voltage at the vacuum release solenoid, the vacuum will be vented immediately, and the servo can not control the throttle.
Now you have checked everything you can easily check. The function of the servo itself can be tested by applying the proper voltages and grounds to the servo and by checking if it holds and vents vacuum. All further checks make the removal of the cruise module necessary. You need to check for the voltages at the connector and repair the wiring or replace the module as necessary. Both of these tests require a little more electrical knowledge. But as I stated earlier, the two most common causes for cruise failure are a defective stalk and problems with the clutch and brake switches. So, don't be afraid of your cruise control system, fix it and you'll be cruising again in no time!