Saturday, February 13, 2016

Don't use unruly cruise control

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Cruise control button.
Cruise control button. iStock

(MCT) -- Q: The answer to this may be the same as the answer to the column of Saturday, July 12, 2014, "Cruise control unreliable." My cruise control will be set at 62 mph and I will find myself flying down the road at 70 or 75. It will run at 62 for awhile and for no reason jump up to 70 or 75. The vehicle is a 2002 Subaru Forester having less than 60,000 miles.

-Kleon Mimis

A: Kleon, your situation is different from the intermittently inoperative cruise control looked at a few weeks back - this is downright dangerous!

Your Forester's cruise control employs a previously common methodology now made obsolete by the electronically controlled throttle. Engine vacuum acts upon a rubber diaphragm within an actuator unit, which pulls on a cable to adjust throttle position. The cruise control cable runs parallel to the gas pedal/throttle cable except it originates at the actuator, in the engine compartment. Diaphragm/cable position is regulated by three electric solenoid valves inside the actuator - one meters vacuum into the diaphragm chamber, one meters an escape path for vacuum, and the third dumps vacuum in a hurry if the brake pedal or cancel button is depressed. The first two solenoids tick rapidly when in use, fine tuning the vacuum level and resulting cable position. For safety reasons all three solenoids must receive the proper electrical signals for vacuum to enter/remain in the diaphragm chamber (it's far more likely for an electrical signal to go missing than appear unwanted - and no cruise is preferable to crazy cruise). Similar to the Buick a few weeks back, an electronic control module receives operator and vehicle speed information and manages the solenoids.

More coverage
  • Buick's cruise control is none too steady
  • It sounds like your cruise control cancels OK when you apply the brakes or you'd really be freaking out! In a follow up message you confirmed the over-speed issue occurs on level ground rather than a downgrade. Only more recent adaptive cruise control systems are smart enough to apply brakes as needed to maintain the selected speed.

    The most likely reasons for the system to mismanage speed would be an erratic vehicle speed sensor signal, problems within the actuator's solenoid valves, or a binding control cable. I would absolutely stop using the cruise control until it's been successfully repaired by a qualified technician! I checked for applicable technical service bulletins and NHTSA complaints and as far as I could tell this appears to be a rare circumstance.

    Q: My driver's door power window sometimes will not roll up. I noticed it sometimes will begin to work when I have opened the door. Any Ideas?

    -Marta Heller

    A: Marta, My hunch is a broken wire in the wiring harness between the car body and driver's door. This bundle of perhaps 15-40 wires flexes every time the door is opened and closed. The driver's door gets more use than any other, so it's often the first or only one to suffer this fate. Repairing the broken wire at the flex point isn't advised as the repair will be less flexible than the original wire and fail again soon. It's best to run a new length of wire and make the splice connections inside the door and body cowl, where there isn't any movement. Plan on a one or two-hour labor charge and a buck or two for a section of wire.

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    ABOUT THE WRITER

    Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at)earthlink.net; he cannot make personal replies.

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    (c)2014 McClatchy-Tribune Information Services

    Distributed by MCT Information Services

    McClatchy-Tribune News Service
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