Cruise control on an R107 is a real pleasure on a long drive, and when it stops working, many owners simply live without it. In most cases the fault is one of a few known items: the brake light circuit, the stalk switch, the actuator or the amplifier.
How the system works
The system uses a stalk on the steering column, an electronic amplifier (control unit), an actuator that moves the throttle linkage in the engine bay, and a speed signal. The amplifier compares the actual speed with the set speed and tells the actuator to open or close the throttle. Pressing the brake pedal cancels cruise through the brake light switch circuit.
Systems changed over the years, so check the diagram for your year. Earlier and later cars use different amplifiers and actuators.
Step 1: Brake lights
If the brake lights do not work, the cruise control will not engage on many cars. Check that both brake lights work and that the brake light switch is adjusted correctly. A blown brake light fuse can also disable cruise. This is the first and cheapest check.
Step 2: The switch
The cruise stalk has contacts that wear. Using the wiring diagram, check that the stalk sends signals to the amplifier in each position: set or accelerate, decelerate, resume and off. A switch that works intermittently should be replaced.
Step 3: The actuator
The actuator is in the engine bay, connected to the throttle linkage. With the system powered, an actuator test can be done by applying voltage as described in the factory test procedure. A seized or jerky actuator, or one that does not return smoothly, needs replacement or repair. Check that the linkage moves freely and is not binding.
Step 4: The amplifier
The amplifier is a small electronic unit. Its location varies by year, often under the dash or behind a kick panel. Common failures are cold solder joints and aging capacitors. Amplifier rebuild services exist, and some owners reflow the solder joints and replace capacitors themselves. Make sure you get the correct amplifier part number for your car.
Speed signal
If the speedometer works but cruise does not, the speed signal to the amplifier can still be faulty. Check the signal wire on the diagram.
Tools
A multimeter and the wiring diagram are essential. See electrical troubleshooting for the general approach. A basic automotive multimeter is enough: digital multimeters on Amazon.
Quick answers
- Why does cruise disengage randomly? A loose brake light switch, a weak actuator or a failing amplifier.
- Can a bad ground affect cruise? Yes, check the amplifier and actuator grounds.
- Are used amplifiers reliable? They can be, but they are the same age as yours. A rebuilt one is a safer choice.
- Will cruise work if the speedometer is broken? On many cars, no.
Related reading
- Automatic transmission and cruise control overview
- R107 Auxiliary Electric Fan Not Working
- R107 Fuel Gauge Not Working: How to Test the Sender
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