This page pertains only to Mercedes-Benz models from 1981 to 1993 and Porsche 944 models. These automobiles have the motor-type cruise control actuators.
Occasionally a customer complains that when the key is turned on there is a continuous chatter coming from the engine compartment! (It is normal for the actuator to click just once when the key is turned on.)
There is a failure mode of the system wherein the Cruise Control Actuator will chatter if the amplifier has power (key on.) The vehicle does not need to be running and therefore there is no speed signal to the amplifier from the speed transducer.
On the Mercedes-Benz this indicates an open-circuited connection to the actuator of either pin 2 or pin 3 or both and a good connection of all other pins (1, 4, 5, 6, 7) Naturally, this can be caused by plug corrosion, a broken wire, or a faulty connection inside the amplifier or actuator.
Initial troubleshooting in the Mercedes-Benz can be performed as follows:
On the Porsche the fault is at pins 3 or 4 or both. The corresponding pins on the amplifier depend on which amplifier
is installed.
For the 944.617.127.00 pins are the same as the Mercedes above; follow that procedure.
For the 944.617.132.00 proceed as follows:
Either the amplifier or actuator (or both) should be immediately unplugged because this "chattering action" can damage both units.
This failure is usually in the cruise amp but can be vehicle wiring, switch or a bad connection in the actuator.