The cruise control on the P38 is a conventional vacuum operated
system with a vacuum-powered throttle actuator. The most common
problem people experience is a gradual loss of response of the system.
You will notice that the throttle will not be fully opened when the
cruise control is trying to accelerate the vehicle up a hill.
Eventually it will not even open the throttle enough to maintain a set
speed on the flat.
"The most common causes of the malady
are leaks in the ends of the vacuum lines connected to the
throttle actuator. This is located at the left
rear corner of the engine compartment behind the Electronic Air
Suspension enclosure (see photo). The rubber tubing gradually
deteriorates and splits where it joins on to the actuator and the nylon
"tee" piece.
The problem is easily repaired by replacing the rubber vacuum tubing.
You can either use the official parts which give you tubing already cut
to length and molded to shape, or generic rubber vacuum line tubing
available in any auto parts store.
