Troubleshooting Mercedes-Benz Cruise Control Systems
Vacuum Actuator, 10-pin Amplifier System (1976-1980)
Please note that these ideas and procedures are for someone who has the time, inclination and technical ability
to understand and follow them. There are other alternatives. We can generally tell you, over the phone or by email,
what is wrong with your system if you let us know what the symptoms are. Or, naturally, you can have your favorite
local qualified mechanic perform the work. Please do not hesitate to email or call us if you have questions or concerns.
Troubleshooting this system is relatively limited and therefore fairly simple. We should begin by saying
that the amplifier is the primary cause of failure in these systems. If you would prefer to not attempt the
following procedures we will be glad to check out the amplifier for you. We do not test the vacuum actuators.
The cruise control system is composed of the following components:
- Control Switch - on the left of the steering column
- Amplifier - the electronic brain of the system
- Actuator - converts signals from the amplifier into mechanical movement of the throttle linkage
- Speed Sensor - on the back of the speedometer
- Brake Light Bulbs - a mechanism for cancelling the cruise control
- assorted wiring and connectors
The first, and easiest components to check if the system is not working at all are the brake light bulbs.
The amplifier gets a critical ground through the filaments and will not work at all if the brake light bulbs
are burned out or missing. Most systems will work fine if at least one bulb is working.
It should be noted that these systems are at least 20 years old. The actuators are vacuum powered and have an
internal rubber diaphram which can have deterioration. If so, the actuator must be replaced. The cable between
the actuator and the throttle linkage has an internal white plastic sheath which can become worn, causing
the cable to bind. The cable is an OEM part as is the actuator. Each can be bought separately from a Mercedes
dealer or parts supplier.
The actuator can and should be tested to see if it holds vacuum and whether the cable pushes and pulls with
vacuum application. 12-14 inches of vacuum should be applied.
The most common symptoms are as follows and each indicates a course of action:
1) The cruise control system works but is surgy or uneven. The cause is probably a bad amplifier.
The only decisive test is for us to look at the signals on an oscilloscope. An alternative not available to
everyone is to install an known good amplifier and see what happens.
2) The system does not work at all. Although the most common cause is a faulty amplifer, the cause can be
anywhere and there can be multiple causes.
***We are still writing this section. For a discussion of details before this page is finished please
call us at 805 772-5588.***
General Development Laboratories
121 Easter Street
Morro Bay, CA 93442
805 772-5588