Troubleshooting Your Cruise Control

The factory-installed cruise control provided on Mercedes-Benz automobiles works very well for the first 4 to 5 years of operation. But with time, the components in the system age and begin to cause trouble. The first indication can be intermittent loss of control or even total failure.

Note: The repair technique outlines here for the printed circuit board has been successful in about 2/3 of the cases that I have encountered, but it is worth a try before replacing this outrageously expensive device.

Three major components exist in the CC system:
a) control unit
b) transducer
c) throttle servo unit.

Control Unit: compares the actual speed of the car and the selected speed. In the event of a deviation from the selected speed, the control unit sends pertinent control signals to the vacuum-actuated or electrically actuated throttle servo unit until the actual and selected speeds are again in agreement.

Transducer serves as a speed sensor that is mounted on the speedometer cable (early version) or on the speedometer itself (later version). The transducer sends the actual speed signal to the Control unit.

Throttle servo unit (early version) a vacuum-actuated servo that positions the engine throttle to attain the selected speed. Later versions utilize an electric servomotor.

What you'll need:
To troubleshoot the system, first check to see if the fuse is blown. If not, you'll need the following:
1. a digital volt ohm meter
2. some test leads with alligator clips
3. straight and Phillips-head screwdrivers
4. metric wrenches
5. trouble light.

Getting started:
1. Locate the throttle servo unit in the engine compartment.

2. Check the vacuum and vent lines for any cracks in the small rubber hose couplings.

Note: Age and heat can deteriorate these rubber parts as well as other couplings under the hood and throughout the car.

3. Check that the linkage is secure from the electric unit (a small metal box) to the throttle.

Note: If you have a vacuum unit, perform the following steps.
1. Pull the 2-pole connector from the throttle servo unit.
2. Connect an ohmmeter to the servo unit pins.
3. If the resistance is not between 10 and 22 ohms, replace the-throttle servo unit.
4. Follow the actuating cable from the servo to the engine throttle linkage while checking that the end of the actuating cable is just touching the throttle lever with the least possible free play, but not exerting any force on it (force could increase the engine idle). If the end of the actuating cable is not touching the linkage, turn the adjusting nut in such a manner that the end of the actuating cable just touches the throttle linkage.

Note: On diesels, turn the idle speed adjuster knob completely to the right and hold the emergency stop lever (on throttle linkage) all the way to its stop before adjusting the nut. This adjustment assures that the vacuum-operated throttle servo unit is operating in the middle of its range. The middle of its range gives the best control and response.
Checking the speed transducer:

1. Remove the left hand cover under the instrument panel.
Note: On early models, the transducer resides in line with the speedometer cable. On later models, the transducer is a small black box, about 1 inch square, that is mounted on the back of the speedometer head.
Note: You may have to push the instrument cluster out of the dashboard to reach the backside of the speedometer.
2. Unplug the 2-pole connector from the transducer.
3. Connect an ohmmeter to the transducer.
4. If on early models you don't see 50 to 106 ohms and on later versions you don't see 650 to 1370 ohms, replace the transducer.
If these steps do not solve your cruise control problem, then the control unit could be at fault.

If the steps in part I of this article didn't solve your cruise control problem, then the control unit could be at fault. The control unit is in an aluminum box, about 1" x 4" x 7", that is secured by a single bolt to the rake pedal bearing bracket.
What you'll need:

To do any repair on the control unit you'll need the following:
1. a soldering iron of not more than 25 watts
2. a small amount of fine resin core solder
3. a magnifying glass
4 a steady hand!

Repairing the control unit:
1. Remove the left hand cover under the instrument panel.
2. Remove the bolt holding the aluminum box to the brake pedal bearing bracket.
3. Carefully bend back the crimps on the aluminum housing so that the printed circuit board can be withdrawn from the box.
4. Inspect both sides of the printed circuit board for burned or melted components.
Note: If any of the components on the printed circuit board are burned or melted, the board can't be fixed. You'll have to replace it. If the board shows no obvious signs of overheating, you may be able to repair it.

Repairing the printed circuit board:
1. Look at the two sides of the printed circuit board noticing the mounted transistors, diodes, and integrated circuits on the component side and the confusing pattern of thin copper foil "wires" soldered to the wire leads of the various parts on the opposite side (the "foil" side).
Note: The control unit generally fails whenever one or more of the soldered connections on the foil side become loose due to vibration or heat. If you're very careful, you can re-solder these connections and get the unit working again.

2. Solidly position the printed circuit board foil side up in a well-lit work area.

3. Starting at one end of the board, carefully apply heat with the tip of the soldering iron to each solder joint on the board. Caution: Apply only enough heat to cause the solder around the connecting wire or lug to momentarily melt then remove the soldering iron and allow the soldered joint to "freeze." Make sure no solder flows to an adjacent connection or you'll have a short circuit. You may add a small amount of solder if the joint appears to be lacking enough for a good connection. The solid state devices cannot tolerate excessive heat, so use care with the soldering iron.

4. Replace the printed circuit board in its housing.
5. Carefully re-crimp the sides of the box.
6. Reinstall the unit in the car.
7. Make sure all connections are secure.
8. Check that the fuse for the unit is in the fuse enclosure.
9. IMPORTANT: Check that the brake light bulb in each tail light unit of your car is an original equipment OSRAM or BOSCH bulb. Do not use U.S. type 1157 bulbs--they can damage the control unit beyond repair!

10. Take the car out for a road test in which you actuate the control unit in accordance with the owner’s manual to ensure it works property.
If the cruise control still does not work properly, no repair is possible anymore on these control amps for the vacuum type cruise control – the components are simply no longer available.

The only solution is to replace the unit with a new purpose-built amplifier. The only manufacturer now making these is Beckmann Technologies, Inc. at 800-742-1021.

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