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Cruise Control Problem 300sdl

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Bart's picture
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Cruise control never worked on my '87 300sdl. Have owned car for about 4 years, looks brand new. This is the only thing that doesn't work on the car. All of a sudden it started to work today. Then later it stopped working. Could be amp, but suspect I should check some of the under the hood components. Any ideas? Anyone know how this system works? I suspect that switch controls amp and then amp controls actuator. How does system gauge the speed of car? When it started working it worked below 45 miles per hour. In fact it worked from the time I put the car in gear. I found this out by accidentally hitting the switch and car took off.

Regards,

Bart

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Robby Ackerman's picture
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Re: Cruise Control Problem 300sdl

Typically the problem is with the amp. You might even open it and look for a loose solder connection or a crack in the board, things you can fix with a soldering iron.

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1960 190 SL
1983 300 D (WVO)
1984 190E 2.3-16 / Mosselman Turbo System & Dinan suspension
1984 190E 2.3-16 / Delsing Motorsport suspension
1992 500SL


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Re: Cruise Control Problem 300sdl

You could try this procedure:

Troubleshooting Your Cruise Control

by George Murphy

The factory-installed cruise control provided on Mercedes-Benz automobiles works very well for the first 4 to 5 years of operation. It is rock steady up hill and down and really a leg saver on long trips. 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. In this article I will cover common problems I have encountered in the 8 years I have owned my 1978 300D and the experience of other owners who have contacted me with cruise control (CC) problems.

NOTE: The repair technique outlined below for the printed circuit board has been successful in about 2/3 of the cases I have encountered - but it is worth a try before replacing this outrageously expensive device.

There are three major components in the CC system: the control unit, the transducer, and the throttle servo unit.
Control Unit: this device 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- or electrically- actuated throttle servo unit until the actual and selected speeds are again in agreement.
Transducer: a speed sensor mounted on the speedometer cable (early version) or on the speedometer (later version). The transducer sends the actual speed signal to the Control unit.

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

In order to trouble-shoot the system, you should have a digital volt-ohm meter, some test leads with alligator clips, plus straight and Phillips-head screwdrivers, metric wrenches, and a trouble light. But first of all, check the obvious - is the fuse blown?

1. Locate the throttle servo unit in the engine compartment. The vacuum unit is similar to that shown in Figure 1. Check the vacuum and vent lines - replace the small rubber hose couplings if they are cracked. Age and heat can cause deterioration of these rubber parts - as well as other couplings under the hood (and throughout the car). The electric unit looks like a small metal box with a linkage connected to the throttle. Check that the linkage is secure.

2. (Vacuum units only) Pull the 2-pole connector from the throttle servo unit. Connect an ohmmeter to the servo unit pins. The resistance should be between 10 and 22 ohms; if not, replace the throttle servo unit.

3. (Vacuum units only) Follow the actuating cable from the servo to the engine throttle linkage. Check 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 (otherwise the engine idle could be increased). If the end of the actuating cable is not touching the linkage, turn the adjusting nut (Figure 2) in such a manner that the end of the actuating cable just touches the throttle linkage. CAUTION: on diesels, turn the idle speed adjuster knob completely to the right and hold the emergency stop lever (on the 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, which gives the best control and response.

4. To check the speed transducer, remove the left hand cover under the instrument panel. On early models the transducer is located in line with the speedometer cable. On later models, it is a small black box about 1" square mounted on the back of the speedometer head. (You may have to push the instrument cluster out of the dashboard to reach the backside of the speedometer). Unplug the 2-pole connector from the transducer. Connect an ohmmeter to the transducer. Early models should read 50 to 106 ohms; later versions should read 650 to 1370 ohms. If these values are not attained, replace the transducer.
If the above steps do not solve your CC problem, then the control unit could be at fault. In order to do any repair on the control unit, you will need a soldering iron of not more than 25 watts, plus a small amount of fine resin core solder wire. (These can be obtained at Radio Shack for a few dollars)

1. Remove the left hand cover under the instrument panel. The control unit is contained in an aluminum box about 1" by 4" by 7" and is secured by a single bolt to the brake pedal bearing bracket. Remove the bolt, unplug the electrical coupling from the unit, and remove the unit from the car.

2. Carefully bend back the crimps on the aluminum housing so the printed circuit board can be withdrawn from the box.

3. Inspect both sides of the printed circuit board for burned or melted components. If there are any, the unit will have to be replaced. If the board does not show any obvious signs of overheating, it may be repairable.

4. Look at the two sides of the printed circuit board - mounted on the component side are various transistors, diodes, and integrated circuits; and on the "foil" side is a confusing pattern of thin copper foil "wires" soldered to the wire leads of the various parts on the opposite side. 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 are very careful, it is possible to re-solder these connections and get the unit working again. For this task, you will need a steady hand and the 25-watt soldering iron (and possibly a magnifying glass to inspect your work).

5. Solidly position the printed circuit board foil side up in a well-lighted work area. 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 flowed to an adjacent connection or you will 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.

6. After you have re-soldered each connection on the board, closely inspect for solder "bridges" between connections that can cause a short circuit. The connections may appear slightly discolored from your re-soldering efforts, but no harm should occur if you were careful with the heat.

7. Replace the printed circuit board in its housing and carefully re-crimp the sides of the box. Reinstall the unit in the car and make sure all connections are secure. Be sure to check the fuse for the unit in the fuse enclosure.

8. IMPORTANT: If you are not sure, 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! The correct bulbs are available from your M-B parts supplier.

9. Take the car out for a road test and actuate the CC in accordance with the owners manual to make sure it works properly.

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87 300SDL
95 E300D

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vanagonjon's picture
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Re: Cruise Control Problem 300sdl

I have found that sometimes when the weather is hot, a cruise that would not normally work, will work intermittently. Must be some electrical reason. Best bet is to have the amp and actuator rebuilt at the same time by someone that is a specialist for the task. The actuator is a little harder to get out on the SDL. The actuator can be tested for amp draw, but if orig. it is 21 years old.

John F

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John Fitzgerald

Lakeville, MA

20 year experience diesel owner / mechanic

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