My cruise control starts to surge after about 10-15 minutes of driving. Has anyone here had this problem before? I suspect that it is the Cruise Amp that is at fault but, I would like to be sure before spending the money for one. Thanks, Cliff
when i was undertaking he nightmare of adjusting ALL my various linkages, and one point, when i thought i was done, and tested, i got surging cruise control, because the rods werent quite right yet.....when i got them into "spec" (lenghts), the surging stopped....
When my cruise control went out I replaced the amp and everything worked. But I also had some slight surging and still do. I would describe it as if the car was trying to stay at whatever was the exact speed ... it was constantly trying to keep the speed within 1 mph of the set speed by speeding up and then letting off.
So my input to you is that it may not be the amp but other components trying to maintain the speed. I'll be interested to see what other folks say.
I've been using cruise control on the W123 meet and I noticed that it would let off when the car got too fast, even on a flat plane. My theory is that the ALDA has something to do with this because if you input too much fuel (tweak the ALDA for more fuel) the car will respond with a calibrated measured "thrust" not linked to the amount of the fuel squirted into the engine. Meaning, it's programmed to input a certain amount of throttle linkage movement in order to achieve the desired speed.
Now if you adjust the ALDA, this throws the cruise control system off by giving more fuel than it's programmed to put in, therefore it will simply surge as it tries to maintain balance.
Another idea is to adjust the throttle linkage back to factory spec in order to minimize freeplay.
For some reason I've managed to fix the surging without even digging into the CC amp! There was some oil around the valve cover gasket so I removed the cover and cleaned up the area then when I reinstalled everything, I adjusted the throttle linkage to produce max power (throttle stop) but with a part of the linkage that wasn't set to max (to allow room for wear and tear because of 1 bushing).
I think that the cruise control system works by the actuator inputing a small amount of movement into the throttle linkage. If it's too much, it will surge. If it's too little, it will continue inputing until the desired speed is reached. If it's small enough to not provide too much of a burst, then it will be smooth.
It's like hammering down a nail on a fragile piece of wood. You don't want to break the piece of wood (finish) so you hammer the nail ever so gently that the nail goes in almost unnoticed.
Before this I attempted to adjust the linkage for max throttle so the throttle stop on the IP linkages is reached. I've attempted a DIY on it but it's very hard to explain. Here's another go at it:
Follow the throttle input from the pedal. In the above diagram on my car, ROD 1 will turn Clockwise from the view of a person in front of the car. This will turn LEVER 1 CW, and in turn pull ROD 2 to the RIGHT. This ROD 2 will then pull LEVER B1 Clockwise from the diagram's point of view. LEVER B1 and B2 are connected via a bushing on their axis AND the indicated BUSHING. This BUSHING is the one I mentioned that has some slack in it, so what I did before was to compromise for the slack which led me to put too much input on the cruise control part. Anyway back to the diagram, when LEVER B1 turns, it will pull LEVER B2 and consequently, ROD 3 in the same direction, CW for the lever and to the RIGHT for ROD 3. ROD 3 will then push on LEVER C (emergency shut off) to push onto ROD 5, which will then push a lever on the injection pump and into the throttle stop via 2 or 3 more linkages through a spring.
Adjustment was at ROD 3 near where the BUSHING is. If you look at ROD 3, there are two ends. From the viewpoint of the diagram, the upper balljoint is the one I adjusted. There will be two nuts, a 12 and an 8mm. The 8mm is the locknut, which is on the bottom. The 12 is the top one for the ball joint head. What I did is loosen the 8mm bolt on the bottom and pull LEVERS A and C to FULL THROTTLE or towards THROTTLE STOP. You can see it when the throttle stop pin contacts the bolt on the IP. It is best to have somebody hold this for you while you adjust the bolt (8mm). When adjusting this bolt ensure that the BUSHING is at its other end of the slot. Then back it off a little to compensate for wear. LEVER B2 and ROD 4 is part of the cruise control linkage mechanism so that's where you focus on.
This is where the mystery part comes on, how much adjustment is necessary. Well I did not intend to remove the surging, I just felt I overcompensated for the throttle stop so I maxed it out. The power was quite raw and I wanted the smoothness back so I just backed it out a bit.
Make sure to keep the LEVER A and C at throttle stop!! Otherwise any slack in this will not adjust the linkages properly.
I thought it would be in the one unlabled linkage rod? (The one after rod 4 in the chain.) That one actually connects to the cruise control actuator.
I've also read that you need someone to actually hold the pedal to the floor to check if you hit the throttle stop. Apparently you can pull the linkage to the stop, but that still doesn't mean you can do it with the pedal.
I wish I had a helper with all of this! (Who didn't charge $100/hour.)
At one point in time, wasn't there a post that listed the proper linkage lengths?
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.
Disregard #8, it's one of those urband legend myths
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