[W126 Coupe] Re: Cruise Control

[Steve Nervig]

Shayegan, Richard asked:

> On a related note, when the cruise control dies, how do you know which
> part is bad?


by George Murphy, MBCA Technical Director

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.
  --
Entropy Needs No Maintenance