Re: [MBZ] Weeping Servo...Weeping Driver
Bob writes:
I suppose I need to devise a systematic troubleshooting methodology for this
and get it right this year.
Bob,
Here is some information I have accumulated over the years of working on
this system:
If the ACC Servo has been replaced with an aluminum body servo, make sure
that you used nylon attachment bolts, and nylon washers to mount the servo,
or you might encounter intermittent electrical problems with the servo.
Everything in the ACC system is vacuum controlled through the ACC Servo. It
is possible that a vacuum line came off or loose. The switch that turns on
the fan is a vacuum actuated relay. This relay (or vacuum switch) is located
just above the radio and behind the vertical switch assembly. It is the one
on the far right side of the relay bank. Near this relay is a coil of green
wire located near the passenger foot-well near the glove box. This coil of
wire is a controlled vent for the vacuum system. Don't try to find a place
to connect it - one end is plugged into a vacuum line; the other end is open
to allow air to enter the vacuum system at a very slow rate. If this wire is
missing, or has fallen out, the blower will only run in defrost. (The vent
line is a German engineers idea of a practical joke).
Defrost mode, by the way, bypasses all of these circuits.
Assuming that the vacuum is OK, check the servo and servo control system.
This system consist of a temperature sensing resistor loop, an ACC control
amplifier (located behind the glove box), and the dreaded servo (located
under the hood, to the right on the engine).
The temperature is sensed/controlled by a sensor chain. The sensor chain
consists of a series of resisters; the ambient air temperature sensor
(located on the firewall just behind the engine), the temperature dial, the
in-car temperature sensor (located in the dash in the right hand speaker
grill, and the resister in the servo. If there is a problem in the resistor
chain, it is likely with the sensor in the dash. This sensor work by drawing
in car air through a tube connected to the blower. If this tube is
deteriorated, replace it with a piece of water pipe foam insulation.
Generally, a deteriorated sensor pipe will cause erratic temperature
control. The temperature dial resister can have a dead spot near the end.
To check out the resister chain,
1. Remove the ACC amplifier (located behind the glove box) and connect a
ground to pin 3 of the connector plug.
2. At the servo electrical connector, connect an OHM meter between pins 1
and pin 2 on the left of the connector.
3. Place the temperature wheel at max (85 degrees) and the resistance should
be between 2.2K ohms, and 4.7K ohms.
4. Place the temperature wheel at 65 degrees, and the resistance should be
between 1K ohm and 3.5 K ohms.
When you finish, re-install the amplifier.
If the circuit is open at any place in the resistor chain, the result is
heat. If the chain is OK, then the problem might be the amp.
To check the amplifier, connect a voltmeter between pins 1 and 7. With the
ignition switch in the on position, the voltage should be about a volt. Move
the temperature wheel from cold to hot. The voltage should increase to about
5 volts, the servo motor should move, and when the servo finds it home, the
voltage will drop back to about a volt. If it does not vary, then the amp is
bad. If you listen to the servo, you should be able to hear the servo motor
running to find the new position.
To check the motor in the servo, disconnect the electrical connectors on the
servo, and connect 12 volts between pins 4 and 5. The motor should run in
one direction and stop. Reverse the voltage and it should run in the other
direction. If it runs, only run it momentarily in each direction. Pin 1 on
the connector is on the left (towards the engine) side of the servo.
There is an inline fuse, located in the relay box on the fender, as I
recall. The in-line fuse powers the amplifier when the car is turned off to
run the servo to a parked position. The purpose of the parked position is to
avoid having the system start up where it was when you last turned off the
car. If the fuse is in there and the servo is operating properly, the servo
will run to PARK and shut itself off after you turn off the car. If the
servo is seized, the servo will run the battery down overnight for no
apparent reason - because the motor in the servo is always trying to move
the servo to a parked position and can't, thus, the system runs the battery
down trying to park the servo all night. It does not hurt to leave the
inline fuse disconnected. The system will work correctly, but the servo will
not park. For most people, this is generally not a problem.
While you are working around the servo, check the operation on the aux water
pump located beside the servo. The auxiliary water pump tends to short when
they stop working. If the pump is bad, it can keep the system from working.
This electric pump is located just in front of the servo. If it does not hum
happily when connected to 12 volts, leave it disconnected. The brown wire on
the pump is ground. If you ever had the push button assembly on the console
out and noticed melted or burnt connectors, it is likely a result of a
failing aux water pump. The purpose of this pump is to keep hot water
flowing through the heater while the car is idling, such as when you are
stopped at a red light. You don't need it in the summer, and unless you live
in a really cold climate, may not need it in the winter.
If you need vacuum diagrams and schematics of the system, please let me
know.
Wiring Diagram for the ACC system:
Amplifier Connector (Amp)
Pin Color Use
1 Grn Pin 1 (TP)
2 wht Push Button Pin 3
3 Bk/Bu Pin 2 (TP)
4 Brn Ground
5 Vi Servo Pin 3
6 yel/grn Park fuse
7 vi/wht Pin 6 (TP)
8 rd/wht Pin 7 (TP)
Ten Pin Test Plug (TP)
Pin Color Use
1
2 bl Ambient Temp Sensor
3 bl/ge Ambient Temp Sensor
4 gn Pin 3 (TP)
5 gr In Car Sensor
6 vi/ws Amp Pin 7
7 gn/ws fuse 5
8
9 gn/ws Temp Dial
10 gn Temp Dial
Servo
Pin Color Use
1 brn Ground
2 gn TP Pin 9
3 vi Amp pin 5
4 blk TP Pin 6
5 blk TP Pin 1
6 rt Push Button Pin 4
7 gn Push Button Pin 7
8 gn Fan Speed resister
9 ge Fan Speed resister Vacuum Switch
10 gn Fan Speed resister
Temperature Selector Wheel (TSW)
Pin Color Use
1 In Car Sensor
2 Pin 2 Servo Via TP Pin 9
In Car Temperature Sensor
Pin Color Use
1 Ambient Temp Sensor
2 Temperature Wheel
Ambient Temperature Sensor
Pin Color Use
1 TP Pin 3
2 In Car Sensor
Hope this gets you started.
Dan Steadman
1980 300D
1980 300TD Euro
