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Who didn't hear it? "My automatic air condition is broken and I was told I should change the heater control valve. Mercedes asked for 1100 Euros and still it does not work. What shall I do?" Even A/C professionals refuse to repair 1st series automatic A/C system of Mercedes-Benz.

We will just analyze the electrical part of this type of air conditon and remove possible failures. I assume that you have a WIS-CD (workshop information CD)  and have already applied test 83-601 „Quick test without tester and remedies“. If this testhas told you to try and replace the heater control valve or the amplifier (comparator) then this article is right for you.

Pic 1: Control Panel from a MB R107 500SL
Pic 1: Control Panel from a MB R107 500SL

Even if labels are in English and °F degrees, it was also installed in European cars! Merceds-Benz also used an US supplier for that!

There is a quick test on WIS-CD with number 83-601 „Quick test without tester and remedies". That is a really good test procedure, but often enough it demands that you replace control valve or amplifier for a try. Even chapter "A/C and automatic A/C control" on WIS-CD mix all different systems that were installed in MB R/C107, W116 and W123 at any time. So that includes my manual A/C of my 73 450SL as well as A/C automatic control systems of late 70s and 80s. We will investigate in detail the root problems of 1st series automatic A/C systems.

What are heater control valve and amplifier? Heater control valve continuously opens and closes the hot water valve and controls fan speeds. The amplifier measures the resitance of temperature sensors together with the temperature preselection wheel of control panel and positions the heater valve from Heating 4 to Cooling 5. When switching off ignition it parks the heater control valve in Cooling 2. Below heater control valve is a temperature controlled vaccum switch, that disables your fan when Auto-Lo or Auto-Hi are selecte as long as cooling water temperature us below 40°C. This will avoid that you get a cool air shock when you start your cold car in winter.

We start and swirch off ignition. Then we open the lid of the heater conrtol valve, but please only 4 screws of the lid. Who continues to diasseble further needs braveness and knohow hot to synchronize gearbox, potenitometer and the heater valve. Now we see a number of vacuum pipes, that may not be stripped or interchanged. You can unscrew them with their connector and thus achieve more room for movement. Should you still have a loose hose, use magnifying glasses to read digits on both connectors for reconnecting them. Below vacuum connector you have 2 electrical connectors, both 5 pins. We leave them connected and count pin 1 and pin 2 (both green cables connected), which lead to our potentiometer. Now we measure whether we see a voltage for parking at pin 3 after switching off ignition.  Picture 2 shows the opened heater control valve (vacuum hoses removed, leave them on) with measuring cables connected to pin 1 (blue), pin 2 (yellow), pin 4 (red) and pin 5 (black).

Pic 2: Heater control valve opened
Pic 2: Heater control valve opened

1. Voltage for parking position  after switching off ignition

Pin 3 control valve: 5V – 8V versus ground (position control valce in parking position Cooling 2)
What to check if you see too little or no voltage here:

  • Measure same voltage at pin 5 (connected) of amplifier to verify if you have a cable issue
  • Check voltage between pin 2 and pin 6 of amplifier whether you find 12V. Amplifier is supplied here with 12V after switching off ignition.
  • If this voltage is not present, we either have a broken additional fuse 2A or a ground problem on amplifier
  • If cables are okay and fuse supplies voltage, amplifier is broken.

2. Check whether motor / gear box of control valve hangs

Now remove both electical connectors from control valve and measure resistance of potentiometer (pin1 and pin 2).
RPoti: 1300 – 1500 Ω

If this is not the case, control valve is not in parking position. We note measured value. A value of just below 2000 Ω or around 200 Ω means that our control valve is in mechanical end stop. If you saw a voltage on pin 3 and potentiometer shows one of these vaules, it means that amplifier has tried to park control valve but it is in mechanical end stop.  Now we have to check whether our valve hangs in end stop.

3. Check whether control valve moves by external voltage

Pic 3: Driving control valve externally
Pic 3: Driving control valve externally

Get an external battery with a voltage of about 4V. I use three sequential AAA accumulators which make 3.6 V. Now we connect them to pin 4 and pin 5. Now we can move the motor in the valve to the left or right, depending on positive or negative voltage. We apply +4V or -4V with the red and black cable on pin 4 and pin 5. On the right we can see the potentiometer move and also hear it or measure changing  potenitometer resistance on pins 1 and 2. 

If all this works, your control valve is electrically okay. If not, replace your heater control valve.

In the end we park our control valve: +4V to pin 3, ground to pin 4 or pin 5, depending on which position it is in.

4. Check whether amplifier sends voltage to drive control valve

We reconnect both electrical connectors and connect our multimeter to pins 4 and 5. Now we aplly 85°F at tempreature selector and press DEF on control panel (full heating and full fan). Now our potentiometer should move left in counter-clockwise direction. And we should see a voltage of 4-6 V. (except our valve was already in position heating 4 and we saw 400 Ω or less on potentiometer). Now we select 65°F and our valve should move in opposite direction, meaning minus 4-6V now.

Check errors:

Measure the same at pin 1 and 7 of amplifier to eliminate possible cabling problems between amplifier and valce. If there is still no voltage, measure resistance of all temperature sensors.

5. Resistance of sensors and temperature selector wheel plus voltage for amplifier

Pic 4: Amplifier with connections
Pic 4: Amplifier with connections

Take out glove box and find 10 pin test connector and open it. Now we put multimeter in resitance mode and now measure in male and female connector as follows:

Where to measure?Reference value
Male Pin 2 und Pin 3 Outside temp sensor acc. WIS-CD 83-609: 300 Ω at 77°F
Male Pin 4 und Pin 5 Inside temp sensor acc. WIS-CD 83-609: 1720 Ω at 77°F
Male Pin 9 und Pin 10 Temp selctor wheel from 300 Ω (65°F) to 1480 Ω (85°F)
Female Pin 7 +12V to ground when ignition is on

If they are all okay, we do not have a sensor problem. That leaves cables to amplifier or the amplifier itself as a possible cause. Check all cables according to WIS-CD 83-605 and also verify proper ground at pin 4 of amplifier.

Analysis of heater control valve control:

  • If you cannot drive control valve with external batteries, it is broken: Replace or Repair
  • If control valce can be driven externally but amolifier sends no driving voltage (no voltage on pin 3 and 4) and also all sensors have proper resisitance and all cables are okay: Replace amplifier.

For all other issues follow repair instructions 83-601 "Shot test withour testing device" from WIS.CD. There are many other relais, vacuum actuators and switches, cables, heat water pumps, attenuation resistors, fan motor, ... Who's interested will e.g. find resistance values of potentiometer in várious positions of control valve or can check how fan speeds are operated,

Repair of control valve

Yes there is a repair instruction available in German for this type of heater control valve. It recommends to put a disc of 1mm into the gearbox to prevent it from hanging in mechanical end stop. I had a number of control valves in my hands, but none of them had this defect, so I do not add this disc. These instructions are partially helpful as they describe how to reassemble the control valve. Hiwever I recommend to do the synchronisation betweeen gearbox, potentiometer and valve in a different way: Close valve completely and then open it just a little. Now add gearbox in according mechanical end stop. But beware: I already had completely broken and therefore scrap valces in my hands from people who tried to repair it themselves.

You should check a reassembled control valce as follows: Put it in end stop heating and blow air through valve from hot water pump connection. It should easily flow through to the opposite pipe connector. Now move control valve to cooling end stop and no air may pass through to opposite pipe connector. Adjust potentiometer accordingly in these positions.

In my experience most problems arise from cracks in housings that let heating water pass to gearbox and motor. Some people in the US replace the middle housing by an aluminum hosuing. While this is a good idea, they seem to fail on properly reassembling the valve. At least I had two of them in my hands and both did not work properly, even that one was just overhauled before the car went back to Europe. That is why I do not advice these overhauled control valves.

Pin out of 10 pin test connector behind glove box

For measuring with F or M, open connector and measure directly on male (M) or female (F) connector:


Where to measure?
M=male ,F=female
1-6 Drive motor of control valve (+ or – 6V measured, tested with 3.6V)
2 Rtotal to ground (only when connector applied)
2M-3M Outside temperature sensor NTC 300 Ω
4M-5M Inside temperature sensor NTC 1720 Ω
7F Ignition (15) from relais air condition (not while starting engine)
8F Fan motor voltage
9F Resistance of potentiometer in control valve to ground
9M-10M Resistance temperature selector wheel 300-1480 Ω (cold-warm)


Pinout connector amplifier

Where to measure?Signal
1-7 Out: Drive motor in control valve
2 In: +12V Defrost
3 In: Rtotal to ground
4 Ground
5 Out: Drive control valve in parking position (Cooling 2)
6 In +12V: Plus from addtl. fuse 2A directly from battery. Amplifier needs this voltage to drive control valve into parking
8 In: Ignition (15) from relais air condition (not while starting engine)


Pinout connectors heater control valve

Where to measure?Signal
1,2 Out: Position signalling potentiometer in control valve, pin 1 externally to ground
3 In: Voltage for going to parking position
4,5 In: Drive motor in control valve (+ or – 6V measured, tested with 3.6V)
6,7,8,9,10 Control of attenuator resistors of fan motor


Pinout of control panel

Where to measure?Signal
3 Out: +12V signals DEF to amplifier, connected to pin 4 via a diode
5, 6 Ignition (15) from fuse 6


Attenuation resistors for fan (3 x 0,55W, 1 x 0,25W, 1 x 0,15 W)

Where to measure?Signal
2 Ignition (15) from fuse 6
2-4 0,55W
4-1 0,55W
1-3 0,55W
3-5 0,25W
5-6 0,15W


Resistors measured by amplifier

Rtotal Totaler Widerstand aus Innentemperaturfühler, Außentemperaturfühler, Wählradpoti und Positionspoti im Regulierventil
Inside temperature sensor Resistance decreases with increasing temperature, 1720 Ω at 77° F
Outside temperature sensor Resistance decreases with increasing temperature, 303 Ω at 77° F
Temperature selector wheel
Resistance increases when higher temperature selected
300Ω +50-30Ω bei 65 °F
900Ω bei 75 °F
1480Ω +140-110Ω bei 85°F


Position signalling potentiometer in heater control valve 1300 - 1500Ω in parking position, adjusted to 1370 Ω

  • Heating 4 400 Ω
  • Cooling 5 2100 Ω

Values see WIS CD 83-609, 83-610

Limitation of liabilities

I exclude all liabilities if you follow these instructiosn or if there is an error in them. If you use them, please do on your own responsibility. Otherwise I wish you fun and success with this work. And those, who prefer easier things, start with checking your fuses.


I retain all rights on this article. Copying, distributing - in parts or as a whole, electronically or in paper - ar not permitted.


Yours, Volker

Measuring protocol tracing fauls as of 26. March 2010

Heater control valve A0008300384

  • RPositionspoti control valve 1800 Ω (End stop cooling). Pins 9, 8 and 6 connected. Fits to this position end stop cooling.
  • Externally driven control valve and adjusted resistance in parking position to 1347 Ω (parked from heating) repsectively 1411 Ω (parked from cooling)
  • After revision control valve moves automatically in parking postion but stays there. Removed cooling water and then heater control valve. Valve did not open and close despite of moving gearbox. Reassembled with new screws and resynchronized everything.
  • Testes heat water pump. Moves and gets voltage to drive it.
  • Disassembeld control valve and glued housing. Reassembeld and readjusted everything. Tested with external voltage, everything okay.
PositionPotentiometerPin 3 is in contact with pinsPin 6 is in contact with pinsRemark
End stop cooling 1834 Ω      
Cooling 5 1788 Ω 4 8, 9 error in wire drawing
Cooling 4 1633 Ω 4 7, 8 error in wire drawing
Cooling 3 1536 Ω 4 7, 10  
Park position
(=Cooling 2)
1411 Ω
(from cooling) /
1347 Ω
(from heating)
4 10  
Middle position
(=Heating 1 /
Cooling 1)
1028 Ω   -  
Heating 2 757 Ω 5 10  
Heating 3 575 Ω 5 7, 10  
Heating 4 414 Ω 5 7, 8  
End stop Heating 162 Ω 5    


Wire drawings on WIS-CD have 2 mistakes: According to WIS pin 6 is in contact with pins 8, 9 and 10 in position Cooling 5. For Cooling 4 pin 6 is described to be in contact with pin 8 only. This is both wrong.

Reapplied vacuum hoses reapplied.


Sensor tests

  • Temperature in garage 13°C
  • Measurements at test connector
  • Inside temperature sensor 2.483 kΩ okay
  • Outside temperature sensor 451 Ω okay
  • Temperature selector wheel 73Ω, 740Ω, 1480Ω bei 65, 75, 85°F
  • Ignition (15) on pin 7 Yes


Control panel connector 6 pins for buttons (allother connectors removed)

Diode test (pin 3 - 4) okay. Pins 5 and 6 are connected internally.

between pins
1, 2, 4, 5, 6 2, 5, 6 1, 2, 5, 6 (5, 6) (5, 6)


1 lamp 1,2W defect, replaced.

Pic 5: Resistor on upper right was loose!
Pic 5: Resistor on upper right was loose!

Connector amplifier A0008350157

  • Pin 6 +12 V without ignition okay
  • Pin 8 Ignition (15) okay

But no driving voltage for heater conrtol valve! Amplifier is broken. I opened it and resoldered a cold soldering of a resistor 3 Ω to ground.

Finally I get driving volatage a control valve! However Auto-Lo and Auto-Hi still do not operate the fan.

Vacuum test on heater control valve

There is no vacuum on black vacuum pipe. I detected that the two hoses (black and yellow) leading to bimetal switch underneath heater corntol valve are misplaced. That means that the tongue of bimetal switch is always kept in Close position and no vacuuum is passed on to yellow hose and main vacuum switch. This would never enable the fan. I wish the one who has misplaced those 2 hoses to ... - you know what I mean.

All errors found and repaired. Now it runs like a charm! Finally I went to a/c service to reifill R413a. Here they finally found out that expansion valve was also blocked and had ti be replaced.