In-Car Temp Sensor Test/Modified Aux. Fan Circuit (83/50)

Publication date: 1993-05-01
Reference number: 83/50

IN-CAR TEMP SENSOR TEST/MODIFIED AUX. FAN CIRCUIT

TECHNICAL SERVICE BULLETIN

Reference Number(s): 83/50, Date of Issue: May 1993

A. MODEL 124.066 IN-CAR TEMPERATURE SENSOR (B10/4) B. MODEL 140 MODIFIED AUXILIARY FAN (M4) CIRCUIT

Model(s): 1992 Mercedes-Benz 400 SE; 1992-93 Mercedes-Benz 300 SD, 300 SE, 500 SEL, 600 SEL; 1993 Mercedes-Benz 300 CE Cabriolet, 400 SEL, 500 SEC, 600 SEC

Group: 83 - Climate Control System, Automatic/Tempmatic A/C

Bulletin No.: MBNA 83/50

Date: May 1993

A. MODEL 124.066 - IN-CAR TEMPERATURE SENSOR (B10/4)

TESTING THE IN-CAR TEMPERATURE SENSOR (B10/4)

The in-car temperature sensor signal is routed via the power soft top control module (N54).

The A/C temperature regulating circuit provides two separate operating modes (soft top raised/soft top lowered). When the soft top is raised, the temperature regulating circuit operates in the same manner as in other 124 models. However, with the soft top lowered, the in-car temperature sensor is bypassed by a 10 kohm resistor and the A/C pushbutton control module (N22) receives a simulated fixed 75°F/25°C temperature.

Measuring resistance at the pushbutton control module is not possible due to electronic circuitry in the power soft top control module. Instead, when testing with a socket box tester and test cable, refer to the voltage measurements listed in Fig. Fig 2 :

ERRONEOUS IN-CAR TEMPERATURE VALUES

In cases of customer complaints on some early production vehicles of "the in-car temperature is 5-85°F higher than set temperature" modify the power soft top control module harness by soldering an 82 kohm, 1/4 watt resistor between pins 9 and 11 of connector "Z" (see wiring diagram in Fig. Fig 3 ).

WARRANTY INFORMATION

Operation: A/C Temp. Control, Install Resistor to Power Soft Top Control Module, between pins 9 & 11.

WARRANTY INFORMATION

Damage Code	Operation No.	Time (hrs.)	Model Indicator(s)
77302 73	02-1718	0.2 hr.	65

MODIFIED POWER SOFT TOP CONTROL MODULE (N54)

As of February 1993, the power soft top control module was modified to incorporate an 82 kohm resistor. The modified control module is recognizable by a green dot and "K8" stamped onto the housing.
As of March 1993, the electronic circuitry of the power soft top control module was further enhanced and is recognizable by a green dot and "K9" stamped onto the housing.

NOTE: When replacing a power soft top control module with a modified version, it is not necessary to remove the 82 kohm resistor installed on connector "Z". The resultant error is negligible.

PARTS INFORMATION

PARTS INFORMATION - SECTION A

Part Name

Part Number

Power soft top control module

124 820 40 26 ⁽¹⁾

⁽¹⁾	= Current stock supplies contain only the modified version (K9 or higher).

SPECIAL TOOLS

124 589 00 21 00
124 589 16 63 00

B. MODEL 140 - MODIFIED AUXILIARY FAN (M4) CIRCUIT

As of the chassis end number listed below, the auxiliary fan was changed from 3-speed to 2-speed. As a result, the auxiliary fan preresistor (R15/1), auxiliary an relay (K9/1) and related wiring have been eliminated (see wiring diagram in Fig. Fig 4 ).

NOTE: On some early vehicles, the preresistor was not removed, instead it was wired in series with the other preresistor.

EFFECTIVE SERIAL NUMBER

Model	As Of Chassis End Number
140	1A- 123446

The auxiliary fan switch points have subsequently changed as follows:

Fig 1: AUXILIARY FAN SWITCH POINTS

The current version A/C pushbutton control module (N22) does not recognize the new auxiliary fan circuit and therefore generates erroneous DTC's.

Until a modified pushbutton module is introduced, DTC's 98 and 99 should be ignored on vehicles as of the chassis end number listed above.

Fig 2: Pushbutton Control Module Voltage Measurements

Fig 3: In-Car Temperature Sensor Wiring Diagram

Fig 4: Auxiliary Fan Circuit Diagram