A/C - ACC Intermittent Heat
83mercedes03TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS
DATE: August, 1983 REF. NO. MBNA 83/9
SUBJECT: INTERMITTENT HEAT CAUSED BY MONO-VALVE AUTOMATIC CLIMATE CONTROL (ACC)
When dealing with any ACC problem, it is advisable to always first check the monovalve Plug connection for tightness or corrosion, and correct as necessary. It is not necessary to replace the mono-valve insert when the problem is only a poor connection.
Complaints of erratic ACC operation such as intermittent heat from center outlets, etc. may be caused by a poor electrical connection at the two pole mono-valve plug, pin number 2 (left plug) or 4 (right plug) of the pushbutton switch, at the fuse, or main chassis ground.
If corroded, new pin sockets can be soldered-in and pinched tight for a better fit.
The mono-valve is either fully open (no electrical current) or fully closed (energized) to regulate the flow of coolant to the heater core. As shown in the ETM, one pin always has voltage and the other pin connects into the pushbutton switch and Temperature Control Unit for completion of the circuit to ground. With a poor electrical connection in the mono-valve circuit, either it cannot be closed (energized) or it is intermittently opened (de-energized) allowing coolant to flow. If the system is calling for "cooling", the center flaps will be opened and hot air can exit.
11 PLUG COVER:
Mono-Valve Function Test
1. Remove 2-pole plug at mono-valve and inspect pin sockets for corrosion.. (Clean, replace or reduce inside diameter for tighter fit as necessary).
2. Reconnect the plug to the monovalve, remove the plug cover and connect a voltmeter, in parallel, to the pin sockets.
CAUTION: Do not allow the connections to touch each other!