Heater - Heat Output Insufficient
83mercedes03TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS
DATE: April, 1983 REF. NO. 83/8
SUBJECT: COMPLAINTS OF INSUFFICIENT HEAT OUTPUT
A. DEFECTIVE MONO-VALVE
B. CONTAMINATED HEATER CORE
Complaints of reduced heat output may be the result of either a defective mono-valve or a contaminated heater core.
A. Defective Mono-Valve
A defective mono-valve is indicated by good output at low engine speeds and/or immediately after warm engine start-up that decreases after a few minutes at higher rpm (i.e. intermittent heat).
The mono-valve can be quickly checked as follows:
1. Relieve coolant system pressure at cap.
2. Remove electrical plug and (4) mono-valve insert mounting screws.
3. Pull insert out of valve and plug valve opening with shop rag.
Closely inspect the diaphragm for cuts while pulling down to its extended position. Also, check that after being extended, the valve completely retracts to the "fully open" position. If damaged, exchange entire insert with a new one from a new mono-valve. The mono-valve body need not be removed.
All inserts found defective in this manner must be reinstalled into the unused, new mono-valve body. The new body with defective insert installed may then be returned for Warranty consideration.
Time Allowance - Mono-Valve Insert Replacement
83-S08-01 Major Operation 0.4 hour
83-S08-02 Related Operation 0.2 hour
B. Contaminated Heater Core
NOTE: Verify the poor heat output by measuring the defroster air outlet temperature after running engine at 2000 RPM for 5 minutes and with "Defrost" button pressed. Minimum acceptable temperature is 55~C (131~F).
A contaminated heater core will be indicated by heavy gel deposits on the filter screen of the insert. Chemical and pressure flushing or heater core replacement in severe cases are the only remedies. (A gel coated insert can be reused after it is flushed clean with clear water). Contact your Field Service Manager for special core flushing instructions and materials information before core replacement.