Cooling System: Description and Operation
NOTE: From June - November 1981 the vent line (arrow) between coolant pump and cylinder head on engine 617.91 and from October 1984 on engines 615,616,617 has been dispensed with.
In case of complaints for overheating phenomena and/or cavitation at coolant pump. The vent line may be installed. For this purpose, remove screw plugs from the connecting bores.
Production breakpoint: June - November 1981
Production breakpoint October 1984
Thermostat
A thermostat, with a thermostat opening at 80 °C, is installed in all engines.
In the time September-November 1981, the coolant thermostat of another manufacturer was installed in engine 615.940.
Identification: Black stripe or dot on cover of coolant thermostat housing.
This coolant thermostat cannot be obtained from stock.
Employment: September - November 1981
From April 1983 to January 1984 engine 615.940 has been provided with a coolant thermostat housing made of plastic material. On engines 616.912 and 617.912 as standard equipment starting July 1985.
The cover is inked black for identification.
Tightening torque of bolts: 6 Nm
Breakpoint: April 1983 to January 1984
A to radiator
B from radiator
C bypass line
D from cylinder head
The main valve is closed up to a coolant temperature of approx. 80 °C and the bypass disc is fully opened. The flow (B) from radiator is thereby interrupted and the coolant flows via bypass line (C) directly to coolant pump.
A to radiator
B from radiator
C bypass line
D from cylinder head
At coolant temperatures of approx. 80 °C to 94 °C, the main valve as well as the bypass valve are more or less open, depending on engine load. The coolant flows via radiator (A) and bypass line (C) to coolant pump.
At coolant temperatures above approx. 94 °C, the bypass line (C) is closed by the bypass disc. The entire coolant volume should flow through radiator. It is therefore wrung to remove thermostat for better cooling of engine. Diagrams
Venting of thermostat housing
a from radiator
b to radiator
The thermostat housing is suspended and has an integrated, independent venting system.
The air in the cooling system can flow to the radiator and the expansion tank through a bore which bypasses the thermostat
This venting method has the following advantages:
a) When filling-in the coolant, the coolant circuit will be independently vented.
b) Better continuous venting of coolant circuit when engine is operating.
Engine cooling
The spring-loaded closing cap (code number 100 or 120) on radiator establishes a gauge pressure of approx. 1 bar or 1.2 bar in cooling system.
The cooling system is filled ex factory for use throughout the year with a coolant which comprises approx. 55 % by volume of water and 45% by volume of anti-freeze.
The anti-freeze provides protection down to -30 °C and by means of additives in anti-freeze prevents corrosion in cooling system. Since the additives are subject to ageing, the coolant should be changed every three years.
To prevent corrosion the concentration of the antifreeze should not drop below -20 °C anti-freeze protection (30 % by volume).
If no antifreeze is available and only water is filled in, be sure to add 1 % of treating compound (anti corrosion oil) (10 cc/l water).
On model 115.1 always add 1% =10 cc/l of treating compound for lubricating heater valves, also when using antifreeze.
The anti-freeze increases the boiling point, which amounts to approx. 118 °C for water at 1 bar (atu) gauge pressure, to approx. 125 °C by the mixture filled-in at the factory.
The red mark on telethermometer begins at 122 °C (since the middle of May 1975, formerly at 115 °C).
When driving under full load, on mountain roads and bumper to bumper, or following a fast ride on an express highway with subsequent traffic congestion, or when driving in areas with high outside temperatures, the coolant temperature indicator may rise up to the red mark if an antifreeze of at least -30 °C is filled in, without any ejection of coolant or faulty running of engine.
When the engine is operated for an extended period with the vehicle stopped, e.g. vehicle congestion, it will be of advantage on vehicles with automatic transmission to move the selector lever into position ,,N". This will reduce the heat development in transmission and thereby additional heating up of coolant by way of the transmission oil cooler.
When coolant is lost by leaks in cooling system or by ejection as a result of overheating, add a pertinently prepared coolant.
Losses caused by evaporation can be compensated by adding tap water.
A Standard cap
B Spare part cap
A radiator cap with a higher opening pressure is installed since June 1983. The pressure amounts to 1.2 ± 0.1 bar. The modified cap has the code number 120. It can also be installed in vehicles manufactured
at an earlier date.
This cap guarantees min. 1 bar gauge pressure in cooling system, also in older vehicles. Increased protection against coolant ejection will be attained.
A Standard cap
B Spare part cap
The spare part cap differs from standard cap by a resilient sealing disk (rigid on standard cap), as well as by an additional rubber sealing ring. It is also identified by a blue color dot on top.