OM 617 – Coolant circuit and engine cooling

A. Model 115.1 and 123.1 (version up t o February 1979)

The thermostat layout on model 123.1 ( 1 s t version) differs. On vehicles without air conditioning system, the thermostat housing with thermostat i s located horizontally and on vehicles with air, conditioning system vertically for reasons of space.

Layout on vehicles without air conditioning

The ID’S of connections in direction of radiator were reduced from 35 mm to 29 mm.
When filling, unscrew vent screw (arrows) on thermost at housing cover until coolant flows out.

Layout on vehicles with air conditioning

The main valve is closed up to a coolant temperature of approx. 80c 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 water pump.

At coolant temperatures of approx. 80c to 94c, the main valve as well as the bypass valve are more or less opened, depending on engine load. The coolant flows via radiator ( A ) and bypass line ( C ) to water pump.

At coolant temperatures above approx. 94c, the bypass line ( C ) i s closed by the bypass disc. The entire
coolant volume should flow through radiator (refer to overall diagram). I t is therefore wrong to remove
thermostat for better cooling of engine.

If the thermostat is replaced, make sure during installation that the punched-on arrow is pointing upwards or in driving direction toward the rear.

B. Model 123.1 (2nd version starting February 1979)

Thermostat
The thermostat installed on all engines begins to open at 80c.

The main valve i s closed up to a coolant temperature of approx. 80c and the bypass disc is fully opened.
The flow (0) from radiator is thereby interrupted and the coolant flows via bypass line (C) directly to water pump.

At coolant temperatures of approx. 80c to 94c, 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 water pump.

At coolant temperatures above approx. 94Oc, the bypass line (C) is closed by the bypass disc. The entire coolant volume should flow through radiator (refer to overall diagram). I t is therefore wrong to remove thermostat for better cooling of engine.

Venting of thermostat housing
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 cap (code No. 100) on radiator establishes a gauge pressure of approx. 1 bar (atij) in
the 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 -30c 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 two yerars.

If no anti-freeze is available and only water is filled in, be sure to add 1 % of treating compound (anti corrosion oil) ( 1 0 cc/i water).
On model 1 15.1 always add 1 % = 10 cc/l of treating compound for lubricating heater valves, also when using anti-freeze.
The anti-freeze increases the boiling point, which amounts to approx. 118c for water at 1 bar (atu) gauge pressure, to approx. 125c by the mixture filled-in at the factory.

The red mark on telethermometer begins at 122c (since the middle of May 1975, formerly at 115c)
This point must be specially observed when only water and treating agent are filled in. In such a case,
water may be thrown out before the indicator of the coolant thermometer is at the red mark.

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 anti-freeze of at least -30c 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 selctor 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 i s 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.

 

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