Keeping Your Cool Under the Hood
Comprehensive periodic maintenance is the key
Old-Time Technical
by Pierre Hedary
The cooling system is one of the major systems that must be kept in good operating order if your Mercedes-Benz automobile is to be reliable for a long period of time.
But periodic maintenance is an easy way to keep your cool under the hood. In particular, our cars use a number of aluminum and, more recently, plastic cooling-system parts. If these parts are not serviced properly, major failures can occur that could have been entirely prevented with sensible maintenance. Let’s take an in-depth look at all the basic parts of the cooling system, how they can fail, and what to check and do to prevent premature failure.
As a rule, perform a coolant service on your Mercedes every two years or 30,000 miles. This service should include a thermostat replacement and an inspection of the cooling system’s plumbing.
Radiators
Starting at the front, Mercedes-Benz radiators are made by Behr, though Nissens and Auradia both make great replacement units. As a rule, all Mercedes cars had complete metal radiators up to 1980. The exceptions are the W123s, some of which had plastic units from 1977, and the 107s, which used full metal units up to 1985. The plastic radiators are notorious for upper radiator neck failure. If a neck fails, you must replace the entire radiator.
Over time, deposits can build up in the water channels in any radiator, which will constrict coolant flow and hence cooling efficiency. Signs of a constricted radiator include brown coolant, calcification (white powdery buildup on cooling-system parts), and overheating in traffic or at high speeds when all other items are in order.
Older Mercedes-Benzes -- this is a 240D (W115) -- came with durable metal radiators,but they're vulnerable to hose failures and corrosion
You can have a good radiator shop remove the tanks (solid metal upper and lower sections) from your vehicle and clean them with a rod, or replace the core (the finned section) if it is rotten. Plastic radiators are different. Cleaning is difficult due to the frailty of the plastic parts, and the aluminum core can contract if overheated. The Mercedes-Benz flushing agent may help your plastic radiator, but very constricted radiators should be replaced.
A classic case of plastic radiator failure on a 1985 280SEL
Fan Clutches
Mercedes has used fan clutches on cars with air conditioning since the advent of the M189 engine in 1962. From 1968 to 1972, all Mercedes cars had viscous clutches. Afterward, lightweight plastic fans were adopted for the 4-cylinder models.
Mercedes-Benz fan clutches fall into three classes: thermostatic (coupled by temperature of the fan-clutch oil), viscous (continuous coupling by fluid, governed by speed), and electromagnetic (engaged by a thermo-switch). By 1965, M-B engineers became huge fans of viscous clutches, which were used up to 1991 on M117 engines. Six-cylinder gasoline engines, starting with the M103, used a thermostatic clutch that engaged at 100 C. Four-cylinder 190 sedans, gas and diesel, used a magnetic clutch, activated at approximately 100 C by a temperature switch on the engine. All 602, 603, and 606 diesel engines use a visco-clutch. All visco-clutches should be checked for excessive free-wheeling, and if the engine does not cool down from traffic under acceleration, or the car overheats at speed, you may need a new one.
To prevent fluid loss, Mercedes Visco fan clutches, like these, should always be stored in an upright position when removed for service.
Also, we should mention the electric auxiliary fans in front of the a/c condenser. These fans were developed to help keep the condenser cool in traffic and to dissipate engine heat. This program started in 1970 on most models with air conditioning. Beginning in 1990, all Mercede-Benzes had dual electric fans. These fans help keep the engine cool, especially important because emissions requirements have continuously demanded that engines run much hotter. A switch on the a/c drier turns these fans on at 52C, or if the engine temperature reaches 115C. From 1985, there was also a high/low-speed function.
Circulation
Inspect your cooling hoses every six months and check them carefully on any used Mercedes you plan to buy. They should not show any evidence of cracking and should be firm when squeezed. Also, you should always use the hose clamps designated by Mercedes-Benz for each specific application, especially on plastic fittings. All Mercedes-Benz hoses with formed curves (illustrated at right) should be examined carefully for failure. However, many M-B hoses are good for decades.
The water pump at the front of the engine should also be checked for operation and leakage. With the engine running, make sure that the water pump shaft is not wobbling and that the weep holes are dry. If a weep hole leaks, replace the water pump immediately. During water-pump replacement, especially on aluminum V-8 engines, watch out for broken bolts.
On the water pump, inspect the weep hole the little hole in the snout -- up toward the pulley, indicated by the arrow -- for leakage, especially when there is a mystery coolant leak.
Molded or formed hoses are vulnerable both to leaks and to deterioration of the inner walls, which can cause them to collapse and restrict coolant flow.
On branched hoses, inspect hose unions for leakage or deterioration, even if they are hard to see. This hose is for a 1987 300D (W124).
Thermostats
Our last area is the thermostat department. These must be replaced every two years and as a standard practice on any used car you buy that doesn’t have service records. If you haven’t done this job before and perhaps can’t even locate the thermostat – it’s tucked out of the way on some models – find a workshop manual for reference or ask an experienced friend for assistance.
The thermostat, installed in a housing like this, should be replaced every two years as a standard precaution.
The thermostat housing should be inspected prior to replacement, and, if it has been sitting for a while and your housing is held together with 10mm bolts, carefully use an impact-drive wrench and penetrating oil to ensure that the bolts don’t break off. Always make sure to install a new seal ring with each thermostat.
I’ve included with this article a chart of the different thermostats for the more popular Mercedes-Benz engines.
Coolant
The characteristics of the coolant are important, and coolant generally isn’t drained and replaced as often as it should be. Changing it every year is a good idea. Mercedes cars with metal radiators have a 19mm radiator plug, a plastic plug on the driver’s-side bottom on aluminum radiators, or a petcock on the right-side tank on cars with plastic radiators with tanks on each side. The engine block should also be drained. The engine block plug will be a 19mm fitting on the passenger side, toward the back and bottom of the block. V-8 cars have plugs on both the left and right sides, and both sides should be drained.
After flushing (you may use water or the Mercedes-Benz citric acid agent), refill the coolant until there are no bubbles left in the system. (Be careful to dispose of your radiator fluid in accordance with local environmental guidelines – it’s a contaminant and should never be drained into a sewer.) If you have a car with no separate radiator tank, and a thermostat that points upward, there will be a number of plugs in the thermostat housing that you can remove to bleed the system of air. When refilling, if you do not have a plug to bleed from, then run the car and fill the radiator slowly, allowing the top radiator hose to burp until you have filled the system to capacity. If you have a separate tank, fill the system from the top radiator hose first, and then reattach it. Next, fill the overflow tank. As you’re filling, check carefully for leaks.
Always use a 50/50 mix of coolant and distilled or tap water. Mercedes-Benz coolant, specifically formulated to be compatible with the materials in your engine, is recommended. If you doubt that M-B coolant is really necessary, remember that having a cylinder head deck-welded is expensive, but buying a $20 gallon of coolant from your dealer once a year that is specifically designed to protect your aluminum or plastic engine parts is cheap insurance.
Happy motoring!
A classic case of plastic radiator failure on a 1985 280SEL
To prevent fluid loss, Mercedes Visco fan clutches should always be stored in an upright position when removed for service
On the water pump, inspect the weep hole – the little hole in the snout, up toward the pulley, indicated by the arrow – for leakage, especially when there is a mystery coolant leak.
Molded or formed hoses are vulnerable both to leaks and to deterioration of the inner walls, which can cause them to collapse and restrict coolant flow.
Inspect hose unions for leakage or deterioration, even if they are hard to see. This hose is for a 1987 300D (W124).
Engines and Thermostats
Engine Type Thermostat temperature MB part numbers
OM621 79C 100 200 0715
OM615 87C, with fan clutch 110 200 0515
80 C With solid fan 617 200 1815
or similar
OM616 80C 617 200 1815
OM617 80C 617 200 1815
OM617A 80C 617 200 1815
OM601 80C 602 200 0015
Updated by Mercedes
OM602/OM 602A 80C 602 200 0015
OM603A 80C 602 200 0015
OM606/OM606A 80C 602 200 0015
M180 80C 617 200 1815
87C 110 200 0515
M127 80C 100 200 0515
617 200 1815
M108/M129 80C 100 200 0515
617 200 1815
M130 80C 100 200 0515
617 200 1815
M110 87C, cold regions 110 200 0515
80C, warm regions 617 200 1815
M102, incl. 16 valve 87C, cold regions 110 200 0515
80C, warm regions 617 200 1815
M103 87C, cold regions 110 200 0515
80C, warm regions 617 200 1815
M104 87C, cold regions 110 200 0515
80C, warm regions 617 200 1815
M116/M117 75C 116 200 0015
Up to 1980
M116/M117 84C 116 200 0215
up to 1985
M116/M117 80C 116 200 0315
from 1986
M119 80C 116 200 0315
M115 87C, cold regions 110 200 0515
80C, warm regions 617 200 1815
M100 79C 100 200 0715