Mercedes-benz w126 thermostat replacement 1981-1991 s-class pelican parts diy maintenance article static electricity review worksheet


The thermostat helps control the engines temperature. If your car is running too hot and there is the proper amount of coolant in the car and it is not leaking, or your car is taking a long time to warm up, there is a very good chance your thermostat is bad and needs to be replaced.

The thermostat is located on the top of the water pump just to the right of the distributor. If you are going to be working on the thermostat, make sure the car is cool and not under pressure. Working on a hot engine or one under pressure can cause serious harm (coolant is also very toxic) and should never be attempted. Coolant needs to be collected and disposed of in accordance with your local regulations, as pouring coolant down a drain or into the street is illegal. Also, be sure to wash your hands thoroughly after working around it.

With the car cool, begin by releasing any residual pressure in the system by removing the cap on the coolant reservoir. Next, move to the thermostat itself and remove the small hose going from the top of the thermostat to the reservoir overflow line. There may be a little coolant that spills so be ready for it with a rag. Loosen the hose clamp on the large hose coming out of the back of the thermostat housing.

There are three 10mm bolts holding the top of the thermostat to the housing. These can stick a little as coolant can get in them and corrode them. Two of the bolts also hold a bracket with a ground wire on it. If you want to take the bracket to your bench to clean up you will need to remove this ground wire with an 8mm wrench. Remove the bolts and wiggle the top from the housing.

With the top removed you can pull the thermostat up and out of the housing, sliding it off of the hose at the rear. The inside of the housing can get a little corrosion which you should clean up before installing the new unit. Always install a new gasket, even if you are just inspecting and reusing the old thermostat. The thermostat will only fit into the housing one way as there is a hole in one side of the housing to fit the bracket in the thermostat.

Comments: 1989 560SEC, Yeah, it’s not on the top, it’s on the side of the water pump attached to the lower radiator hose. Three 10mm screws release the cover, the rear one is tricky but doable. Not taking the top housing off means you don’t have to mess with cleaning off gasket surfaces and replacing a paper gasket. Sealing the thermostat cover is a breeze with a rubber gasket. Changing the thermostat cleared a lot of pesky issues for me. Cheap fix!

Comments: is there a good reason to insist keeping my car business moved didn’t let me know tracked where business went by trash collector said he had it towed engine blew? said he could fix it has over 5000. and just started fixing now car apart after saying I would tow it home until he had the time after spending three days on it says im having crankcase problem is this a way to justify getting more money or is it possible there is this problem out of the blue 1984 560 sel 126

Followup from the Pelican Staff: I am not sure what happened. If you don’t trust the mechanics opinion, have a different mechanic look the vehicle over, or have your current mechanic show you and explain what happened. – Nick at Pelican Parts

Followup from the Pelican Staff: In heavy traffic, it could be the radiator. You will have to check coolant flow through the radiator using a temp gun. Check for proper cooling as the coolant flows through. if flow is slow, it could be faulty. – Nick at Pelican Parts

Comments: Hello. I have a 77 450 sl and I went to change the thermoststat following all the aforementioned steps and youre not going to believe this but THERE IS NO THERMOSTAT IN THE HOUSING!!! Instead of a round opening between the three bolt holes there are two strangely shaped holes, one larger than the other. The larger hole is somewhat rectangular and the smaller hole is kidney shaped. There is no way that a thermostat can fit into this area. Im obviously missing something HELP!!!!!

Comments: Hello. I have a 1984 500sel and while changing the thermostat I discovered that the reservoir overflow valve was corroded shut. Just to be clear, I don’t mean the reservoir line, I am speaking about the valve depicted by the your yellow arrow in figure 3 above. How would you advise I tackle this problem, other than replacing the housing?

Comments: I have a 1983 280 SEL. It high revs during warm up and takes a long time. If I put it in gear, the revs come down but in park they go back up. I will look for the thermostat this weekend. I know that it is a grey market 280sel but does anyone know which thermostat I would need ??? Also, in Texas traffic it runs very hot and stalls. Any other ideas ???

Comments: I replaced the thermostat in my 1985 380sl and had some trouble getting the housing back on and into the large hose yellow arrow in the procedure, figure 4. Now it’s leaking from that hose connection, I’d like to replace it but I’m not sure what it’s called. I’m a bit of a novice if you can’t tell.

Comments: I imported my Car MB C200 to use in Southern Africa. I have noted that my temperature rises to 90.The area has extreme outside temperatures of 28 – 35. I was advised by a local mechanic to take out the thermostat, will this help cool the engine or what would that help

Comments: On my 1986 560 SEL The housing is not exactly that shown here, and does not house the thermostat, which is in fact located further down, between the alternator and the block. This, after replacing the thermostat, I confirmed visually on two other 560’s. You may wish to investigate further and correct your instructions.

Comments: Hmmm. Mine is a 1986. I removed the thermostat and there was none inside…the new one wouldnt fit. The housing was not the same as described above. There is a large hose going in, and another large hose going to the engine. No small hose to the overflow tank.

Followup from the Pelican Staff: I prefer to put the cooling system into a vacuum, then draw the coolant into the cooling system. There are a few great tools out there for doing it this way. However, if you don’t have access to one, you can slowly fill and allow the air to bleed, or if in you want, try the method of removing the coolant temp sensor. – Nick at Pelican Parts

Comments: I have no ground wire and am having trouble getting the thermostat and gasket to stay in place so that the screw holes line up for assembly. When we took the housing off there was no thermostat which is why it was overheating. My thermostat does not look like the pic but I have verified it is the correct one.