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Installing a New Thermostat on your Mercedes-Benz
 
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Pelican Technical Article:

Installing a New Thermostat on your Mercedes-Benz

Steve Vernon

Time:

1 hour1 hr

Tab:

$45

Talent:

**

Tools:

8mm, 10mm socket, flathead screwdriver

Applicable Models:

Mercedes-Benz W126 (1981-91)

Parts Required:

Thermostat, gasket, coolant

Hot Tip:

Work on a cold engine

Performance Gain:

Engine runs at the proper temperature

Complementary Modification:

Replace older hoses

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.

Next remove the two electrical connectors.

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.

Installation is the reversal of removal. Don't forget to top off your coolant!

PLEASE NOTE: If you have a 1977 450SL, the thermostat is not located in the housing. Instead, it is mounted to the right side of the housing for the top hose adapter you removed. Look on the right side, just below it.


Once the car is cold, begin by removing the cap on the coolant reservoir to release any residual pressure.
Figure 1

Once the car is cold, begin by removing the cap on the coolant reservoir to release any residual pressure.

Remove small hose going from the top of the thermostat housing to the coolant reservoir (green arrows).
Figure 2

Remove small hose going from the top of the thermostat housing to the coolant reservoir (green arrows).

Next remove the two electrical connectors (green and red arrows).
Figure 3

Next remove the two electrical connectors (green and red arrows). The yellow arrow points to where the overflow hose attaches.

Remove the clamp from the large hose coming out of the thermostat housing (yellow arrow).
Figure 4

Remove the clamp from the large hose coming out of the thermostat housing (yellow arrow).

Two of the bolts also hold a bracket with a ground wire on it.
Figure 5

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 three 10mm bolts holding the top of the housing to the base.
Figure 6

Remove the three 10mm bolts holding the top of the housing to the base.

With the bolts removed you can remove the top of the housing along with the thermostat.
Figure 7

With the bolts removed you can remove the top of the housing along with the thermostat. The thermostat will only fit into the housing one way. This photo illustrates the side hole in the housing (green arrow) that the side bracket on the thermostat (red arrow) must fit into.

You should clean the contact surfaces of the housing of any gunk or corrosion (red arrow).
Figure 8

You should clean the contact surfaces of the housing of any gunk or corrosion (red arrow). Always install a new seal or gasket even if you are reusing your thermostat.

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Comments and Suggestions:
Todde14 Comments: Photo of no thermostat
October 4, 2016
  Followup from the Pelican Staff: Someone may have removed it from your engine. Can;t be sure on non_US spec models. - Nick at Pelican Parts  
Todde14 Comments: Guys I have a 1989 420sel and there isn't a thermostats where you suggested, took the bolts and hoses off to remove the housing to find no thermostat or enough room to fit one ???
October 4, 2016
  Followup from the Pelican Staff: Someone may have removed it from your engine. Can;t be sure on non_US spec models. - Nick at Pelican Parts  
Shaun Comments: I have a 76' 380sec and gets too hot in heavy traffic, the cooling fan comes on at 90 degrees and i think that's too late. Is it the thermostat/radiator? please help.
March 24, 2016
  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  
Chef Daddy Comments: Heres a piocture of the inside of my "thermostat housing" PLEASE HELP!!!!!
February 10, 2016
  Followup from the Pelican Staff: Got it, thanks. - Nick at Pelican Parts  
Chef Daddy 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!!!!!
February 10, 2016
  Followup from the Pelican Staff: Yours is mounted to the right side of the housing for the top hose adapter you removed. Look on the right side, just below it.- Nick at Pelican Parts  
Keenan 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?
October 24, 2015
  Followup from the Pelican Staff: You'll need to replace the housing. Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
280 SEL guy 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 ???
September 15, 2015
  Followup from the Pelican Staff: Thermostat is a good place to start with the engine temp issue. Also check for vacuum leaks if the idle is high. I’m not the best with part numbers.

Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need. - Nick at Pelican Parts
 
Capta76 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.

Anyone know the name of that hose?
Love this site, it's helped me more than once.
April 11, 2015
  Followup from the Pelican Staff: Just a coolant hose.

Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
adam Comments: Thanks for the advise, Nick
March 20, 2015
  Followup from the Pelican Staff: No problem. - Nick at Pelican Parts  
Adam Comments: If my mercedes 560sl, 1988 gets hot in heavy traffic, should I remove the thermostate or should I change it? I live in florida, how much should the temperature for the thermostate be?
March 19, 2015
  Followup from the Pelican Staff: I don't know the operating spec of your thermostat.

Check that the cooling fan is coming on when in traffic, if not that may be your issue. - Nick at Pelican Parts
 
Esperenzo 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
January 13, 2015
  Followup from the Pelican Staff: It could keep the engine cooler. However, there may be an underlying issue. Check the water pump, etc, also. - Nick at Pelican Parts  
Annoyed as hell Comments: This is not where the thermostat is at on a 1987 560SL its on the side. Its the bypass housing. Due to this error now I have to change the damn water pump cause one of the bolts broke half way and its impossible to take it out.
September 21, 2014
  Followup from the Pelican Staff: Thanks for the feedback. I will have someone look into and update the article as needed. - Nick at Pelican Parts  
Tony Comments: 1975 Mercedes 450sl

Thanks for the help...got the new thermostat and replaced it successfully. I also ordered and received the coolant sensor but can't seem to find the location. Any ideas on how to trace it?
August 8, 2014
  Followup from the Pelican Staff: This is going by memory, but I recall it being next to the thermo-time switch. Near the cylinder head and intake manifold. - Nick at Pelican Parts  
Tony Comments: Thanks for the help...got the new thermostat and replaced it successfully. I also ordered and received the coolant sensor but can't seem to find the location. Any ideas on how to trace it?
August 7, 2014
  Followup from the Pelican Staff: What vehicle are you working on? - Nick at Pelican Parts  
fastx19 Comments: This does not apply to the 420sel range. The thermostat is not located on the top, instead it is on the side where the lower hose from the radiator connects to the water pump.
August 4, 2014
  Followup from the Pelican Staff: Thanks for the help. I will have the article updated. - Nick at Pelican Parts  
Tony Comments: Thanks....I'll go out right now to see if I can trace it...Once again, thanks.
July 30, 2014
  Followup from the Pelican Staff: No problem. - Nick at Pelican Parts  
Tony Comments: Does anyone know where the thermostat is on a 1975 450sl. Not sure which hose to follow.
July 28, 2014
  Followup from the Pelican Staff: Shoud be at the front of the cylinder head. Follow the hose that goes to the cylinder head. - Nick at Pelican Parts  
Jakli123 Comments: HI Please can you tell me what are the 2 electric connectors are for Figure 3? I know one sends signal to the temp displayshown in red arrow, but what is the other one used for Yellow Arrow ?
May 24, 2014
  Followup from the Pelican Staff: The yellow arrow is pointing to the overflow hose connection. - Nick at Pelican Parts  
Clint 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.
November 21, 2013
  Followup from the Pelican Staff: Thanks for the heads up and we will look into our tech article - Kerry at Pelican Parts  
Clint 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.
November 15, 2013
  Followup from the Pelican Staff: I would double check the part number for the thermostat you purchased and be sure it is for your vehicle. - Nick at Pelican Parts  
fasterthanu87 Comments: also curious on the 83' 380 sec how to properly purge the cooling, I've heard remove the highest coolant sensor but does this car not have a purge port? Won't auto purge just heating up so far.
October 23, 2013
  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  
fasterthanu87 Comments: ummm fyi, missing a thermostat wont cause an overheat..it was likely removed due to an overheat issue. not having one will make it cool faster but reach operating temperature a lot slower
October 23, 2013
  Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help. - Nick at Pelican Parts  
Deb 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.
September 23, 2013
  Followup from the Pelican Staff: Try using the silicone sealan1t to glue the gasket to the housing and let it cure for awhile. Then you should be able to position it and get the bolts in. - Kerry at Pelican Parts  

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