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  #1  
Old 09-26-2020, 03:27 AM
cmac2012's Avatar
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Expansion tank, '84 300D

Anybody know if a tank is made that doesn't have that second large outlet on the side? The stopper kit is only $10 but I don't like the idea of having another potential leak spot. I'm trying to think if I could fab a more permanent plug.



The one on the side, you can see the opening. Used on some rig, but not mine.

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  #2  
Old 09-26-2020, 09:46 AM
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The hole on the side is for the coolant level sensor, used on the W126. Once plugged, that bung shouldn't ever leak. The sensor that goes in that hole is sealed with an O-ring and retained with a snap ring.
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Old 09-26-2020, 11:23 AM
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Assuming that stopper is threaded; degrease the inner and outer threads and use the slow curing JB Weld Epoxy and screw it in and it will likely out last the rest of the tank.

Loctite that can take the coolant temp on the threads I think would also work to seal it off.
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  #4  
Old 09-26-2020, 02:50 PM
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You could also fit a sensor and retrofit a light on the dash or the cluster to tell you when your coolant is low.

What I really miss on 85 and older benzes is that they don't have low oil or low coolant sensors (lowers the chance of your motor blowing up) and no light control module light (keeps you from being pulled over for having a light out)
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  #5  
Old 09-26-2020, 04:52 PM
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Quote:
Originally Posted by christuna View Post
You could also fit a sensor and retrofit a light on the dash or the cluster to tell you when your coolant is low.

What I really miss on 85 and older benzes is that they don't have low oil or low coolant sensors (lowers the chance of your motor blowing up) and no light control module light (keeps you from being pulled over for having a light out)
Interesting idea. I was wondering where the collant level sensor was on this one. My 300SDL was saved by the coolant light last fall. I saw that light blink on - miraculously - and glanced up at the temp gauge which was headng in the wrong direction. I pulled over immediately - the engine bay was full of steam. Way better than full of stinky smoke.
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Last edited by cmac2012; 09-26-2020 at 05:04 PM.
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  #6  
Old 09-26-2020, 04:53 PM
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Quote:
Originally Posted by Diseasel300 View Post
The hole on the side is for the coolant level sensor, used on the W126. Once plugged, that bung shouldn't ever leak. The sensor that goes in that hole is sealed with an O-ring and retained with a snap ring.
I was starting to go there. Why fight it, this tank must have a small crack underneath. Need a new one.
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  #7  
Old 09-26-2020, 07:42 PM
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I have serviced these Mercedes diesels over the last 30 years, thousands, have never even
seen one leak, at this place. Buy the plug and be done with it.
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  #8  
Old 09-26-2020, 10:05 PM
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Smile

Quote:
Originally Posted by hercules View Post
I have serviced these Mercedes diesels over the last 30 years, thousands, have never even
seen one leak, at this place. Buy the plug and be done with it.
What he says. Buy the plug kit with confidence, and install.
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  #9  
Old 09-26-2020, 11:39 PM
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Quote:
Originally Posted by Diesel911 View Post
Assuming that stopper is threaded; degrease the inner and outer threads and use the slow curing JB Weld Epoxy and screw it in and it will likely out last the rest of the tank.

Loctite that can take the coolant temp on the threads I think would also work to seal it off.
If you don't want to spend on the plug kit take some time and take the tank to the Hardware store and see if there is an regular metric Blots that have the same threads and use one of the methods I suggested to seal the blot threads.
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  #10  
Old 09-27-2020, 01:13 AM
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Quote:
Originally Posted by Diesel911 View Post
If you don't want to spend on the plug kit take some time and take the tank to the Hardware store and see if there is an regular metric Blots that have the same threads and use one of the methods I suggested to seal the blot threads.
There are no threads. If you took the time to read the rest of this thread or look at the picture of the tank in the OP, you would know that by now.
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  #11  
Old 09-27-2020, 02:31 AM
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Quote:
Originally Posted by Diseasel300 View Post
The hole on the side is for the coolant level sensor, used on the W126. Once plugged, that bung shouldn't ever leak. The sensor that goes in that hole is sealed with an O-ring and retained with a snap ring.
Odd thing is, I can't find a coolant level sensor for a W126 for sale, at least not for a 300SD. It's a different coolant tank than my SDL. I could just buy one of those. I dunno, maybe the idea of putting a sensor in is overkill.

The SDL sensor looks lke it would work.
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  #12  
Old 09-27-2020, 07:08 AM
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Quote:
Originally Posted by cmac2012 View Post
Odd thing is, I can't find a coolant level sensor for a W126 for sale, at least not for a 300SD. It's a different coolant tank than my SDL. I could just buy one of those. I dunno, maybe the idea of putting a sensor in is overkill.

The SDL sensor looks lke it would work.
300SD never had a coolant sensor since they stopped selling it in 85. Coolant sensors appeared on gen 2 cars (86 and up)

A 300SDL has a very different tank to what's in your 1st post. Would have to look up the epc to see if it's the same sensor.

It looks like a tank in my 300SE. Try looking up a sensor for a 300SE (1990 for example) if you want to do the retrofit

Last edited by Usaguy; 09-27-2020 at 07:19 AM.
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  #13  
Old 09-27-2020, 09:29 AM
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Quote:
Originally Posted by hercules View Post
I have serviced these Mercedes diesels over the last 30 years, thousands, have never even seen one leak at this place...
My experience comports with the above.
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  #14  
Old 09-27-2020, 09:31 AM
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Quote:
Originally Posted by cmac2012 View Post
Odd thing is, I can't find a coolant level sensor for a W126 for sale, at least not for a 300SD. It's a different coolant tank than my SDL. I could just buy one of those. I dunno, maybe the idea of putting a sensor in is overkill.

The SDL sensor looks lke it would work.
If you decide to do some sort of retrofit with a sensor, I hope that you'll consider posting the details of your experience. Sounds like it could be an interesting and relatively straightforward undertaking.

Also:
I'm thinking that if a retrofit was done, it could be optimal to connect the trigger wire (I don't know if it is the positive or negative) to the buzzer under the dashboard instead of to a light. This would be the quickest way to get the driver's attention.

Last edited by 5cylinder; 09-27-2020 at 01:15 PM.
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  #15  
Old 09-27-2020, 08:16 PM
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Quote:
Originally Posted by christuna View Post
300SD never had a coolant sensor since they stopped selling it in 85. Coolant sensors appeared on gen 2 cars (86 and up)

A 300SDL has a very different tank to what's in your 1st post. Would have to look up the epc to see if it's the same sensor.

It looks like a tank in my 300SE. Try looking up a sensor for a 300SE (1990 for example) if you want to do the retrofit
This is where my confusion began. Diseasel said the extra port is for the coolant sensor on the W126, and he knows these cars a lot better than I do. My SDL tank is way different. I relaced that sensor on that about a year ago and it looks a lot like the plug kit piece - size and how it's held in. Some parts are so basic one thinks they are probably used on various different models for years, such as the level sensor.

Good advice on the 300SE, I looked it up, it uses the coolant tank advised for the 300D and the level sensors are the same for the SDL. The price difference is a bit puzzling, the Febi goes for $6, that's less than for the plug kit ($10.94)! I don't know if I'm a fool for thinking the higher priced units will be better quality. It's often the case but who knows.

Speaking of the SDL, while I was searching on this issue I found this thread from 5 years ago:

300SDL coolant expansion tank retrofit, tank from 300SD

Good food for thought. My SDL doesn't leak (yet) but it is as opaque as hell. With my really bright little LED work light I can see the coolant level but only with that. The SDL tank looks like it might be larger capacity. Who knows, I suspect the tank in question, the other W126 tank would be big enough.

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