W126 heater stuck on full

[bob1179]

Morning all, just a quick question for all you w126 gurus out there.

As the title says, the heater on my 300SE is stuck on full and no amount of control knob fiddling makes any difference.

As I understand it, there is a heater control valve which can fail with age, does anybody have any tips on replacing it? Is it a fairly straight forward job? I don't suppose anybody knows wher I can get hold of one at short notice if this is the problem?

I need to fix it this week as I have a 300 mile drive on Friday which will be sweaty if I have to do it in a boiler room!

Thanks in advance.

[r129sl]

It is a straight forward job. There is a valve on the bulkhead called a duo valve. The hot coolant comes from the engine side in a single pipe and then this paired valve regulates how much is allowed through onto the cabin side, one valve and one pipe for each side, left and right. The valve itself consists of a plunger with a plasticky membrane round it which moves up and down. The membrane splits. Easiest way to fix it is with new plungers (cheaper, and you don't have to remove the valve). Takes about ten minutes. There are quite a few how-tos on the web. In the US it is a mono valve (because they have climate control there which is not two-sided) but it works in the same way. The plungers are avaliable from the dealer at about £75 each. Or you could get a used valve on Ebay and dismantle it. Apparently you can buy them from Bosch as well at a much reduced price.

[bob1179]

Cheers for the helpful reply!

I shall take a look at lunchtime today, from reading a few threads on the MB forums it looks like this may well be the issue.

Just need to get some spare parts now.

[somnv]

For a used duo valve try Rob - [e-mail] . They break all sorts of Merc, and his prices are usually pretty good. Tell him Norman sent you.

BTW - There is also a W126 club if you are interested - www.126-sclass.com

[bob1179]

somnv said:

For a used duo valve try Rob - [e-mail] . They break all sorts of Merc, and his prices are usually pretty good. Tell him Norman sent you.

BTW - There is also a W126 club if you are interested - www.126-sclass.com

Cheers for the link! I appreciate the help. Due to work and it being dark all the time up here I still haven't got round to even taking a look under the bonnet yet. I will be doing this tomorrow...

The W126 club looks great too, I shall take a look at it.

[bob1179]

r129sl said:

It is a straight forward job. There is a valve on the bulkhead called a duo valve. The hot coolant comes from the engine side in a single pipe and then this paired valve regulates how much is allowed through onto the cabin side, one valve and one pipe for each side, left and right. The valve itself consists of a plunger with a plasticky membrane round it which moves up and down. The membrane splits. Easiest way to fix it is with new plungers (cheaper, and you don't have to remove the valve). Takes about ten minutes. There are quite a few how-tos on the web. In the US it is a mono valve (because they have climate control there which is not two-sided) but it works in the same way. The plungers are avaliable from the dealer at about £75 each. Or you could get a used valve on Ebay and dismantle it. Apparently you can buy them from Bosch as well at a much reduced price.

Sorry for the delay in replying to this! Now Chrimbo and New Year are behind us I am trying to get his fixed. I spoke to Mercedes and they want £377.00 for a new valve I have a part number for a replacement kit (0008350644) but it looks like this is for the mono-valve plunger and seal. I have been trawling the internet and various forums and I'm coming up blank with a part number for a duo valve repair kit. I don't suppose you'd know anybody who knows the part number or where to get a duo valve repair kit from?

I'll be buggered if I'm paying £377.00 for a new valve!

[r129sl]

I've got the part number for the insert or cartridge, described as a magnet in the EPC, at home in the history of my 126. I'll have a look. I think it is A0008350644, which is currently listing at £93.96 including VAT and two are required. I seem to recall paying about £150 for a pair of them maybe four years ago. Much easier to change the inserts than the whole valve.

You might want to investigate a used valve (try German SPOB) or buying the inserts from a factor as the part is made by Bosch and I believe is available other than through Merc.

[bob1179]

r129sl said:

I've got the part number for the insert or cartridge, described as a magnet in the EPC, at home in the history of my 126. I'll have a look. I think it is A0008350644, which is currently listing at £93.96 including VAT and two are required. I seem to recall paying about £150 for a pair of them maybe four years ago. Much easier to change the inserts than the whole valve.

You might want to investigate a used valve (try German SPOB) or buying the inserts from a factor as the part is made by Bosch and I believe is available other than through Merc.

I appreciate your help, I had an inkling you might be able to help.

I will definitely buy new as there doesn't seem much point buying a used valve. I've been quoted £605.10 all inclusive to replace the old valve with a new one!

[r129sl]

Replacing the inserts is a dead easy DIY. I mean, I managed it and I'm a total spaz.

These sites are interesting:

https://mercedessource.com/node/570 (the insert in the monovalve is the same as the insert in the duo valve except inthe latter case there are two of them)

http://www.partsgeek.com/w9m81gd-mercedes-300sel-m...

There's quite a lot of internet stuff about these parts.

Edited by r129sl on Wednesday 16th January 14:10

[bob1179]

A further update on this.

I just placed my order with Mercedes Parts online at the advertised price of £93.96 which was processed and a confirmation sent. I have just had an email back telling me the order has been cancelled as they 'made a mistake' and the parts should be £142.56! That is an increase of more than 50%! They've offered me a 15% discount but it is still a massive hike.

I'm currently not a happy bunny.

[r129sl]

A deal is a deal but it may well be that their terms and conditions provide for this eventuality.

One question: do you need both sides? I know on my 124 the passenger side seems to work fine and just the driver's side is stuck on hot.