Monovalve Problems
 

Thanks for the quick response to my heater problem. How much of a job is it to change the monovalve in a 420sel? Is this inside or under the hood? and would this valve effect the air cond. in any way?

The valve just controls the flow of coolant through the heater core. Not likely related to any a/c issue.

You don't need to change the valve, just the insides. Buy a rebuild kit. You'll see it between the inner and outer firewall, just a bit to the passenger side of the middle. Heater hoses lead to it on from both sides. The kit you buy has a piece of paper that describes how to change it, or at least the one I bought did. It takes more time to find all the right tools and drain some antifreeze than relacing the parts. A really simply DIY job.

I'll second the advice on just buying the rebuild kit. It's basically the moving piston part. I replaced the guts of mine on my '91 420SEL (W126) a year ago, more or less, at about 170k miles. Symptoms were basically intermittent periods of no heat.

The monovalve is basically a pulse-width modulation system, that is basically either fully open or fully shut. Full open corresponds to full heat, full shut corresponds to no heat at all. For anything in between, the valve opens and closes at the appropriate duty cycle. You can do a fair amount of diagnosing by placing your finger on the top of the valve body - you can feel the valve "cycling" if it is working properly. In my case it would sometimes open up if I set my ACC "temperature wheel" to full hot, but it wouldn't cycle at normal termperature settings. What tends to fail on these is that coolant seeps past worn seals, leading to corrosion. No power corresponds to full-open (i.e. full heat) = a fuse failure of the ACC system results in full heat. If you put the ACC wheel in the opposite state, you should definitely feel the valve move when you remove and re-apply the electrical connection.
I think it was this forum where I searched and found a step-by-step set of instructions to installing the rebuild kit, but it was fairly intuitive.

A couple of tips, from memory:

- I recall moving one or more things out of the way, to get more access or working room above the valve. But now I can't remember what I moved. Maybe it was a switch that detects the hood being opened? I might also have moved the windshield washer fluid reservoir out of the way, and maybe even taken the battery out to get more working room. But I can't remember for sure.

- You want to remove the coolant reservoir cap to release pressure from the cooling system before taking the top off the valve. BUT (and this wasn't mentioned in the instructions I found), put the coolant reservoir cap back ON, after releasing the pressure but prior to actually opening up the top of the valve. Otherwise a lot of coolant will drain out onto the floor. With the cap put back on, very little coolant will leak out.

- I forget the details now, but be very careful with removing the electrical connector, so as not to crack the plastic of the retaining clip.

It is a 1 on a scale of 1 to 10 - Easy as eating pie!

Here: A picture is worth a thousand words:

http://mbdieseldiy.tripod.com/monovalve.htm

http://www.peachparts.com/shopforum/showthread.php?t=131227&highlight=monovalve

rschleicher,
Even though this post of your was 2 years old it helped me when I took my Monovalve out yesterday. You are right about previous posts or instructions NOT telling you to replace the coolant cap prior to taking the monovalve out. first time I lost quite q bit of coolant. Second time (today)(dummy here reassembled the monovalve with the coil cover upside down with the slots at the bottom) I had read your post and uncapped the coolant overflow tank, and then recapped it. The loss today was less than half of yesterday's loss.
It pays to read these DIY hints and posts...even if there are some age to them.
Thanks.

JohnHef just did mine and it looked fairly easy. Buy a new one. I have a 420 SEL as well. Mine car is toasty warm now.

Would this being faulty cause the A/C to blow cold for about 30 minutes, then blow out hot air? Our 88 W126 does this. The heat works fine in the winter.

I was told that the evaporator drain could be clogged as well. I am at a crossroads. :confused: