Folks,

I have a '68 250 and I'm trying to sort the heat/AC system.

The AC part is easy -- the compressor's been removed, but all the wiring, etc. is intact. It's not a big problem since the car is only driven on mild days and I'm happy to leave it off.

I just rebuilt the heater monovalves and reconnected them to the red-triangle sliders on the dash (the control cables were broken and the valves were locked up in the off position). The blue sliders and the demister knob/motor work fine, too.

Here's the dilemma: Even with the red slider controls off, I still have a lot of heat bleeding through the defroster vents and out of the round vents at the outside edges of the dash.

Is there something I've missed with the AC controls (the lever control on the console or the round AC temp gauge? Is there some way they should be set or blocked off?)

Basically, I'd like to have a nice cool cooled-by-airflow cabin, with the heat available for windshield clearing, etc. on the rare occasion.

Any help you can give me will be greatly appreciated.

All the best,

Matt

Hello Matt,

Which chassis? Is it a W108 or a W114? Can you post a picture?

__________________

1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o
1957 Ponton 220S
2001 S600 Daily Driver
The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com

Have you confirmed that by attaching the red temperature control cables to the levers on the heater valve you actually see them move when the sliders are moved? Next, are the valve innards installed so that the opening the slider is opening the valve? I don't recall whether it is possible to install the valve guts only one way, or not. Finally, make sure there is full movement of the cable when you move the slider, could be that the cable is deflecting or slipping in its clamped restraint rather than fully shutting off the valve inside the valve body.

First, here's a photo of the car:

It's a '68 W114. I turned 100K last summer just before putting it away for the winter.

I added new o rings to the monovalves and reinstalled the cores in the full-open position (with the hole in the valve center lined up with the inlet port in the side of the valve housing). I then set the control sliders to full open and reattached the lever arms on the valves. The controls are stiff, but they do have full travel. I seem to have the heat-all-the-time problem even when I manually close the valves on the firewall.

I'll keep experimenting, but so far, I'm still stumped.

Matt

Nice car!

__________________

1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o
1957 Ponton 220S
2001 S600 Daily Driver
The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com

Thanks!

It really is a sweetheart. It may look conservative from the outside, but the interior is fire-engine-red MB tex. It surprises a lot of people when they look inside!

Matt

Also stumped...no good reason for the hot water to flow if you have the valves closed. A thought...are you waiting long enough after closing the valves to assure yourself that the flow is genuinely stopped. I ask because heated air will still be delivered for some time. It takes time for the hot coolant in the heater core to be cooled down by the fresh air passing over the core. Might take a couple of minutes before it is completely cooled down. Other than that I cannot think of why you would have this problem. When you find it, I'm sure it will be a head-slapping-why-didn't-I-think-of-that solution.

Good luck.

Problem solved!

As I suspected, the problem was with the unused AC controls, particularly the large horizontal slider in the console below the "regular" climate controls.

Since the AC compressor was removed by the previous owner, the lever and the temp. control knob had been closed up inside the console. The slider was set to "off" instead of "AC" (which I suppose makes some sense with no AC in the car). Now with the ability to adjust it I can control the fans and the air-control valve down by the accelerator pedal.

But sheesh, what a complicated system!

M.

Matt,

Minor point, but to avoid confusion in the future, you do not have a monovalve. A monovalve is used in the auto temp cars and is elctrically controlled.

Your car has a pair (for left and right) of mechanical cable operated heater valves that are considered as one unit. Older style is metal, but I've seen some later ones that had plastic housings.

A previous poster mentioned a lag time before the heat dissipated. MBs work the heat modulation by flow, vice moving a circulation air flap. So, the latent heat in the core must be removed for you to get ambient airflow.

Another problem I've seen on these valves is limited travel wrt the cables.
This can prevent the valve from completely closing.

Nice car, we're pretty partial to our 250 as well.

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

The car has the brass housing for the combined left/right valves. They were stuck in the closed position, but after opening them up and cleaning all the parts, all I had to do was install new o-rings.

The control cables were pretty stiff, but several gallons of penetrating oil loosened them up. Out of curiosity, has anyone purchased a set of these from MB? I never can tell if replacement parts will be $3 or $3000.

The real weak point in the system turned out to be the cast-metal control levers in the dash unit -- the left and right heat control levers were snapped off. I picked up a used unit on eBay and swapped out the broken bits. The eBay unit was a few years newer, and although the lever geometry is identical, the arms had been reinforced where the old ones had broken. Apparently a big enough problem to warrant a little redesign.

Matt