I guess I don't understand. From reading the previous posts in this thread, I understood that the problem is that pressure past a torn diaphragm inside the monovalve causes it to close - not an electrical problem. So I guess I don't understand how grounding and energizing it ("it" being what I presume to be an internal solenoid) is going to make it stay open.

And that is the plan - once I can get one. But as previously stated I don't see how I can get one AND find time to install it before around 10 am Thursday morning.

On the other hand, loosening two hose clamps, removing the hoses from the valve, and re-joining the two hoses with a plastic elbow - which is something I can pick up at any local auto parts store - seems more do-able to get the immediate result - until I can get the rebuild kit.

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

My monovalve was bad and here are some pictures of it.

Attached Thumbnails

     

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1983 300SD

The boot tearing is the most common cause of no heat. A 5 cent rubber part. The boot can be taken off via a clip on top and you just replace that if you have a boot that is not torn. Used monovalve with a good rubber boot will do the same thing. The cylinder and pin in the cylinder is not defective, just the boot for 99% of the time. Head to your local junkyard and pull monvalves until you find one with a good rubber part and you're good to go. Takes all of two minutes to remove and replace. Gas engines have the same part.

I believe another simpler solution would be removing the "innards" of the monovalve. The only drawback is no control over heat and you may need to open windows.

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Sam

84 300SD 350K+ miles ( Blue Belle )

DAMN you guys are GOOD! That was going to be my next question! I thought of that as a possible temprorary solution last night after I went to bed! If I did this, wouldn't I still be able to control the heat by turning the fan switch to high/low/off? No way to change the temperature of the air output, but if I can turn the heat to high/low/off, that will do for our trip tomorrow.

That's an easy one to test - I can just unplug it before I head home from work tonight and see what it does. If that doesn't do the trick (temporarily) then I'll pull the innards out tomorrow morning.

BTW, I did an eBay search on "Mercedes monovalve" and only got one hit for a rebuild kit - for 56 bucks!

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

You might want to carry a clamp so that you can make a primitive adjustment (partially or completely clamping a heater hose) to the flow through the heater core. Full heat can be unbearable, even with the fan off.

Good news and bad news.

The bad - unplugging the power from the monovalve did nothing. The one on my car apparently needs power to OPEN, not to close. The rest of the bad news is that the diaphragm in it is totally shot - ripped all the way around where it attaches to the plunger.

The good news is that removing the "guts" worked! All I had to do is remove the e-clip from the end of the shaft and slide the plastic spacer ring, rubber stopper, and filter screen off the shaft and slide the soleniod and the rest of it back into the body of the monovalve. I left the outer portion of the diaphragm in place because it didn't look like it would seal up water-tight without it.

Took it for a test drive and VIOLA' I have all the heat I need! With the blower off it barely puts out just a tiny bit of warm air, but with the blower on high it warms the interior up real nice and real quick, and with the blower on low it still blows enough warm air to keep it nice and comfortable inside.

For anyone else in this situation, this is just a temporary fix, but one that will work quite well until you can get the parts to fix it permanently...

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

Don't bet on it. With a torn diaphragm, electrical control is fairly irrelevant.

Well, I'm just guessing by looking at the way it is made, but since it is solenoid operated, and the at-rest position position for most solenoids is with the center bar sticking out of the coil, it seems like the at-rest (de-energized) position seems to be closed - with the rubber stopper on the end of the plunger pushed down into the hole where the bottom hose attachment tube is. It at least LOOKS like it would be closed with no power applied to the coil. But I certainly could be wrong - wouldn't be the first time.

When I replace it I'll mess around with the old one I take off to see what it does with and without power.

__________________
1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown