Just to confirm symptoms, I took the car out of gear at 80mph, to see what happened at high speed and low rpms. As soon as it was out of gear, the heat quickly came on.

Since this obviously does sound like leaking monovalve, but mine is new, when I take it out to look at it, if the rubber is intact and the plunger/movement function is good, is there some thing else about the monovalve that can cause this?

thanks

So you're driving with a warm (on the temp gauge) car, at highway speed. The blower fan is blowing but the air temp from the vents is dropping. You go out of D and into N, the RPMs drop, the blower fan keeps blowing, the temp (on the gauge) stays about the same but the temp of the air from the heater rises?

Maybe a problem with the fan clutch fully engaging, rapidly cooling the coolant in the radiator at speed and with higher RPM? But if that's the case, I'd expect to see a corresponding drop in the temp on gauge. I guess it's possible that the engine temp could appear more stable since the head isn't cooling down much and your thermostat acts quickly to shut off the flow to the radiator, and then also the flow of hot coolant to the heater core?

With lower RPMs, the airflow drops, the temp increases incrementally, the t-stat opens and you get a pulse of hot water?

I'm just theorizing how a disconnected and therefore wide open monovalve could shut off heat, or if there's something non-monovalve involved.

When the thermostat opens, it's the radiator that gets the hot water, not the heater core.

If you want to prove that the problem is the monovalve, bypass it and see if you have heat all the time. You will. And then you discount all the theories about fan clutches, thermostats, water pumps, etc.

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I inspected the new monovalve insert and confirmed that it's good, no tears, and moves freely. So now all I can do is either bypass the monovalve as suggested, or take out the whole monovalve and pump assy and see if I can find anything wrong with it?

Im battling my own heat issues. I was having no heat at highway speeds --- replaces the monovalve and now I have a lot of heat. The mono valve I took out looked fine except for a small nick on the inside. The screen looked a little clogged.

Now I have too much heat which I believe is related to the cabin air sampler motor that blows air to the CCU thermostat.

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daw_two
Germantown, TN

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New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

300SDL

So I rechecked the new insert plunger/valve that sets in the monovalve coil. It looks perfect. When I apply 12v to it out of the car, the plunger extends fully. However, with moderate pressure, I can push it back in while the 12v is still applied. The way I understand it, the valve is leaking because at high rpms, the coolant pressure pushes the plunger back in the same way I did by hand. Doesn't that tell me that the problem is not a torn seal(it isn't torn), but rather a failure of the plunger to hold the pressure? (My memory of electromagnets is that they're quite difficult to defeat). If that's so, do I assume I got a bad part & try again, or do I try to measure the coil to see if it's producing what it should to move and hold the plunger? (Measured across the monovalve terminals, I get no resistance through the coil, which I understand should be correct.)

Thanks

Your theory would explain why the valve would not remain closed. Your problem is that it won't remain open. In other words, it's closing when it should remain open, not opening when it should be closed.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

This is going to be a longshot, but the symptom can also manifest itself if you've got low coolant level in the tank.

Sometimes you cannot properly see the level on an old tank..............

The FSM shows a nominal impedance of 15 ohms. I thought most coils had some measurable impedance; for example the AC compressor is about 4 ohms.

I also thought electromotive force in a solenoid is proportional to the current, not the voltage. So depending on the 12 v source, it might be easy, or may be very very hard to push it back. Also, if I remember the monovalve coil design, when extended while out of the car isn't it mostly out of the coil and therefore out of the electric field so it should be easy to push?

Regardless, Tango's right - if the plunger moves easily, then it should provide heat when none is demanded, not vice versa.

I go back to your original troubleshooting: with the thing unplugged, you should get heat. Since you don't, you probably have some non-electrical problem. Maybe a torn diaphragm or poor seal that permits coolant to flow in a direction that closes the valve, maybe low coolant that creates suction and pulls it closed.

I'd look very closely at the hydraulic parts and the hose connections: is everything clean with no corrosion, are the hoses (all 3) to/from the heater routed correctly and not pinched or clamped?

Your aux fan isn't hot-wired on, is it?

An extremely worn water pump would essentially cavitate at higher rpm's and cause low flow. That would explain his (up to) 100c temps in cold weather and loss of heat flow at highway speed.

I am new to MB coolant/heating strategies and wanted to toss some pump theory into the mix.

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'20 GLE 450

Thanks for the new help.

"Your problem is that it won't remain open. In other words, it's closing when it should remain open, not opening when it should be closed."

Yes, I was confused. But with the voltage source connector disconnected, in order for the monovalve to block the heat, wouldn't the plunger have to be somehow sucked out of the coil so that it could block the coolant flow? Without negative pressure in the cooling system, how could this happen? In other words, how could increased pressure from higher rpms cause the valve to close (= pull the plunger out of the coil so that it could seat and block the coolant)?

As for the coolant level, what I can report is that the tank, though original, is clean, with a clear view of the coolant level. I have to add a cup or two of coolant each week, but I keep the level within an inch of the "full when cold" line.

The aux fan is NOT hot-wired on.

I'll have to re-inspect the hoses, etc. but all the visible ones were clean and tight when I did a flush a few thousand miles ago.

Water pump idea is interesting, will have to investigate, although at steady highway speeds around 3000 rpm, the water temp gauge stays steady below 90 degrees in moderate weather.

Soldiering on.

The monovalve is spring loaded. When there is NO voltage ( connector unplugged ) then it is fully retracted by the spring. I cannot possibly see a scenario that it can be closed due to coolant temperature or anything else. Something has to work against the spring to compress it - what can it be? Only current thru the coil.

The other thing you can do is to remove the valve and make a cover plate on top, screw it tight, to make sure no blockage in coolant at all time and check it again.

Assuming the valve is full opened then I can only think of the following scenarios with no heat at highway speed.

1) Air in coolant system
2) Aux pump not running
3) Somehow coolant coming out of engine block thru aux pump is cold.

I am sure you can fix the problem if you eliminate one issue at a time. This is not rocket science.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

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1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

90 deg (F or C ) is too low. Can you verify with a infrared thermometer?

Did you have this problem before the cooling system flush?

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What does the check valve at the base of the monovalve housing do? How does it close when the engine is off? Does it float to the top under low pressure (no pump), but get pushed down under pressure?

In this case, might it be worth removing the housing and flushing or cleaning out the housing?

There are some pics of a removed housing here: Mono Valve Housing leaking!

Attached Files

monovalve cutaway.pdf (129.7 KB, 135 views)

By 90, I meant that the temp gauge stays around 90. I'm not sure if this problem started before I flushed the system.

In any case, when I fill the cooling system, I fill it both in the expansion tank and in the top hose from the block to the radiator, so I don't think there's any air in there. This sounds like I need to look into the aux pump. Does anyone know a clever way of testing it in place?