I just took apart a good monovalve and the spring is quite strong. The range of motion on the plunger is about 1/2 inch. I can't imagine that spring can stretch that much and loose effectiveness. Easy to check. take it out and pull on the plunger and feel how much tension is on that spring.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

monovalve connector to correct that problem. Make sure your connector is gripping the terminals tightly and 12Vdc is present when control is at the cold range.

"Good" is the operative word. It does not apply to some aftermarket models.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I would appear that the proper operation in the cold range has already been confirmed.

12 volts should be present anytime the key is on; the monovalve is controlled on the ground side of the circuit.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

You mean the MTC kit the op used? got a pic of that kit?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Found this MTC kit. Do these have weak springs?

MTC Mono Valve Repair Kit : Amazon.com : Automotive

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

See post #31 here:

1983 300td No heat at highway speed

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Read your link. Last post from ytmtnman:


"Phil sent me a new monovalve. I popped it in last night. My drive to work this morning was warm and cozy even at high rpm. It seems that the problem was that the monovalve that I put in last month was defective. My guess is that the spring was getting sucked down, but the new and the old seem to have the same resistance. Problem solved anyhow."

He said new and old springs seem to have the same resistance.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I guess was focused on the part that said "Problem solved...". Whatever the cause of the problem, replacing the part made the problem go away.

Maybe the isssue isn't spring tension itself, but some other failing that allows the increased coolant pressure to defeat the spring. Or maybe the measurement of spring tension lacked objectivity.

This forum contains numerous accounts of "no heat at highway speeds." The overwhelming solution to the problem is a new monovalve or monovalve insert.

There are also reports of new monovalve inserts being defective.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

This is a pic of my good used OE monovalve.

The rubber plug at the tip is the part that moves (opens or close flow). The second piece of rubber (diaphragm) flexes but the non magnetic metal cylinder visible in the pic does not move (there is another iron core cylinder inside that moves). The diaphragm keeps coolant out of the spring cavity and more importantly, keeps coolant out of the electromagnet windings. There is a copper mesh between the 2 rubbers. Do you know what it's supposed to do?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I have not figured that one out.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Coolant flow is horizontal into the mesh, then makes a 90 degree turn downwards. Maybe the mesh acts as some kind of buffer against side loading the rubber plug?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If this has not been resolved ?

Verify there is no air trapped in the cooling system (heater core).

I use many of the MTC units, because most owners accept the higher defective unit trade off versus MB cost.

That said, I would not be even slightly surprised if the new MTC unit was defective.


.

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Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
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Technical Quality Auditor.
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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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An easy way to check if there is air in the heater core or other problems is after a high speed drive with heat on, pull over and feel all the hoses going in and out of the fire wall. If they are cold, there is air or blockage.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Not resolved as of yet. What would be the best way to check for trapped air?

What was intersting about the MTC kit is that now the heat at low speed seems better, but transitions sharper to cold as the speed increased -~45mph.

The original diaphram was torn. Thanks.

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2012 B Class

1981 300D - Now with greazzer