300SDL

Wrestling with no heat at highway speeds. Just installed new monovalve. Did the test suggested by the venerable ah-kay, to wit:

1. Unplug the connector to the monovalve. This will cause the plunger to retract and call for heat ALL the time.
My result: heat all the time, until highway speeds. As speed increases, temp decreases to no heat.

2. Then put a constant 12v on the monovalve such that the plunger will be fully extended to block the coolant and there will be NO heat.
My result: no heat ever.

I also tested the new monovalve before installing it and it operated perfectly. BTW, cooling system is flushed and clean with proper fluid.

So if the monovalve is 100% open as in test 1, is something keeping the coolant from getting to the heater core at high speeds? Is the only reason that there is heat at idle and low speeds because of the aux pump? Is it possible that the valve(not the cartridge part that gets rebuilt) at the bottom of the monovalve chassis is blocking the flow at higher speeds?

Thanks for looking.

Try bypassing the aux pump with a plastic 90° from the hardware store. Assuming the aux pump is frozen-they're pretty easy to rebuild.

Possibly check your thermostat?

FWIW, I replaced my monovalve with some pipe for the winter (but I like hackjob fixes)

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It looks like the monovalve is doing its job. It is strange that you have no heat at highway speed and heat at idle when plug removed. I suggest you do the following tests as I am out of ideas. I would do some un-intrusive tests first before doing hacking.

1) What is the gauge temperature when at highway speed?
2) When it happens at the freeway with no heat, exit freeway and find a convenient place to stop asap. Touch pipe getting in and out of engine to cabin and check for temperature, or use IR temp gun. If it is cool to touch then may be the aux pump is faulty. Do it at idle first to get an idea.
3) I have never by-passed the aux pump nor have the need to check it. So I have no experience here. May be you can measure the pump for continuity to make sure it is OK. Or hardwired it 12V to make it runs at all times to try out. Polarity is important here.

Good luck.

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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.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Aux pumps are flow-through impeller style, which allows coolant to flow even when unpowered, or motor is ceased. Unless there is material caught up in the impeller, it might block flow. But then it would block flow always and you'd never get heat because it's a serial flow through the aux pump to the heater core.

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Cheers!
Scott McPhee

1987 300D

What is the repair history?
Is it possible there could be a low coolant issue?

.

As S=B did, I removed my monovalve altogether, and put a small 90* piece of PVC pipe in its place as a temp fix. I'll rebuild the mv this Spring when it's warmer out. The result is that I have all heat, all the time. I just have to control it by turning the fan on and off. It's not very convenient, but at least I'm warm!

Is this happening regardless of temperature wheel position?

I did my monovalve and flushed the system in November, but I still lose heat at highways speeds if I don't keep the temp wheel inside the red zone. If it's into the red zone it stays hot.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

THANKS! You lot are the best.

To answer some of the above questions:

1. Thermostat is new and cooling system seems to work correctly.
2. The temp gauge varies from about 80 to about 100, depending upon conditions and how I'm driving. It always goes a few degrees lower when air is passing over the heater core and entering the cabin hot.
3. As far as repair history, coolant system was flushed recently, thermostat replaced. I was losing coolant and had a hydrolock problem about 1000 miles ago. Having no time or money to pull the head to fix coolant leak, I used a block/gasket sealer treatment (don't hurt me). It dramatically reduced the coolant loss and have had no hydrolock since. It know it's just a patch.
4. Loss of heat seems to happen regardless of temp wheel, but I will recheck that. However, when I pull the voltage plug at the monovalve, the monovalve can't get any instructions from the ACC unit, so shouldn't the temp wheel be irrelevant in that case?

Will try the touch-test suggested above and report back.

Maybe the use of that treatment blocked the aux. pump abit or yjr pump impeller got stuck due to the treatment. Can you verify if the aux pump impeller is running?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

Define exactly what you mean by "new monovalve."

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

My 85SD does the same thing. Heat at idle, usually but always no heat at highway speeds. Monovalve is good and unpluged.

I know that my ccu is not functioning correctly. I had 2 of them on the bench to re-solder.

At least part of the problem seems to be with intermittent push button response. Sometimes the button works, sometimes not. (solder joints aren't the problem - at least not all of it) I may pull it a part to look for switch repair or at least enough good switches.

I was following these continuity readings (scroll down): Middle-aged Mercedes Electrical Repairs

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

That's a classic symptom of a leaking monovalve diaphragm. The increased coolant pressure at higher engine rpm acts on the back side of the diaphragm, overcoming the solenoid and forcing the valve to the closed position. At lower rpm's, the solenoid becomes effective and the heater functions normally.

That is highly unlikely.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I did the touch test, but don't think I learned much. The large coolant line that comes out of the firewall below the monovalve is hot at all times. The smaller coolant line that goes into the firewall above the monovalve is very warm, but 10-20 degrees cooler than the bottom line, and it's consistent in all conditions.

I did notice that following freeway driving, the heat very quickly comes on to full heat (in about 5 seconds) at 40 mph. If air flow becomes very quickly hot, doesn't it HAVE to be the monovalve opening?

Interesting point about the impeller, I'll have to figure out how to test it, hopefully without taking the whole area apart.

By new monovalve, I just mean a new valve insert that goes in the coil at the top of the monovalve body. Didn't do anything to the rest of the monovalve body.

I also know that my ACC pushbutton unit is probably bad, but by cutting power to the monovalve, aren't I taking the pushbuttons out of the loop?

The problem isn't a pump; it's the monovalve.

Textbook leaking diaphragm.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

However he claims it is new..

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