Stupid question, I looked it up. Move the battery aside and the plug leading to the aux pump is right next to the monovalve. All the hoses around the Aux water pump and into the block and into the heater are tight and show no leaks. I put 12v to the Aux pump and it turns easily. Do I have to actually measure the flow to know if it's good? If 12 volts makes it turn, what will be the sensors or devices to look at to see if it gets 12 volts when it's supposed to? Or do I need to go to that step?

thanks

Why don't you unplug the monovalve, put 12v to the aux pump, leave the pump on and take the car to the freeway. If you have heat then may be the CCU is turning it off somehow, may be a sensor is faulty. If you do not have heat then it may be the coolant temp.

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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.

I drove my 300 d turbo with a bad aux pump for a few years and never had problems with getting heat at idle or at highway speeds. I installed a good used aux pump a year ago and it does not seem to make a lot of difference. Still getting plenty of heat. My temperature gauge stays steady and seldom moves. I never changed the thermostat or did a citric acid flush even though I bought both. If it aint broke why fix it?

I would suggest putting your hands on the various hoses -upper and lower radiator, heater hoses in and out and feel the differences in temperature. You can't measure temperature with your hands but you can feel which hose is hotter than the other. By doing so you may figure out what is going on.

It sounds like you may have a clog somewhere in you cooling system.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If you do power the aux pump independently, make sure you get the polarity correct otherwise the pump will run backwards.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Let add my name to the list (1986 300SDL). Plenty of heat at speeds below 60. Ice cold shortly above that. I am pretty sure my CCU is good as I was using it in my 420 with no problems. So, unless it crapped out on me since the swap, its good. My engine temp is always steady at just above 80. Coolant level is good.

I do seem to have an erratic blower fan. Sometimes it works great, sometime it does not come on. Even when it is not on, you can feel a wisp of ice cold air from the vents.

Unknowns:

Auxiliary WP condition/functionality
Main WP (seems fine as I have no overheating issues)
Thermostat (seems fine as I get a steady engine temp)
Air sampler (not sure of condition or functionality)
Monovalve (thought it fine all along till this highway speed issue. This will be the first thing I check).

I just noticed this this past week so I have not had a chance to troubleshoot.

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1986 420SEL W/1990 560 SEL Full Power Interior Swap
1995 E300D
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Just pulled my monovalve. Indeed the diaphragm is torn. Just ordered a repair kit. Well see if that fixes it in a few days when it gets here.

__________________
1986 420SEL W/1990 560 SEL Full Power Interior Swap
1995 E300D
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So I did the suggested test:

1. unplug the monovalve,
2. put 12v to the aux pump, leave the pump on and take the car to the freeway.

I warmed the car up to the gauge reading 80, where it usually sits on mild nights. At idle, the air blows very hot, as in the past. Getting directly on the freeway, the temp immediately starts to lower, hitting no heat after a few miles. The engine temp gauge stays steady throughout.

I can't feel the hoses at speed (obviously), but at idle, the three large hoses that go into the bulkhead next to the aux pump are hot, like all the other coolant hoses. The two smaller hoses above going to the monovalve are also the same heat.

It seems that either hot coolant is blocked to the heat exchanger based upon rpm, or the ACC airflow is redirected away from the heat exchanger based upon rpm. (This is with the monovalve disconnected and the aux pump continuously on.) Having felt my share of radiators in operation, I'd say the way the airflow cools feels exactly like how a high fan airflow would cool a radiator that had no hot coolant supply.

I don't understand the ACC system well, but could poor vacuum or a failing part under the dash route the ACC blower air away from the heat exchanger? Final note: with the ACC system off, when the car is warmed up, there is noticeable hot air leakage from the windshield defrost vent.

thanks!

Since you're confident the monovalve diaphragm has no leak, and you've re-investigated the electrical system six ways from Sunday, I'd refer back to the pdf I posted that says there's a check valve below the monovalve in the housing. I don't know how it works, it's not referred to much or described very well, but it's a part there in the system that blocks coolant. Sometimes. Allegedly.

If it were me, at this point, I'd drain some coolant, remove the monovalve and housing. THOROUGHLY inspect the rubber and the sealing lips on the insert. Clean and flush the housing to make sure there's no debris trapped under the checkvalve that could block coolant at higher pressures.

Then put it back together.

If you're adding other variables now like there might be hot air, but it's not coming out from where you expect it, that's a different issue and NOT related to the monovalve. You've got to isolate your symptoms. Take a passenger along. Have him or her listen for moving flaps or venting noises.

Yes, I do have multiple problems; I know that the ACC unit doesn't work right. I tried to take it out of the loop for these tests, but to really eliminate the monovalve, I suppose I do need to take it out, bypass it and inspect it.
thanks

Audible, I'm sorry you're having so many problems with your climate system. Keep at it- you'll find the fix.

After I had my mono valve bypassed for nearly 2 months, I got to replacing the inner diaphragm today. I am happy to report that I now have great heat at any speed, and the auto temp control seems to be working well. It's hard to believe the problems that such a small part can cause. Now if only I could get the vent buttons to work properly...

I replaced the monovalve with a new one and it changed nothing on my 90 350SDL, but I have a different problem: when I push the defrost button, I get a flood of heat, but none of the other bvuttons do anything and the fan control does nothing either. I'm thinking multiple problems.

I took out the non-working radio and found that the wiring had been previously butchered by the owner previous to the seller of the car (therefore, at least two owners ago).

I don't think I have an electrical problem or the fan would not work at all, I'm betting some kind of vacuum problem, but I need to get into the dash and find the stuff. Plus I need some diagrams, both electrical and vacuum.

This is for later in the year, when the weather is not storming and my building is set up. I can drive the 240D to work and take my time on this one.

__________________
Strelnik
Invest in America: Buy a Congressman!

1950 170SD
1951 Citroen 11BN
1953 Citroen 11BNF limo
1953 220a project
1959 180D
1960 190D
1960 Borgward Isabella TS 2dr
1983 240D daily driver
1983 380SL
1990 350SDL daily driver alt
3 x Citroen DS21M, down from 5
3 x Citroen 2CV, down from 6

What is the "inner diaphram" and how do you get to it?

I finally found some conclusive info about the silly monovalve. I "rebuilt" mine twice with two different replacement parts, and both parts turned out to be defective. (MTC brand from Pelican, who were great about it all, but don't try the $16.50 MTC, it's a defective design).

I proved this by finally bypassing monovalve and learning that heat with the valve bypassed was good in all conditions. The way I bypassed it was to use an old monovalve insert and remove the rubber/brass end & screen so there was nothing to seal against when the coil pushed it down. Since this worked, it's obvious that the increased water pressure from higher rpms sucked the defective valves closed.

I can see from endlessly screwing around with this that the spring-action inside the monovalve insert has to be fairly robust to work right. In the bad parts I got, the spring action was quite light, and felt like the old one I took out, which is why none of them worked, I suppose.

Thanks to all

Well, as I said, upon inspection of my monovalve, it was torn. I got the repair kit and installed it this weekend. No joy. Got great heat at speeds around town, went on the interstate, heat slowly went away till it was gone. Engine temp stayed steady at just above 80. I may try another ACC unit this week. However the one I just put in was known to be good as I mentioned before. Is it possible for the electro magnet that controls the MV to go bad and not be able to hold against the pressure?

__________________
1986 420SEL W/1990 560 SEL Full Power Interior Swap
1995 E300D
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Might you, by any chance, have installed the same "good" type of new monovalve insert that didn't work for the OP? Maybe there's a current production lot being sold that just doesn't work.

The OP troubleshot his a$$ off and ended up swapping in a "real good" monovalve.

You might be chasing geese if you're looking at weakened electromotive force, since that holds it CLOSED and you want it OPEN.

Return the insert you bought, order one from a different manufacturer and try again.

Someone correct me if I have it backwards, but the coolant flow goes IN at the top of the monovalve and exits, when the valve is open, out the bottom, going to the aux pump and from there to the heat exchanger. The monovalve open is the default with no current to the electromagnet inside, letting the coolant go to the heat exchanger. The electromagnet shouldn't be in play in this condition.

If the monovalve isn't holding OPEN, the only reason that the spring inside isn't strong enough. Supposedly a $50 Bosch or $125 dealer part will work better.