Never turns off the heat...

[fatimafg]

Greetings everyone. The heat won't go off, and I'm trying to figure out why. Using the URO kit, I was able to fix the monovalve.
It is functional. Upon removal, I physically verified that it responds to toggling with the acc. I have attached an LED light to the monovlve so I can see when it receives 12v. During testing, everything appears to be in order. The light is turned on, but shortly it will start to emit heat. When I turned on the defroster, the temperature shot up and the fan sped up even more. Based on my observations thus far, it appears that the valve is not closed properly, allowing warm water to enter the heat exchanger. Does the circulatoon pump have the potential to exacerbate a valve leak? Do you think I can unplug it? Is a leakage from the monovalve likely? Except for the URO rubber replacement pieces, the valve is brand new. In order to make a diagnosis, what do you suggest?

[omarwalkerah]

Is this the identical issue that you were seeing prior to installing the URO component? Finding an original monovalve: how do you do it? Was it ever effective?

As a design principle, Mercedes ensured that the monovalve would automatically switch to maximum heat in the event of a failure or control failure. It seems like they didn't account for the possibility that the spring would break the seal and allow coolant to leak out of the valve.


The monovalve requires 12 volts at all times. To switch off the heat, use the climate control supplies. The valve is electrically "off," open, and the heat is on if you probe both sides of the monovalve coil with regard to ground and discover 12 volts on both sides. Modulation of the heat occurs when one side alternates between 12 and 0 volts. It is believed that the heat is off and the valve is electrically "on" and closed if one side is consistently 12 volts and the other side is always 0 volts.

As far as I remember, turning off the heat (valve on, shut) should also turn off the circulation pump. Disconnecting it probably won't make a difference, though.

Max heat is required for defrosting.

To turn off the heat and activate the valve, connect the LED to the two ends of the monovalve.

Is that sufficient?

[fatimafg]

The actual problems have long eluded me. This vehicle had its engine hoses disconnected and plugged when I purchased it. Either the heat core was leaking or the climate control was malfunctioning, I thought. I made sure there were no leaks in the heat core by testing it. In order to replace the diaphragm and rubber seal (the Uro replacement package), I removed the solenoid from the valve and placed an order. It was verified that the power to the valve is functioning properly. I am now correct in my hypothesis. 1. The URO kit is of low quality, which causes the valve to not seal off the hot coolant adequately. 2. The solenoid moves, but not with enough force. The voltage is OK, but the amps are not. c) Inadequate rubber fit because of damaged internal seating of the valves (?). 2. Is it possible that the coolant is leaking from somewhere else and escaping the valve?

How can I know that this single-valve engine is unique? It appears to be at least 30 years old, if not older, based on the patina and worn rubber inside. I can't say for sure, though.


The next thing I'd do is disconnect the solenoid and see if it works better when connected directly to the battery.
At this very moment, I'm wondering if the length of the original moving "arm" and the repair kitl are same. Will have to verify that, although I would think it might leak if it were shorter.

[fatimafg]

[omarwalkerah]

If the heat still doesn't turn off after that, I'll try connecting the valve straight to 12 volts and ground. If the fault is with the valve's mechanics or the controls, that will reveal it.