Monovalve

What exactly does the monovalve do? How does it break down? How can I tell if it needs to be repaired? I'm having heating problems ie. the fan only blows luke warm! Thanks

[LUVMBDiesels]

The Monovalve regulates the amount of hot coolant that enters the heater core. Usually the symptoms of a bad monovalve are hot or cold air all the time. If you think that the Monovalve is your problem, or if you just want to do some PM and make sure it is working correctly here are some tips.

1) With the engine on and the Defrost button pushed in, take a volt meter to the two wires that plug into the valve. There should be no voltage across these wires.

2) Press the EC button and move the temp ring to "MIN" then look at the voltage on the wires. There should now be 12 volts across the wires. The Monovalve defaults to full hot so if the electrical conenction is broken you get nothing but super hot air inthe system.

If you want to replace the valve, buy the insert made by Bosch. There are plenty of people selling it some for around $35 US. Do not buy the entire valve as that can cost upwards of $200.

With the engine off and cold, remove the four screws on top of the valve. Pull the top of the valve off and remove the insert. If you just want to look at the existing insert without replacing it, check it for a clogged screen, deteriorated rubber and free movement (pull on the end)

The insert simply pulls out of the end of the valve and is replaced very easily. It took me under 1/2 hour to do it.

One trick I found helpful is to put the valve back together, but do not tighten the screws all the way. Turn on the engine and run it up to operating temperature. Let some coolant run out of the top of the valve to make sure you did not trap an air bubble in the system. Then tighten the screws. I had a bubble trapped in the valve and had no heat at all until I did this.


Good luck

[vstech]

My buddy changed his monovalve, and after he finished he started a 200Mile trip, and blew a head gasket 100 miles from me. I had to get up and go get him at 11:00 at night... so be sure you have coolant in the head. this motor is not real easy to get all the coolant back in, so be carefull.
John

[Cateaux]

You didn't mention which model car you have. The 123 amd 126 cars with ACC (probably others, too, but I'm not sure) have an auxiliary water pump that activates when you turn on the heater. If this pump fails you will have poor coolant flow to the heater core at low RPMs. If this is the problem the air should get warmer at higher RPMs.

Electrical problems involving the monovalve are usually the result of poor/corroded connections. This will cause lower than required voltage to be sent to the monovalve, which will make the air warmer than it should be, not cooler. Like another poster said, 12 volts to the monovalve and it should be all the way closed; 0 volts and it should be all the way open. If you have more than 0 volts with the heater on maximum, you have ACC issues (pushbutton unit, maybe?).

More than likely you have a torn monovalve diaphragm, though. This part is a frequent failer, and some people in colder climates than mine go so far as to rebuild the monovalve as regular maintenance. When mine failed I had heat at idle but the air got cooler at higher RPMs.

I pulled the plug connection off the monovalve and got instant heat. Thans everyone! now how do I get at the top of the monovalve in my 1986 sdl?

[TX76513]

Originally Posted by djoyce93

I pulled the plug connection off the monovalve and got instant heat. Thans everyone! now how do I get at the top of the monovalve in my 1986 sdl?

Take the four screws out, pull off top - being careful not to drop one in behind the noise shield - not that I would know anything about that

[Brian Carlton]

Originally Posted by djoyce93

I pulled the plug connection off the monovalve and got instant heat. Thans everyone! now how do I get at the top of the monovalve in my 1986 sdl?

If you got full heat with the plug disconnected, the monovalve is working properly, if the heat goes away when the plug is connected.

The issue is the CCU. The system isn't signaling the monovalve to open properly.

Make sure all tests are done with the thumbwheel all the way to the click stop on the hot side.

If you don't get full heat with the thumbwheel at the click stop, replace the CCU.

[Brian Carlton]

Originally Posted by vstech

My buddy changed his monovalve, and after he finished he started a 200Mile trip, and blew a head gasket 100 miles from me. I had to get up and go get him at 11:00 at night... so be sure you have coolant in the head. this motor is not real easy to get all the coolant back in, so be carefull.
John

I'm not following this.

If you pull the top of the monovalve, you lose about three ounces of fluid because the valve is very close to the top of the system.

If you fail to replace that fluid, I find it impossible to blow a head gasket from a loss of that amount.

There must be more to the story.................

[mplafleur]

The CCU doesn't command the monovalve open/closed. That is done by the temperature control module behind the passengers right kick panel. The temperature wheel input also goes to the temp control module. Up through 85, there are no brains in the CCU, just a bunch of relays, push buttons and the temp wheel. Excessive current through the relay paths can cause problems though: burnt traces, solder reflow, cold solder joints, etc.

So do I need to replace the temperature control module?

[mplafleur]

Originally Posted by djoyce93

So do I need to replace the temperature control module?

Not quite yet. Since you got instant heat when unplugging the monovalve, it doesn't mean that it wasn't told to cycle on and off. If you can get a voltmeter on the connector to the monovalve, it should read 0V when heat is required. Make sure it doesn't cycle on and off. It shouldn't if the temp control is full heat. If you place it in the middle of the temp range, you should see the voltage switch between 0V and 12V. This is with the pushbutton on economy.

Report back what you find.

I can't get a voltmeter on the connector, it's just too small. Here is a complete description of the symptoms my car is experiencing. If I disconnect the monovalve, instant heat, very hot. With the monovalve connected after startup I get a very slight increase in heat eventually. However if I stop the car for a few minutes ( to get diesel for example) when I start back up, much more heat is blowing through the fan! If I continue to drive for another 30 minutes or so, all of a sudden the heat drops back down again! It doesn't matter whether I have the setting on defrost, economy or whatever, the results are the same. Thanks for you help!

[mplafleur]

Originally Posted by djoyce93

I can't get a voltmeter on the connector, it's just too small. Here is a complete description of the symptoms my car is experiencing. If I disconnect the monovalve, instant heat, very hot. With the monovalve connected after startup I get a very slight increase in heat eventually. However if I stop the car for a few minutes ( to get diesel for example) when I start back up, much more heat is blowing through the fan! If I continue to drive for another 30 minutes or so, all of a sudden the heat drops back down again! It doesn't matter whether I have the setting on defrost, economy or whatever, the results are the same. Thanks for you help!

You don't have to have the connector plugged in to the monovalve to test the signal. Just pull the two-wire connector off and stick the voltmeter probes in the sockets.

Once the signal is verifed, it sounds like you may be having a problem with the temperature control. Either the module is not cycling the monvalve correctly, or the module is getting bad input from the cabin temperature sensor, the heater box sensor, or the temperature selection wheel.I'm no inclining toward the sensors though. The monovale should be commanded to supply heat if the wheel is turned to the click stop at full heat. Check the voltage on that connector with the wheel at the full stop, on economy.