New monovalve - still no heat - PeachParts Mercedes-Benz Forum

eatont9999 · #1 10-09-2012, 06:45 PM

Hey guys,

I thought the lack of cabin heat was due to a bad monovalve diaphragm, which mine had, so I replaced it with a new Bosch unit. I have hot heat when the electrical terminal is disconnected and no heat when it is connected. I am seeing a constant 12v from the + connector on the terminal. It is not pulsing like I believe it is supposed to. I have the climate wheel all the way on max heat and set to the vent setting third position in from the right side.

Any ideas? I have a spare CCU but I was wondering if anyone had a suggestion before I start that process to find out it may not solve the problem.

Yes, I did search but I did not find much relevant to these symptoms.

I am referring to my 300SD, btw. Ford IDIs don't have monovalves.

qwerty · #2 10-09-2012, 06:54 PM

Quote:

Originally Posted by eatont9999

I am seeing a constant 12v from the + connector on the terminal.

That is as it should be. The monovalve is controlled by switching the ground side of the circuit.

Quote:

Originally Posted by eatont9999

It is not pulsing like I believe it is supposed to. I have the climate wheel all the way on max heat and set to the vent setting third position in from the right side.

In the "Max Heat" detent position, no switching occurs. The ground circuit should be constantly open, so the monovalve remains open. Evidently, you have continuity to ground when you should not.

Does the monovalve open when Defrost is selected? (Alternatively, check the ground circuit for continuity to ground in the Defrost mode.)

Graplr · #3 10-09-2012, 07:04 PM

You should see a change in the voltage at the monovalve wire connections as you move the dial from hot to cold on the PBU. If not the PBU unit is bad.

qwerty · #4 10-09-2012, 07:06 PM

Quote:

Originally Posted by Graplr

Have you simply UNPLUGGED the wires at the monovalve yet?

"Disconnected" and "unplugged" are reasonably synonymous.

Graplr · #5 10-09-2012, 07:07 PM

Quote:

Originally Posted by qwerty

"Disconnected" and "unplugged" are reasonably synonymous.

Yes, I read too fast the first time. I edited it out.

qwerty · #6 10-09-2012, 07:22 PM

Quote:

Originally Posted by Graplr

You should see a change in the voltage at the monovalve wire connections as you move the dial from hot to cold on the PBU.

The voltage output from the temp selector does not go to the monovalve; it goes to the electronic unit for temperature control. Based on that input and others, the EUTC switches the monovalve ground circuit off and on to maintain the desired temperature. Exceptions to that are when max or min temps are selected and when Defrost is selected; in those cases, the ground path is opened or closed mechanically.

Graplr · #7 10-09-2012, 08:44 PM

Quote:

Originally Posted by qwerty

The voltage output from the temp selector does not go to the monovalve; it goes to the electronic unit for temperature control. Based on that input and others, the EUTC switches the monovalve ground circuit off and on to maintain the desired temperature. Exceptions to that are when max or min temps are selected and when Defrost is selected; in those cases, the ground path is opened or closed mechanically.

What is the "electronic unit for temperature control" and where is it located? All I know is when I unplugged the monovalve and put a voltmeter on the wires and moved the dial on the PBU the voltage would change. You are saying I am incorrect? If he notices no change in voltage when moving the wheel from max to min something is wrong, agree?

qwerty · #8 10-09-2012, 09:03 PM

Quote:

Originally Posted by Graplr

All I know is when I unplugged the monovalve and put a voltmeter on the wires and moved the dial on the PBU the voltage would change.

The only electrical event which occurs at the monovalve is the switching (on or off) of the ground side of the circuit. There is no "variation" of voltage, per se.

Quote:

Originally Posted by Graplr

What is the "electronic unit for temperature control" and where is it located?

It's the brain for the monovalve. On most models, it's under the dash in the vicinity of the glove box.

Looks like this:
http://www.pelicanparts.com/catalog/SuperCat/3303/MBZ_3303_VENTHT_pg1.htm#item0

eatont9999 · #9 10-09-2012, 10:09 PM

Well, gentlemen, I think y'all have lead me in the right direction. Once again, it is all about the grounds!

The PO of this car installed a scotchlock (those things are evil) on the ground terminal with a wire coming out of it connected to nothing. Thinking my failed monovalve was an electrical issue, I connected the ground terminal to the battery. This of course did no good but with warm weather coming, I let it be. Why someone would scotchlock a ground cable and then not connect anything to it is beyond my imagination. I'm going to cut that ground circuit and hope that the factory original ground wire is still connected to the rest of the system. I will test in EC mode and move the dial all the way cold and I expect not to get any heat. Then I will try in one of the other vent modes. My CCU has been working perfectly this summer in controlling the A/C minus pulsing the monovavle for accurate temps.

eatont9999 · #10 10-09-2012, 10:35 PM

OK. I disconnected the extra ground wire and I got full heat throughout all the possible combinations of CCU settings. Max cool still gives me full heat. I used the extra ground wire attached to the monovalve to check continuity to the negative terminal on the battery and the valve cover. I did not get continuity between either test. According to my meter, there is no connection to ground, pulsing or not. I had the CCU on max cool and the terminals connected to the monovalve during this test. I touched the extra ground wire to the negative terminal on the battery and I could feel the monovalve solenoid engage each time I touched the wire to ground. With the extra ground wire not touching ground, I did not feel the monovalve pulse at all.

Does anyone know where the ground wire for the monovalve is supposed to connect inside the cabin? I feel the next step is to test continuity in that wire to determine if it is broken or cut somewhere.

Thanks!

qwerty · #11 10-09-2012, 11:02 PM

Quote:

Originally Posted by eatont9999

Does anyone know where the ground wire for the monovalve is supposed to connect inside the cabin?

To the push-button unit. C-1, terminal 4. (That's per a schematic for an '82 300SD.)

Odds are good that the problem is a poor solder joint at the aforementioned terminal.

vstech · #12 10-10-2012, 09:11 AM

ok, I think I am following all this.
OP had no heat.
he traced out the circuits, and found someone has installed an additional ground wire to the circuit feeding the monovalve.
when this wire is disconnected, he gets heat.
when tracing the original wire to the ccu there is no ground on it, no matter the position of the temp wheel.
so, in all likelyhood, assuming the wire is not damaged in between, the ccu ground terminal is defective.
yes?

SD Blue · #13 10-10-2012, 03:28 PM

No, the ground side of the monovalve goes through the pushbutton controller to the temp controller. The temp controller compares the in-car temp sensor to the temp dial on the controller. It also checks the heater temp sensor.

A few possibilities are:
Heater temp sensor has poor/no connection which makes the temp controller think the coolant temp is low.
No power to the temperature controller. (uses the same fuse F10 as the monovalve)
Poor ground to the temperature controller. (same ground as the pushbutton controller)
There is also a heating "enable" that goes from F10 to the pushbutton controller to pin 3 of the temp controller.

I would check the temp sensor first, then access the temp controller to check for power and grounds.

qwerty · #14 10-10-2012, 03:33 PM

Quote:

Originally Posted by SD Blue

...the ground side of the monovalve goes through the pushbutton controller to the temp controller.

Whether the temp controller has any connection whatsoever with the monovalve ground circuit is dependent on the mode and or temp selection.

For example, in DEFROST or MAX setting on the temp selector, the ground is opened in the push button unit. In OFF or MIN, the ground is closed in the PBU; that doesn't appear to be happening.

Quote:

Originally Posted by vstech

so, in all likelyhood, assuming the wire is not damaged in between, the ccu ground terminal is defective.
yes?

If the push button unit ground was open, there would be more than just the monovalve that didn't work. For example, if the fan runs in high speed, that's not it.

SD Blue · #15 10-10-2012, 04:08 PM

The pulsing "removal" of the ground allows coolant flow through the monovalve. The previous owner installed a "scotchguard" ground, most likely to get better cooling of the air-conditioning. In OFF mode or MIN temperature settings, the ground is applied to the monovalve by the pushbutton controller, effectively blocking coolant flow and therefore no heat.