First, my friend was helping me tinker with my car, and he accidentally touched the two wires leading into the monovalve together. That shorted something, because now they're the exact same voltage all the time. Does anyone know what it would've shorted?

Second, does anyone know how the monovalve works? I know that there's an electromagnet in there, and the Benz manual says that when it's energized, the valve is closed. Now, when I looked at the monovalve when I took it out, in resting state it's short, but you can pull it out so that it's longer. What I'm guessing is that when the electromagnet is powered, it repels that top part, but I'm not sure that that's case.

What I'm trying to do is rig it so that the monovalve stays closed, so that the heat won't get mixed in with the cold a/c air.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

In an unpowered state the monovalve allows coolant to flow to the heater core. I don't know how this translates to movements inside the monovalve mechanism.

Rather than override the function of the monovalve, put another valve upstream of the monovalve. I don't know what car with ACC has a valve that's closed by default but I imagine MB being what it is is one of the rare manufacturers that uses a default open monovalve.

You might think of fixing whatever's wrong with your ACC so the monovalve isn't open when it shouldn't be.

Sixto
95 S420
87 300SDL

What do you mean put another valve upstream? I tried clamping the heater supply hose, but the clamp started to cut into the rubber before it could tighten the hose enough.

The default position for the valve is to be open, and when the valve is "energized" (from the Benz manual), it becomes closed. I think that means that when the electromagnet is on, the valve is closed, but I'm not sure.

I'd love to fix what's wrong with the ACC, but I don't know where to look next. I don't know what shorted when I touched those 2 wires together that go to the monovalve, and I don't know why the monovalve wasn't working properly before. I was hoping someone here had some ideas.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

When you apply 12V to the monovalve, it will stop the flow of coolant to the heater core. Actually, it gets 12V anytime the key is on, the ACC system provides the ground as necessary. In defrost mode, it gets a solid ground signal from the PBU; in other modes with the temp wheel on min, it gets a solid open(?) signal from the PBU. In other modes, the PBU passes through a signal from the temp controller. Maybe you can isolate the problem with this info.

Sixto
95 S420
87 300SDL

Thank you, that's something to get me started. Can you clarify a couple of things? From what you said I'm getting that the monovalve needs 12V on one pin, and ground on the other for it to be closed (which would stop the coolant from going in, which would stop the heater core from blowing heat). Is that correct?

What I'm interested in (at the moment anyway, for the summer already underway) is for the valve to be closed when the wheel is on min... so is the PBU is the problem, because it's not sending it the solid open signal?

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

You got it.

Out of curiosity, what were the ACC settings when the wires were crossed? If it was on 0, DEF or MIN, the temp controller should have been isolated.

It would seem the PBU is not sending a ground signal. The diagram suggests that it's handled by mechanical switches with a diode thrown in here and there. The monovalve is on the same fuse as the ACC so it can't be a bad fuse if the rest of the ACC works.

If you want to block coolant flow to the heater core while the weather's warm, fit a mechanical valve before the monovalve. A brass valve from Home Depot should work. Or you can run a switched ground wire to the monovalve. You want it switched or the monovalve will continue to draw from the battery when the car's parked.

Sixto
95 S420
87 300SDL

Thanks a lot, that's great info.

The ACC was on min (I'm 99% sure) when the wires touched. So, then it sounds like the PBU was shorted?

A helpful mechanic told me (before the wires crossed) to check the voltage accross the 2 pins of the monovalve with the PBU on min. He said if the voltage was less than 1 volt, the valve was closed. If it was around 12 volts, the valve was opened and there was something wrong with the PBU. I did that right before my friend crossed the wires, and the voltage was 12 volts. So from what you and him are saying, there was a problem with the PBU before, and now it's a different problem, but still with the PBU.

By the way, where did you get your wiring diagrams from?

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

From what the helpful mechanic was saying, there was a problem with the PBU. His contention is that the monovalve gets power to open. I don't agree.

From what I'm saying, there wasn't a problem with the PBU. You should see 12V across the monovalve pins with the ACC on MIN because it takes 12V to CLOSE the monovalve.

Here's another test - what voltage do you get with the engine running and ACC set to 0 (off)? This setting sends a full ground signal to the monovalve so it should close.

The diagrams are available in the MB CD manual set. Available from FastLane if you're interested.

Sixto
95 S420
87 300SDL

Actually, I spoke to the mechanic today and he told me he was wrong before, that it should be 12V across the 2 pins when the A/C was on min.

But if the voltage was correct, and the monovalve was closed, why was I still getting hot air?

I have the MB CD, which part has the diagrams you were looking at? I looked in the Automatic Climate Control part, and they weren't very helpful.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

Monovalves can leak. The typical symptom of a worn monovalve cartridge is acceptable heat when idling fading to barely any heat when moving.

Forget the intefrace. Explore the CD - go to ETM\ACC83-85.pdf.

Sixto
95 S420
87 300SDL

The old monovalve's diaphragm was broken, so it could have been leaking.

I just installed a new monovalve insert, so I doubt it's leaking. Since my friend touched the two monovalve wires together, they're the same voltage (a little under 12V), so today I put one of them on the monovalve power pin, and I put a ground wire on the other pin, and the heater quit coming on.

However, the A/C isn't running cool. It's running about outside temp, maybe a little cooler. Before the whole heater ordeal, it was blowing very cool.

ACC83-85 is the one I was looking at, but the only place where I could find the monovalve listed was page 308, and it was somewhat helpful, but not very. Do you know what the numbers with an asterisk mean, like 11*?

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

Is the compressor engaging?

81howto.pdf says that a number with an asterisk is an ACC training number. I have no idea what that means.

Sixto
95 S420
87 300SDL

I think the compressor engages. I can't tell from the sound, because the engine is too loud, but the high pressure side is hot, which tells me that it's working. However, the low pressure side is not cold.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

Is the front hub of the compressor turning? That will tell you if it's engaged. The only connection between the ACC controller and the compressor is a 12V wire to engage the clutch. You could not have done anything to the refrigerant system by touching two wires together.

Sixto
95 S420
87 300SDL

No, the front hub of the compressor isn't turning. The belt's spinning, but the front of it is stationary.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled