While searching for CCU posts I found a comment that the monovalve doesn't work properly. My aux pump seems to be seized up... so I would like to know whether everyone disagrees with this.

I shut down the car and ran a + and - jumper from the battery straight from the aux pump, not even a sound, felt it, no vibrations or anything.

I think my monovalve isn't working correctly. When I remove it from the system and hook it straight to the battery the piston VERY SLOWLY moves out just a tiny bit. I tried moving around the casing around the electro magnet and when I did that it did come most of the way out, but VERY slowly. There was no *CLICK* like people say you should hear.

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1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

I believe the monovalve is not designed to work on a straight 12volts. There is some modulation of the current that activates the valve. Someone else might confirm this.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

There is some relationship between the pump and the monovalve. It has to do with the CCU in some fashion.

When the pump draws too much current, the CCU does some funny things with the monovalve.

I had replaced the pump with an aftermarket unit, and, within six months, the monovalve started making quite a racket........very loud cycling. Disconnect the pump and the monovalve returns to it's normal function.

What's going on inside the CCU is certainly a mystery.

The aux pump does not directly affect the operation of the monovalve, it simply circulates more coolant to the heater core to give you decent heat at an idle. From what you've described, it sounds like your monovalve is the problem (try a rebuild kit). However, that does not rule out additional problems, including the push button unit. A seized aux pump can also cause damage to the push button unit, so you should either replace it or unplug it.

That's for sure.

We posted at the same time, but shouldn't the monovalve cycle correctly when he jumpers 12V directly to it?

I don't think so. When I put a multimeter on my functioning monovalve, I did not get a 12volt reading. It jumped around.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Did you measure the voltage across the connections while it was connected to the CCU cable, I've never tried that?

I thought it saw a full 12V, but was cycled open and closed by the CCU to maintain the correct temperature, resulting an a "pulse" to the monovalve. I may be wrong. As Brian said, the CCU is magic.

As I recall, I've directly connected 12V across the monovalve contacts (unplugged) and heard a distinct click when it changed position. I would not necessarily recommend that, I'm not really sure the valve is designed for a full 12V.

I actually disconnected my aux pump weeks ago because I suspected disconnecting it might reduce the amount of heat that gets pumped into my cabin when I run the AC.. it didn't work, but I saw no reason to hook it back up. My CCU has already has some resoldering done to it.. Someone replace a whole trace with solder, that's probably the one that the aux pump already burned up once There are signs of A LOT of heat on the board, but all the solders look to be in good condition. I already bought and replaced a monovalve, because the gasket on my old one was pretty much ripped off. Does a rebuild kit include the magnet coil? That's what I suspect to be the problem.

This would be a last resort, but does anyone see any problems with just installing a twist valve and shutting down the heater loop during the summer?

__________________
1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

refrigerant pressure seems ok, and I know that the compressor is running because the pressure drops as I increase the throttle.

When I turn on the AC I get lots of *burn your face off hot* air. Every now and then the AC works for a short time when I turn on the car. but within 20 minutes it's blowing hot air again.

When I first got the car (before I changed the mono) it actually blew cool-ish air. I wouldn't call it cold, but it was definitely colder than ambient outside.

I was asked to hold my hand on the low pressure pipe and see if it got cold.. it doesn't.. not at all.

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1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

I don't own a multimeter at the moment.

What you said about the pulsing open and closed to maintain correct temp is what I have always read.

__________________
1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

This is the repair kit:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ424D193CSQ&year=1985&make=MB&model=300-DT-001&category=All&part=Mono+Valve+Repair+Kit

I've never bought one, I just replaced the entire valve. You could shutoff the hose in the summer, it would be "less than elegant" but effective.

It sounds like you have more than one issue. If the AC is blowing hot, the monovalve is open for some reason. If the AC line is not getting cold with the compressor engaged, they is also a problem with the AC system (maybe a low charge and/or a leak?). It also sound like your CCU was toasted at some point, so who knows if that's working correctly.

I never measured it, but AFAIK, the monovalve always gets a 12V supply and the CCU switches the ground on and off to maintain cabin temperature.

I'm not sure of the frequency and the results that Kerry observed might be the normal switching of the valve. I have a suspicion that it does quite a bit of shuttling back and forth.

When the pump was marginal, you could hear the monovalve cycle inside the cabin. That prompted me to immediately disconnect the pump and the monovalve then became quiet again.

That, more or less, agrees with my understanding of the monovalve design (but I didn't know the switching was on the ground side).

I don't have a clue how/why the pump and the monovalve interact through the CCU, I still think it's magic.

That repair kit is the exact one that I bought, only I got it off of ebay for 15 bucks. it was in an oem box with a part number and some documentation... I'm thinking it's the same thing. I am thinking that the problem is the electromagnet around the valve though. I have a buddy that's going to send me a whole valve assembly, and an aux pump for 30 bucks. He's also going to send along a CCU to tinker with.

__________________
1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

It's been over a year since I measured it so I can't recall the exact results but it definitely was not going from 12 volts to 0 and back again. I was getting no fixed voltage number.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13