Hello all, I just spent a large chunk of change getting my climate control system restored since the car had no heat when I purchased it.

When I got the car back from the shop, the heat worked perfectly, nice and hot, heated up quickly. I've been driving the car for about 3 weeks, and the heat is starting to fade. When I'm moving, it seems to blow hot, but when at idle, the air cools down and starts to blow cool/cold. The monovalve was completely replaced, and a remanufactured climate control system (the box with the buttons in the dash) was installed. The work was done at Don Poole mercedes in Arlington, a very reputable mercedes shop in the DC area (they are great to work with and highly recommended).

The symptoms sound like a monovalve problem, but is this possible after it was just replaced with a brand new one?!!!

Thanks in advance for your advice!

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1984 300TD Wagon

The heat works when the rpm's are up? Could be a water pump issue.
Was the cooling system (acid) flushed at the time of repair (real long shot, calcium/rust clogging up the heater core) ?

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Mercury is a boxer

1984 300d
1999 Prelude
Medici road bike
Klein touring bike
Klein mtn bike

Sounds like the auxilliary water pump is out. I'd unplug it ASAP because a seized pump can take out your climate control amp(?). People recommend fusing the auxilliary pump to stop this from happening.

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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

+1

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

After reading your suggestions, I did some further searches, and yes, that does seem to be the problem (auxilliary water pump).

Any advice on how to test the bugger and see if it's working? When you say "unplug" do you mean the electical connection, or unplug the hoses running in and out (and re-attach them using a temp connector until I can get a replacement).

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1984 300TD Wagon

I can hear my pump running when I wait for the gloplug cycle-

IE; when the key is on and the heat is on, but the engine is not running.

I hear it from the driver seat, myself. or pop the hood and check it out- w/ key on, heat on. should be on the pass. fender, down low and to the front of the turbo.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

This may be a dumb question, but, if/when I replace it, can I simply unplug the hoses (and block them to keep coolant from running out), install the new pump and then plug the hoses back in, or do I need to bleed the coolant system?

It's been a while since I've been under the hood... my first "old" car in 10 years.

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1984 300TD Wagon

First, unplug the electrical connection.

You could clamp each hose while you replace the aux water pump, or just replace the hoses. You should not have to bleed the system if you clapm the hoses off. If too much coolant os lost, you will need to "burp" the system though.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

I Have /had the same Aux Water Pump problem (Corroded stuck)w/ my 87 300D turbo ( love the car).
I eliminated the problem by removing the dead Aux pump electrically and Mechanically.
Reconnected the hoses together by using an Old Alum. Tent pole , cut to 3" and Smoothened the edges, plugged into the hoses and clamped it tight.
No more Aux pump. The Heat works fine when engine is above 1200 rmp (%95 of the my driving time).
And yes , it does not pump hot water when engine is sitting Idell , but I can live with that. I can always put in Neutral and Rev up. :-)
This can be a temp. or long term solution.
Simplify!

I agree. Pre 1986 Climate control pushbutton units can be burned out very rapidly by a malfunctioning aux water pump. Unplug the electrical connection to the pump as soon as possible.

John

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John
1986 300sdl
1985 380se

I agree also, just replace the auxiliary water pump. if you put your hand on it with full heat selected you can feel if it's running. I replaced mine a couple of weeks ago because it was starting to get a little noisy (I could actually hear it running from inside the car before I started the engine). If it locks up it can toast the push button unit. Some folks put a fuses in the wire to the pump, but I've never bothered.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42BE0QGIMM&year=1984&make=MB&model=300-TDT-001&category=All&part=Auxiliary+Water+Pump

BTW, when I replaced mine I only spilled about a pint of coolant, just work fast.

I eliminated both the aux. pump and monovalve. (replaced the monovalve with a 1/2" manual gate valve from Home Depot). It works perfectly. No more heat coming and going.

Simpler is almost always better.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG

Why not just build a campfire in the back seat?

Reminds me of a friend who used to paddle an aluminum canoe. He'd throw in some sand, build a fire and cooks his lunch on it while floating down the river.

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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

Ok, auxiliary water pump replaced. Pretty easy, took about an hour. Lost about 1/2 quart of antifreeze in the process, not too bad. Splashed some antifreeze in the engine compartment and cleaned it up the best I could... is that stuff corrosive? Does it eat paint or cause rust?

Thanks for everybody's help.

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1984 300TD Wagon