Check all the vac lines in the engine compartment for leaks or breaks or loose connectors. The AC is the green plastic line.

The control unit isn't hard to replace, but won't be cheap, I don't think. There are two -- the Pushbutton control unit in the dash (remove the console wood, unscrew, etc) and the electronics box behind the glove box.

A hand vac pump will make it easy to troubleshoot the vac system -- it must hold vac unless a servo is actually moving. There are a number of places that can leak, but the usual complaint of no air out the center vents is often a bad line or check valve in the engine compartment.

No fan and heat up the windshield all the time is a blow fuse, most likely, or just corrosion.

Heat instead of AC (or lukewarm air instead of cold) can be the monovalve.

Describe the problem more and we can be more specific.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

My sdl has a vaccum problem and my old mechanic before i left LA said the main circuit box is fried and needs to be replaced is this hard easy and is there a source for a repair manual. Thanks in advance for your help.

Details are

Cool not cold air out of side and defroster
No air at all out of center vents
Heat is out of defroster but is not hot
Fan blows well
Looked for green line cant find it
Behind glove box are flappers that are always shut
To the right of the flappers is a partially moved hose that is blowing air.
Iwant to do the work myself because local dealer wants $250 just to look under hood. My wife can help because she almost a rocket scientist.

Vac supply line is beside the brake booster, there are several lines there. Look for a missing rubber connector or broken line, you have no vac to the system.

Reconnect the hose behind the glove box, it should be obvious where it goes. Check the small foam hose to the temp sensor while you are in there, and replace it if it is crumbling. The temp sensor is down by the AC box, and the cabin air is sucked down from the dome light through a plastic hose by a small fan. If the foam part is missing, it sucks air from around the heater box, will cause wild temp fluctuations when you get the rest fixed.

Check the temperature of the heater hoses in the engine compartment while the AC is running -- if they are warm or hot, the monovalve is leaking. It's under a cover (two screws on the front firewall) near the battery on the passenger side. Easy to pull and replace. It is closed when energized, so you can check that by jumpers from the battery -- if it stays closed and you get cold air with it jumpered, the pushbutton control unit is bad.

Make sure the AC compressor runs, and that you don't see foam in the sight glass on the reciever/dryer up by the radiator -- if you do, you are also low on charge.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Pete,

How did you reply before he asked the question?

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Peter is psychic In the last couple of days, a lot of threads have posts out of sequence.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

I must need to reset my clock. It is still 1980 isn't it?

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter I looked for the temp sensor. if it is on the right side of the glove box and is held in place by a u shaped clip the sensor is missing. I couldn't find the site glass. The lines appear to be in good shape. Is the monovalve along thin box in the center. Thank you for your help. Photos or a diagram would be helpful if you know where i could get them.

The sight glass is on the receiver dryer -- a black semicylindrical cansiter up beside the radiator on the driver's side. There is a small glass window on top, just above the two wires that connect there, I think. You will probably have to clean it off, it will be dirty. Shoudl be clear with AC off, get foamy bubbles in it when you first switch the AC on with the engine running, and then go clear again. If it stays foamy, you are low on freon.

Temp sensor is a thermistor under a cover on the left side of the glove box, low down and by the firewall.

The monovalve it hidden behind a cover between the firewalls under the hood. You must remove the cover to see it, it is about an inch and a half square and about 3 inches tall, I think, with two heater hoses attached.

Best (and most expensive) manual is the CD manual from FastLane or Mercedes. There are some aftermarket ones by Chilton and others, but they aren't always useful.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!