Car is blowing hot air on all settings. I've pulled under-dash panel from passenger side and the hose (metal pipe) going to the blower is cold, and I can hear the compressor kick-in when I turn on the a/c. I've checked the thermistor, on top of the dash, it is getting vacuum and the wiring is connected. Any ideas on what may be wrong?

My first thought would be to check the rubber bellows on the mono-water valve insert, I believe this is the system this car uses. If the bellows gets torn, it will allow hot coolant to constantly circulate throught the core.
Gilly

Thanks Gilly,
I've checked and replaced the mono-valve insert, even though it looked okay. As I mentioned, there is cold fluid going through the firewall into the cabin, the tubing into the blower(under the dash on passenger side) is cold. I'm stumped.

The heater valve(mono) must get electrical SHUT-OFF signal from pushbutton. There is power to the valve any time the key is on, so when you ask for A/C the pushbutton sends GROUND signal to shut the valve off. Water flows thru the valve unless it receives a command to shut off. You should check to see if the valve is getting this power & ground signal. HEAT will always overcome the A/C system.

As an expansion to what MBDOC is saying, you can pull the electrical connector off the watervalve, then tap a power and ground to the 2 pins on the valve, you should hear it tap or "thunk" inside the valve when you apply or disconnect the power. If not, the valve itself is "bad", not just the insert. If it seems to be acting correctly, try hooking up a voltmeter to the 2 sockets on the connector and monitor the voltage while running the climate control. In the "hot" side, there should be almost no power. It cycles though, not modulates, so your looking for almost "zero" volts, or almost battery volts, no in-between readings. So on "HOT" it should almost always read near zero, it may show a battery voltage spike from time to time. Just the opposite on max cool, it should almost always show battery voltage, but may cycle to no or almost zero volts occasionally, just to help keep the evaporator from freezing over. If it isn't working correctly, the problem typically is the push button panel/control unit (the part with the buttons on it that controls the climate control). The reason the valve is set this way is so if there is a power failure to the valve, they want to make sure that heat is able to get into the cabin to keep the windshield clear. As a matter of fact the failsafe for the distrubution system is to divert air to the windshield, and for the fan control it is max fan speed. We have had several customers come in with max heat, on the windshield, and full blower speed, on 90 degree days, too! Some came in in the middle of winter too, they didn't complain as much!
Gilly

Okay, these last two posts seem logical. I'm missing a step in the instructions though.

"then tap a power and ground to the 2 pins on the valve, you should hear it tap or "thunk" inside the valve when you apply or disconnect the power."

How do I "tap a power and ground to the 2 pins"?

And then if I determine that the valve is not shutting off, how would I determine if it's the valve itself or the pushbutton unit?

Thank you, gentlemen!

To determine if it's the valve or the pushbutton panel, that's pretty much all my previous post was about. Do the test by jumping power and ground and see if the valve thunks. If it does, the valve is OK, end of test. Then you should monitor the voltages as I suggested in max hot and max cool just to verify the pushbutton panel is in fact faulty. To test the valve, you need a pair of test leads with alligator clips on one end of each. The battery is near the valve, so it's pretty simple. Clip one of the leads to battery positive side, the other to the negative side. The valve is not sensitive to polarity, so it doesn't matter which side of the valve you put the power or ground to, it should "thunk" either way. Anyway, just touch the ends of the test leads to the two pins on the valve, power on one, ground on the other. If your aren't 100% sure the valve is operating, just get someone to either feel the valve or do the wire jumping so you can feel if it's working or not, it's usually pretty obvious if it works.
Gilly

As a second idea, if the valve "thunks" OK, you could rig up a pair of test leads with a clip on each end and keep power and ground applied to the valve, then start the car and see if it's blowing cold air or not, that will tell you that you definitely have a heater valve control problem, probably the push button panel.
Gilly Again

I'm going out to try it right now! Thanks Gilly!

Tested the valve - Thunk!
Had the wife sit in the car w/ A/C on, connected the valve again (no alligator clips, so I had to hold the leads with all four of my hands) and she confirmed cold air was a-blowin'. God I love this web-site!!!
So Gilly, looks like the push button unit is the culprit, but it seems like it could also be a short in one of the wires from that unit to the valve. I'm not real confident in my ability to diagnose anything electrical(I am, however, adept at replacing the batteries in my T.V. remote control), so is there a sure-fire test to eliminate the wires themselves as the problem? If it is the push-button unit, is it repairable? The one in the car now is labeled as "rebuilt", so it's been replaced before I got the car, same as the radio which sucks too. If this is not repairable, guess I'll buy a new one.

Thanks again, Gilly. And MB Doc!

My experience has been that if you can verify cold air by doing what you have done, may as well shop for a push button panel. Sure it COULD be an open in one of the wires. That's the purpose of the test that I gave you, but you'd need a voltmeter, which I believe you could go out and buy relatively cheap at an electronics store or even Sears. But ultimately, I feel 95% certain that your looking at a bad pushbutton panel/control unit. You can check out this website, Fastlane, or call your local MB dealer for a price, unless you're really electronically savvy, it's no use messing with it.
Gilly

Well I guess I failed to fully report my test. Once I got the a/c to cycle on by jumping the power from the battery directly to the mono valve, I did hook up my multimeter to the two pins on the valve - no volt reading with a/c setting on full cold or full heat, but I guess if the push-button unit is not sending a signal that's what I should expect. I thank you for all your help. I'll buy a new or rebuilt unit pretty soon, but since the thing is shot, I think I'll pull it apart to see if I can solder any loose connnections. Can't hurt it any more, and if I don't look busy on my car today, the wife is going to put me to work in the yard.

I had this same problem too. There is some hose behind the glovebox that disintegrates, which is another possible cause of the problem. Strangely enough, my AC started working fine yesterday, blowing ice cold air on a hot California day. I have no idea why. I'll just keep my fingers crossed.

Speedy:
The crumbling hose sounds like the aspirator hose on an older one. It can cause a generally poor operation of the A/C, but not a real warm running like this one. You are probably looking at the same kind of problem, a bad pushbutton panel problem can kind of come and go as you are describing.
Gilly