Crazy Heater Problem?!?! I'm freezing!

[woody429924]

Car: 1989 Mercedes 300SE


So, I bought this car in September. I recharged the AC, and it works great still today.

But, this winter I tried firing up the heater. I rolled the climate control temp to max heat, and I got cool air. It stayed that way, no matter how long I drove. I went 60 miles on the freeway and it still didn't warm up. However, the AC still blows very cold.

After trying to get it to work everyday, it did something.

It warms up a little (definatly not hot) when I am at idle for a few seconds, and then immediatly cools down when I take off.

If I run the fan at th lowest speed, it will come out very slightly warm. If I turn up the fan, it immediatly goes cold.

So what's up? Anybody have this problem before? Is it a temperature sensor, or maybe a leak? Any ideas or thoughts would be appreciated.

[mbdoc]

Try disconnecting the electrical plug at the heater valve..

Also try search for "mono valve"..common failure..

[juribe2]

Also check if you actually have a heater hose going from the engine block to the heater core. Maybe the previous owner had some problem with the heater core and made a bypass in the system.

[POS]

A heater core failure would create these symptoms too.

[david s poole]

does the temp gauge get up to normal [80 or more]?if it does not it's possible that thermostat is broken open.

[woody429924]

The Heater Core is not bypassed.

Engine temperature runs correctly.

I don't think it is a thermostat problem. I'll try the electrical thing.

Any other ideas?

[TX76513]

Bet you a donut it's your mono valve - and may only need a quick cleaning. They get stuck closed after months on AC weather.

Remove the 4 small screws from the top of MV. Don't drop a screw behind firewall Gently remove the internal valve and clean with 000 steel wool. Re-assemble.

You do not have to disconnect MV from car to clean valve.

[woody429924]

I'm such a novice at Mercedes... Forgive me...

Where's the mono-valve in this picture:

[TX76513]

Woody - tough picture bummer somebody put a gas motor in your car! How about I show you what it looks like

The top is the metal (silver).

You can see the four screws mentioned above. If you unplug the wires and you get heat you could have CCU problems

[woody429924]

Where can I find that part? Is it closer to the firewall or towards the radiator. Is it under the CPU?

[woody429924]

Okay, found it. Now how do I get to it around the hose cables? Can I do it with those in place? I removed the electrical and test drove it, no heat. So I'm goin in.

[TX76513]

Originally Posted by woody429924

Okay, found it. Now how do I get to it around the hose cables? Can I do it with those in place? I removed the electrical and test drove it, no heat. So I'm goin in.

Just take the four screw out, remove top and carefully remove the cylinder. Inspect "O" ring for nicks and 000 steel wool the cylinder.

[woody429924]

Okay, I got it out. I looks clean. However, I tested it with 12V and the plunger barely moved at all. The coil itself looks burned out or shorted. I guess I need to get a new one, right?

[TX76513]

There is probably a resistance specific test for the coil and I don't have that information in front of me. But if you leave the plunger out in the interim you will have plenty of hot air -lots

[woody429924]

Originally Posted by TX76513

There is probably a resistance specific test for the coil and I don't have that information in front of me. But if you leave the plunger out in the interim you will have plenty of hot air -lots

So, would it be safe to drive with out the plunger? And would there be a way to shut off the hot air or would I have constant out air?

Hot air is better than cool air in the morning here. I'm guess the plunger works as a regulator. I don't want to damage anything.