w116 heater problem

The valve your talking about is part of the climate control servo. There is an electric motor in the servo that drives a gear assembly from max heat in one direction to max cooling in the other. Along the way, various vacuum and electrical connections are closed or opened to control the system and the valve is moved from fully open to closed. The valve is not replaceable as a seperate unit.

If the system is behaving correctly except for heat, ie, moving the temp to 85 sends air to the footwells with the blower stepping up to high, moving it to 65 turns on the AC with air coming out the upper vents as the blower slowly slows down then speeds up again, then the servo is working. Unless the valve has somehow become seperated inside from the rest of the gear train, it has to be opening and closing.

Is it possible the system is air locked? I think that's unlikely, however, according to the service manual, after refilling the cooling system, it's recommended to manually run the little aux heater pump to help get air out of the heater system. That's the small pump that's located close to the servo. It's function is to pump water through the heater core when the car is first started in cold weather to get heat faster. I've never bothered doing that and never had a problem, but it's easy to do. You just need to disconnect the pump connector and directly connect it to 12Vs, paying attention to polarity so it runs in the right direction. Brown is ground.

As a next step, you could disconnect the hoses to the heater core and try flushing it with water, to make sure there is free flow. You could also try verifying if water makes it through the two paths in the servo valve. With the ignition on and the temp set to 85, the valve should be open. You could connect a piece of heater hose and try pouring water in to see if it runs out the other side of the valve fittings. Another simple test would be to bypass the hoses around the valve temporarily, using two short nipples and hose clamps.

If it turns out you need a servo, I recommend getting one from Performance Analysis in TN. They have remanufactured ones that replace the lower plastic housing with an aluminum one. The plastic cracking, leaking, and ruining the rest of the unit is one primary mode of failure.

Thanks, I now have a couple of things to look at. I have a few fittings and hose pieces to bypass the valve, as that was my first idea.

Bernard

Ok. the blower works but there's no heat. I suggest you first do a test. Run it in DEF mode. That simultaneously engages the A/C compressor and full heat mode (to melt ice and defog the windows). If there's heat in DEF the servo is probably OK and the logic board is suspect.

There could be one of two basic problems. The logic board or "amplifier" could have a solder crack or the "servo" could be dead. The amplifier, a black covered cigarette size pack device with wires connected to its bottom) is located behind the glove box (remove the GB to get to it). Its wire connector has nearly to be pried off. This device can be bought on an exchange rebuilt basis for about $100. In my experience sudden erratic behavior is a symptom of an "amplifier" malfunction.

When you shut off the engine do you hear the servo run for about five seconds? (It's returning to its off position.) If so that's good, or at least better than not, which suggests the servo is dead. The servo can also be bought on an exchange rebuilt basis for about $500.

Of course there could be other causes - the temperature sensor, the temperature setting wheel - but these are much less likely to be the cause than the amplifier or the servo.

Don't bother calling a M-B dealer for these parts for the price will be as much as you paid for the car!

Nope, no heat at any time.

I have not heard this, but will listen

I am planning a dash tear-apart to look at those items.

Thanks for all replies! Bernard

Before you take your dash apart... this is major work...

Try this... look at the heater core nipples that you see in the engine compartment... bypass the servo and hook them up to the engine water pump... if you still feel no heat, then your heater core is plugged up because someone put in the radiator fix thing that clogs everything up... you can try backflushing to see if it works... rig it up to garden hose to clear it out. Otherwise, you will have to replace the heater core.

Before you tear into anything check the fuses. My '80 300SD had a dead climate control and blower because two fuses were blown. That happened after the electric auxiliary water pump, the small pump near the servo, shorted out from coolant leaking into its motor.

I unplugged the auxiliary pump, replaced two fuses and the climate control and blower worked perfectly.

The electric pump helps warm the cabin a bit quicker, in our mild climate I really couldn't feel much difference with it, or without it. Perhaps it matters in cold areas.

Always start a repair by first checking the simple things.

Did you find a good source for the aux water pump? I had to replace the original, which lasted 20 years. Got one from Adsit Company. It stared leaking with a small drip in about 2 months. They replaced it under warranty. Another month, that one has a very small drip too. They replaced it again, 3 months later, that one had a small drip. Since they wouldn't replace it again, I decided to just live with it and see what happens, as it was really barely leaking. Well, last week I'm driving along, and all of a sudden I hear a grinding, rattling sound. Turns out the top bearing in the little motor is shot, the shaft is just bouncing all around.

So, I need a new one, and I'm not gonna get it from Adsit. Also, the pump was running when the coolant was hot, which it shouldn't do. I have to take a look and see what turns it off, but as I recall there is a cut off somewhere, so once the coolant is hot enough, it shuts off.