Not enough heat in the 93 190E's cabin when cold outside

You've already done a lot but the source of the problem hasn't been proven: a control problem or insufficient (hot) coolant.

It should get hot inside when the DEF is selected for a longer period as that's the maximum heat mode. If it produces maximum heat the (hot) coolant side of the equation is satisfied and the problem is in the control system, otherwise I'd suspect a blocked heater core, you having replaced everything but the engine's water pump.

No expert here, but:

I think that small vent up by the dome light is actually a vent for a tube that runs down behind the glove compartment. Down there is a small fan that pulls a sampling of cabin air to adjust for temperature. If that fan has failed, it would no longer be sampling the cabin temperature. It would be reacting to the temperature in the footwell, which is usually much warmer than the upper windshield area. It would lower the heat as soon as it sensed the heat output at the floor, and the whole cabin would feel cool.

I think those fans usually indicate they're failing with a slight buzzing sound (like an insect caught under a paper cup) kind of sound. They can throw everything off when they fail, and they fail occasionally.

Hope it turns out to be something simple like that.

I'd go with the sampling fan too. Had the same problem on a 1985 500SEL. Fan should run anytime the ACC is on so its easy to check. They're only about $30 from the dealer and much cheaper on the web.

Dick

Stupid question, but what's your coolant tmperatute gauge say?

I changed in a new thermostat too and had lousy heat. T'stat was defective right out of the little blue box with stars on it.

Are the windows closed?

mcbrue checklistingly under the bridge in the trailer down by the river

96 S420

The engine temperature goes up to 80C and varies between 80-100C even on really cold days. I swapped it out a couple of years ago. The thermostat begins to open at 87C I believe.

Ok, sounds ok, although it shouldn't really spend much time at 80, 85 is where it wants to be. I've seen estimates as high as one in three thermostats are defective right out of the little blue box, so you have to wonder about how accurate the ones that actually do work are.

Solets assume your coolt gauge reads 85C and your fan works. Barring a plugged up heater core and if your control electronics work you should be roasting.

Step 1 would be to see if the monovalve is opening and letting coolant flow to the ehater core. Pull the connector off and see if thigns are any warmer now.

I don't have a pictrue oc a 201 chassis monovalve (feel fre to send one :-) but here's a pic of a 126 one that should be similar enough to get you started.

With the connector off, the monvalve will be wide open and send a full amount of engine coolant to the heater core. So this bypasses any non-functional temp sensor, contro relays or switches. Let's see if the plumbing works first.

Get back to us and let us know if it's hotter or the same after you did this.

On my car it is called a heater valve and it is vacuum operated via the climate control system. I tried what you said a year ago and there was no difference in the heat. I do get better heat when the engine is at or near 100C but the heat dissipates when I have the blower/fan speed on the highest setting no matter what the engine temperature is. In my opinion it's either the climate control or the potentiometer in the dash.

I just wish I knew the systems of a malfunctioning climate control. That would be the easiest change of all. If the potentiometer was bad it would entail taking the dash out to replace it.

Ok, I asked around and got an answer. You're probbaly not gonna like it though.

" The 201 is notorious for this exact problem. No heat. The heater core is aluminum, and for whatever reason, has fairly bad circulation compared to the 126 chassis. It is so notorious that a class action suit exists in California that makes Mercedes pay for the repair/replacement of the heater core. That only works if you live in Kali fornika. If you don't, you'll pay a mechanic for 15-17 hours of labor to take the entire dash apart just like Deneal got stuck with on his 300E. "

It can't be the control circuitry if you've unplugged the monovalve and it was no better. You can have completley dead or missing control circuitry and still have head if you do this.

"You can have completley dead or missing control circuitry and still have head if you do this."

I'm gonna rip out my control circuitry ASAP so I can get head too.

There is a potentiometer in the dash that controls the air flaps position from the outside. There could be too much outside air coming in to the cabin if the potentiometer is malfunctioning. Also, I forward/reverse flushed the heater core a year and a half ago and there did not seem to be a restriction. Should I try flushing the heater core again???

Be careful what you wish for. I did this and got a stupid shower head.

After two and a half years of not getting enough heat in the cabin and spending $150 on an auxiliary water pump and heater valve, I began thinking about what you said about the heater core and disengaging the vacuum line of the heater valve to get full heat. I had tried that before and saw no difference in the cabin heat.

That triggered a thought to reflush the heater core. I reversed and forward flushed the heater core with high water pressure in both flushing directions. And, voila!!!!! The heat in the cabin is pretty toasty after about 10-15 minutes of the engine temperature being at

80C in the city and even on the highway with cold outside air coming into the cabin. I even had to turn it down with the temperature outside at 28F.

Thanks again Richard !!!!