'74 240D - No heat from blower, only cold air

[250 Coupe]

I was thinking that the AC knob clicks into the off position but could be wrong. It might be that the AC knob is stuck in the on position and that's why the light is on all the time. That would also be why the switchover valve isn't moving.

Michael

[HighDesert Benz]

Originally Posted by 250 Coupe

I was thinking that the AC knob clicks into the off position but could be wrong. It might be that the AC knob is stuck in the on position and that's why the light is on all the time. That would also be why the switchover valve isn't moving.

Michael

Just went out and double-checked mine and you are right. It does click off, but it is a soft detent and not a hard click like a regular electrical switch. Cycling it on and off I can hear it acuate the vents in the dash. If the switch or damper has failed in the on position that would sure explain the syptoms. As a reference, on mine the off postion is in the 9 o'clock position rotating CCW, light comes on as soon as it is turned CW past the detent.

[250 Coupe]

I thought there was some sort of tactile feedback that you'd reached the end of travel.

speace is right about how the lamp is powered. The AC knob is a combo thermostatic control and vacuum switch. There are no electrical connections but there is a cap tube that must be handled carefully.

The soft detent is the vacuum switch. It routes vacuum to several flaps, one of which is the switchover valve. The switchover valve turns on the green light and switches power over to the AC blower. The heater fan speed control continues to control fans speed regardless of which fan is running.

I can email some PDFs from the 114/115 CD to anyone that PMs me an email address. They are too big to attach here.

Michael

[speace]

Originally Posted by 250 Coupe

I thought there was some sort of tactile feedback that you'd reached the end of travel.

speace is right about how the lamp is powered. The AC knob is a combo thermostatic control and vacuum switch. There are no electrical connections but there is a cap tube that must be handled carefully.

Michael

Yes, great stuff Michael!

There MIGHT be a wire to the A/C knob... If so, it is only to provide power for the illumination. I seem to think mine glowed just a bit at night when the dash lights were on. I could be remembering this incorrectly.

Ah yes! The capillary tube! - Handle with care...

[pizzachef]

wow, great info guys.
250coupe, i sent an email about the manual.
Thanks

[250 Coupe]

Geoff,
Your inbox has been flooded with PDFs for the hvac system.

I find that the pictures look nasty on my monitor but print out much clearer.

Michael

PS, for those wondering, the PDFs come from the W114/115 CD-ROM available from mbusa.com
http://www.classictechlit.mbusa.com/MB/Default.asp?cookie_test=1

[pizzachef]

Thank you, Michael!
I think I'll order that CD. I'm hoping to have some time tomorrow to look at this, but I found a bad coolant bypass hose on the wagon (actually it was pointed out to me at the SE PA GTG today)...so I'll be tackling that first.
I'll post results.

[pizzachef]

So after all of that, one of the vacuum lines had come off the back of the AC knob. There is a vacuum switch connected to the AC knob (really a 3-way valve) similar to the master door lock vacuum switch, and one of the lines had fallen off, so it wasn't moving that vacuum pod near the acc. pedal...which in turn wasn't turning the green light off or diverting the air to the floor or turning on the right blower.

So now it seems to work as it should (except for the fact that the air isn't cold when the AC knob is turned on...guess I have some refrigerant issues )...but the heater blower blows nice and strong, I can move the air from the vents to the defrost to the floor, and the AC knob turns on the AC blower. There is a small flat spot on the cam on the backside of the AC knob, so that is the slight click that you can feel when the AC is off.

Thanks for all the help.

[HighDesert Benz]

well happy days! Those are my favorite type of repairs.
Best of all, your defroster is working as it should and we had a very enlightening (at least for me) conversation about the CC on a slash 8, which is probably the most complex part of the whole body to work on. Not that it's complex like most newer ones, but I'm now better armed to tackle a problem should it arise later. It's all good

[amt230]

Trying to see if I can re-ignite this thread to help troubleshoot a potentially similar problem in my '70 220d.

Similar to the OP, I can't get my heater to blow hot air. I've tried all the available combinations of slider positions (which is a puzzle in and of itself), including what I believe should definitely provide some hot air somewhere (all of them toward the outside of car), but still get only cold air.

Unlike the OP, there is no A/C light in the '70 car, so I can't tell if my car is stuck with the A/C on, but I believe that might be the issue. I recently had the car at the mechanic, who I believe was going to take a look at the A/C, but in the end wanted to wait until warmer weather.

The A/C knob turns slowly but smoothly, and there is a very slight click right before it stops in the full CCW position. But, the compressor does not sound like it's turning off (assuming it is on). The knob itself is loose, and actually came off at one point (does anyone have a tool or a recommendation for utilizing those slots on the knob's collar to reattach securely?).

Any help?

Thanks!

[JiveTurkey]

Whenever someone claims that Mercedes Benz has Brilliant Engineers I point them toward the heat / ac on a w114.

[Mike D]

Originally Posted by amt230

Similar to the OP, I can't get my heater to blow hot air. I've tried all the available combinations of slider positions (which is a puzzle in and of itself), including what I believe should definitely provide some hot air somewhere (all of them toward the outside of car), but still get only cold air.


The A/C knob turns slowly but smoothly, and there is a very slight click right before it stops in the full CCW position. But, the compressor does not sound like it's turning off (assuming it is on). The knob itself is loose, and actually came off at one point (does anyone have a tool or a recommendation for utilizing those slots on the knob's collar to reattach securely?).

Any help?

Thanks!

You are aware the A/C and heating systems are completely separate on the car? The heater fan and heater core are actually mounted on the firewall and the A/C fan and evaporator are mounted separately in the central air exchange box. The only connection between the systems is the flap which determines which system is being used. The system is actuated by vacuum lines on the back of the A/C temp control switch. There is a diaphragm on the side of the exchanger which opens/closes the flap. It also clicks the fan select switch (small micro-switch located on the side of the exchanger above the gas pedal) determining which fan unit is engaged.

The heater valves (mounted on the engine side of the firewall just beside the vacuum booster) are known to either clog up or not open. Run the car to temp and feel ALL the heater hoses to ensure you are indeed getting water flow.

I use a long nosed pair of needle nosed pliers to tighten the locks.

[diametricalbenz]

Originally Posted by amt230

Trying to see if I can re-ignite this thread to help troubleshoot a potentially similar problem in my '70 220d.

Similar to the OP, I can't get my heater to blow hot air. I've tried all the available combinations of slider positions (which is a puzzle in and of itself), including what I believe should definitely provide some hot air somewhere (all of them toward the outside of car), but still get only cold air.

Unlike the OP, there is no A/C light in the '70 car, so I can't tell if my car is stuck with the A/C on, but I believe that might be the issue. I recently had the car at the mechanic, who I believe was going to take a look at the A/C, but in the end wanted to wait until warmer weather.

The A/C knob turns slowly but smoothly, and there is a very slight click right before it stops in the full CCW position. But, the compressor does not sound like it's turning off (assuming it is on). The knob itself is loose, and actually came off at one point (does anyone have a tool or a recommendation for utilizing those slots on the knob's collar to reattach securely?).

Any help?

Thanks!

The knob itself has a snap ring which sits around the barrel of the switch which then fits into the socket in the console, it might be missing.

Do check the valves on the firewall, see if they even move or not when the temperature control is slid from cold to heat.

[amt230]

Originally Posted by Mike D

You are aware the A/C and heating systems are completely separate on the car? The heater fan and heater core are actually mounted on the firewall and the A/C fan and evaporator are mounted separately in the central air exchange box. The only connection between the systems is the flap which determines which system is being used. The system is actuated by vacuum lines on the back of the A/C temp control switch. There is a diaphragm on the side of the exchanger which opens/closes the flap. It also clicks the fan select switch (small micro-switch located on the side of the exchanger above the gas pedal) determining which fan unit is engaged.

The heater valves (mounted on the engine side of the firewall just beside the vacuum booster) are known to either clog up or not open. Run the car to temp and feel ALL the heater hoses to ensure you are indeed getting water flow.

I use a long nosed pair of needle nosed pliers to tighten the locks.

Thanks. After reviewing the rest of the thread, I got the general idea that the systems were different, but wanted to confirm for my slightly older car. When the temp control system is shut off completely, but the slides are in the heat position warm air slowly trickles out of the vents when moving. That's one reason my impression was that whatever flap wasn't switching between heat and A/C. I'll look into your suggestions.

A potentially relevant fact: at the same time the heat stopped working, the radio (which is known to cut in and out) also doesn't have power. Is the A/C actuator powered the same route as the radio, and could I just be lacking power to that area of the console? Seems far-fetched, but I thought I'd bring it up.

[amt230]

Originally Posted by diametricalbenz

The knob itself has a snap ring which sits around the barrel of the switch which then fits into the socket in the console, it might be missing.

The snap ring is there - the problem is that the receiving slot for the barrel is slipping around behind the console and trying to hold all those things in place and jimmy some tool into the snap ring's slots requires about 2 more hands than I have. Is there a tool you recommend for working with the snap ring?