1976 Mercedes 240D A/C Question

[BJTX]

Just got a 1976 240D and I have a question about the A/C system.

I am trying to diagnose the A/C and why it doesn't blow cold or at least learn what the various interior controls "should" operate. I am guessing its either a bad compressor or low freon but I will try to explain what does happen when the various knobs are actuated.

My first question is does the car have one or two fans in the dash? I have read in more than one place that they have a main blower fan in the dash for the heat and defrost and another lower and easier to get to fan in the center console for the A/C. I have looked online and I cant seem to find the supposed secondary A/C fan for sale all I can find is the blower fan? I just would like verification that this model and year does have two fans and if so, could someone provide a link online where these might be for sale?

This is what does happen when I turn the various knobs.

When I turn the A/C knob the cutover switch does operate but no fan comes on. Also, this does not operate the A/C compressor clutch.

The DEF knob does turn on the blower and changes blower speeds. This also operates the A/C compressor clutch. Is this wired correctly?

I get heat,defrost and vent and all the slide levers seem to work as the are supposed to.

So, I guess my question is, are the inside controls working as they should?

Any help would be appreciated.

Brian

[Pooka]

I can tell you there is only one fan. The 108 and 109 had different fans for the heater and A/C, but the 123 has only one blower motor for both.

I would start with testing the A/C switch. Since nothing takes place when you switch it on I would guess there is no power coming out of it to send to the rest of the system.

Also test to see if you have power coming to the switch. This could be as simple as a blown fuse.

[BJTX]

Originally Posted by Pooka

I can tell you there is only one fan. The 108 and 109 had different fans for the heater and A/C, but the 123 has only one blower motor for both.

I would start with testing the A/C switch. Since nothing takes place when you switch it on I would guess there is no power coming out of it to send to the rest of the system.

Also test to see if you have power coming to the switch. This could be as simple as a blown fuse.

Thanks for the help.

The 1976 is on the W115 chassis. Would the one fan still apply? Any idea where that fuse might be, I checked the main fuse box under the hood and nothing is labeled for the A/C.

[Mike D]

His '76 is a W115 which does have the separate fans.

The A/C has vacuum and electric control. Turning the knob will cause the vacuum to be diverted to the proper diaphragms and move the internal air flaps and it sounds as though you can hear them move. The turn knob also energizes the electric portion (compressor, external cooling fan,etc.) of the system.

The blower fan (small dual squirrel cage mounted inside the center console) gets its power selected through the "change-over" switch mounted on the left side of the center console. They are easy to check. Remove either of the plastic grills on the side of the console and you will see the blades. Try to spin them using your finger or a small screwdriver with the power off. That is step one.

Crap! I just thought of something. Do you have a big dial which controls the air temperature and a row of push buttons or just one knob mounted center left which you turn to change the system?

[79Mercy]

You have two fans, one fan on the firewall for the heat. Only operates when the green "colder" knob is turned fully counter clock wise. This should move the vacuum actuator near accelerator all the way up so that the system knows to use the heater fan.

When the green knob is turned away from the off position the vacuum actuator should move all the way down and will now allow the AC fan to run if the defrost fan speed switch is turned on one of the 3 speeds.

The knob that says defrost controls both fans, but you control which fan with the green knob.

9 out of 10 W115's with the original heater fan have seized blower motors. The AC fan in the console tends to outlast the heater fan.

[BJTX]

Originally Posted by 79Mercy

You have two fans, one fan on the firewall for the heat. Only operates when the green "colder" knob is turned fully counter clock wise. This should move the vacuum actuator near accelerator all the way up so that the system knows to use the heater fan.

When the green knob is turned away from the off position the vacuum actuator should move all the way down and will now allow the AC fan to run if the defrost fan speed switch is turned on one of the 3 speeds.

The knob that says defrost controls both fans, but you control which fan with the green knob.

9 out of 10 W115's with the original heater fan have seized blower motors. The AC fan in the console tends to outlast the heater fan.

Thanks for help.

Sorry if I'm a little slow on the uptake. So, if the A/C colder knob is turned fully counter clockwise and in the off position and then I turn on the DEF knob and
I get a fan blowing, that should be the heater fan? If I then turn the A/C colder knob fully clockwise, that activates the vacuum actuator and turns on the A/C fan? Would that then turn off the heater fan at the same time? The reason I ask is because I have a fan blowin no matter where the A/C colder knob is positioned.

An earlier post mentioned possibly a fuse associated the the colder switch/vacuum actuator. Could this be a possibility?

Thanks,

[Pooka]

Originally Posted by BJTX

Thanks for the help.

The 1976 is on the W115 chassis. Would the one fan still apply? Any idea where that fuse might be, I checked the main fuse box under the hood and nothing is labeled for the A/C.

Sorry, two fans.

I have seen, and even owned, a 1976 123 240D so I thought that was what you were talking about.

Mercedes once played rather fast and loose with year models since the cars changed so little from year to year. The FTC came down on them pretty hard for this and they cleaned up their act many, many years ago.

But I would think the advice about the switch would still hold.

[Pooka]

If you have a seized heater fan then check out how to lube it through a small tube. There are several posts on how to do this since it is a common problem with all Mercedes of this vintage.

I have replaced these fans. It is a great deal of work. Nothing hard, but it takes forever to remove the dash in order to get to it.