1974 240D W115 AC issue

[Techy-Mechy]

I recently had my AC converted from R12 to R134a. It seemed to work fine until today.

Today, I fixed the parking brake idiot light switch located just above the parking brake push pedal. I had to move some relays in order to get my hand to the switch for removal and install.

Now, when I try to run my AC, the compressor won't engage and air won't come out of the vents. The fan is working (I can hear it) and the vacuum relay on the drivers side is moving and pushing the rod towards the floor correctly. I checked the fuses and they all look OK.

The green light on the AC temperature control knob doesn't turn green either.

Could I have damaged one of the relays by removing it from the pin harness? I removed a relay that had about 20 small pins and one that had about 4 large pins. If I did damage one of these, can you tell me which one I may have damaged and provide a part number?

Or, can you suggest what else I should check?

I"m stumped on this one..and it's rather deflating after the victory over the darn idiot light switch!

Dave

[kraakevik]

Dave

Is it the A/C fan you hear working or the heater fan? It sounds as if the changeover mechanism hasn't engaged in the A/C mode despite the lever position. The changeover switch may be bad.

I'd check all the fuses in the regular fusebox as well as the ones on the passenger-side firewall in the engine compartment.

If the hot wire to the compressor is dead the clutch will not engage--best of luck



Tim Kraakevik
kraakevik@voyager.net

[Idle]

When removing relays on a Mercedes of this vintage it is possible to pop a fuse if you were doing this with the battery still hooked up. Checking the fuses would be the first order of business.

The fuses are not supposed to blow when you remove a relay, but considering the age of the electrical system it is possible because of just normal wear and tear on the system.

[Techy-Mechy]

Hi,

What is the change over switch. I looked at the fuses and they all look good. Do you know which fuse I should focus on?

I have uploaded the pictures of the items I removed while fixing the idiot light switch. I don't know if this is the change over switch.

I can hear the fan running (unless the heater fan will run when the AC fan is running). The vacuum relays under the dash on both the passanger and the drivers side are actuating.

I'm not getting any air out of my vents.

Do you have any suggestions?

Dave

[Techy-Mechy]

Solved! I found a description of the changeover mechanism. My changeover switch was broken off of the mounting bracket. Don't ask me how that happened for I wasn't working in that area. I swaged the plastic part of the switch back on the bracket and all is working well.

Thanks for the advice!

Dave

[kraakevik]

The following is an email I sent to Dave--I include it here in case anyone encounters the same problem--TK

Dave

The black relay is almost certainly the one for your flashers--mine has three live pins and a plastic locator pin.

The changeover switch is physically attached or connected to that driver's-side lever next to the console. It is electromechanical and snaps into position to feed either the blower fan for heat or the squirrel-cage blower for A/C.

It's conceivable that the linkage may need adjustment to ensure that the switch goes through the required arc to engage the A/C. The relays for the compressor and other high-draw components are on the passenger-side cabin behind the glove compartment. I've found these to be very reliable.

The fuses on the firewall are for power windows and the supplementary fan but Mercedes sometimes used 16-amp fuses there for other optional equipment. They are prone to oxidation and frequently open the circuit even though they look fine. You also need to see if you've got voltage to the clutch. It does still sound like the changeover circuit is malfunctioning--good luck



Tim Kraakevik
kraakevik@voyager.net

[Techy-Mechy]

Hi Tim,

Thanks for posting this. You were right, it was the crossover switch. The symptoms I observed were:

1. The compressor wouldn't turn on
2. The blower still operated, but it was the heater blower
3. The air flow vacuum actuators were operating
4. There wasn't any air coming from my vents

What I noticed was the crossover switch, located under the dash and near the console on the drives side, was dangling on the cable. The switch had broken from the metal mounting bracket.

The switch has an arm that will interact with the vacuum actuator rod and turn the switch on (when the rod moves toward the floor and changes the air dampeners). This switch turns on the internal squirrel cage fan and engage the clutch on the air compressor.

The fix was very simple. I epoxied the metal bracket to the switch body and reswaged the rivets to the metal mounting bracket. Then I re-installed the switch and the AC works great.

Thanks to all who gave advice on the issue, especially Kraakevik and Idle. Hopefully, this tread will be useful to others who have experienced this sort of issue.

Dave