'75 300 - How does AC blower fan get switched on? PICTURE

[1975300D]

Hey everyone. I am in the process of putting back my console and I have a problem. Even though the AC blower motor works fine as I tested it, I can not get it to go on by turning the AC switch. How does this thing work?

Also, I did not label the vacuum lines that go to the AC switch. Does anybody know which of the three goes where? What do those vacuum lines do?

Thanks so much!

[1975300D]

Does the AC blower fan get its electrical circuit from the compressor? Does the compressor need to run for the blower to work?

[Mike D]

No. The blower fan supply isn't controlled through the compressor. It has its own power supply. I've never really traced it all the way back to its source but I believe it is the same feed as the defrost fan.

[tram]

Those three vacuum lines control a switchover device under the dash at the console, driver's side. The directed vacuum from the twist switch operates a vacuum pod which pulls or pushes a rod that controls the HVAC flaps in the bottom of the console, and the rod from the vac solenoid has a "foot" attached to it about halfway up the rod. This foot hits a tang on a big boxy looking switch. The switch kicks power to the compressor and separate A/C blower fan, while switching the internal flaps to the A/C evaporator, blocking off the heater core, and interrupting power to the heater blower. It's a real Rube Goldberg setup but it works really reliably. Locate the switch up beside the console. Push up on the metal tang that the foot should engage with the engine running and your a/c switch on and you'll hear the comp kick in and the blowers switch. Once you straighten out the vacuum you'll hear the flaps go "whomp" and feel it change direction, too.

Take your under dash panel out on the driver's side and look up next to the console with a flashlight. You'll see it.

On the vacuum to the switch- there should be paint dabs on it showing what color vac line goes where. Light green goes to the inside nipple closest to the switch. Dark green goes to the one farthest to the left, away from the switch, and the third one ( black, if I recall) goes on the nipple in the middle. Trace the middle one to the engine and make sure it's plugged into a vacuum source.

The bracket holding the vacuum directional switch has little slots where the philips head screws hold it to the switch- this is for adjusting vacuum flow one way or the other. Don't touch it unless you absolutely need to.

Hope this helps.

[1975300D]

Thanks Tram,

When I turn the AC switch on, does that send an electrical signal to the compressor to start up? I can't get the compressor to come alive. Right now I don't have any vacuum either, but I can bypass that by manually pulling the rod down. The HEATER blower shuts off, but the AC blower does not come on. So what could be stopping the electrical circuit to the AC blower? I cleaned the contacts at the switch you talked about over the accelerator pedal. Where else can the circuit be interrupted?

Mark

[Palolo]

Originally Posted by tram

Those three vacuum lines control a switchover device under the dash at the console, driver's side. The directed vacuum from the twist switch operates a vacuum pod which pulls or pushes a rod that controls the HVAC flaps in the bottom of the console, and the rod from the vac solenoid has a "foot" attached to it about halfway up the rod. This foot hits a tang on a big boxy looking switch. The switch kicks power to the compressor and separate A/C blower fan, while switching the internal flaps to the A/C evaporator, blocking off the heater core, and interrupting power to the heater blower.

I'm tackling W108 A/C soon.

Reading that makes me want to kiss my car.

[TylerW]

Originally Posted by 1975300D

Thanks Tram,

When I turn the AC switch on, does that send an electrical signal to the compressor to start up? I can't get the compressor to come alive. Right now I don't have any vacuum either, but I can bypass that by manually pulling the rod down. The HEATER blower shuts off, but the AC blower does not come on. So what could be stopping the electrical circuit to the AC blower? I cleaned the contacts at the switch you talked about over the accelerator pedal. Where else can the circuit be interrupted?

Mark

I don't know if a 300 is different, but on my '73 220D, you have to turn the AC knob to activate the system(the compressor comes on), then use the fan switch up higher to operate the fan, which you may already know. Your AC fan might be seized up if it hasn't been used ina while...mine squeals pretty good for a few seconds.

Also on my car there are two fuses in a holder mounted on the passenger side firewall, and evidently one controls the fan motor. If that isn;t complicated enough, there is a resistor pack located on the DRIVER SIDE of the fire wall.

I would start at the fan itself to see whether it's even getting power, and if not, work your way back. Are you sure the AC system is charged? Most systems have a low-pressure cut-off switch that kills the compressor if the charge is too low.

I had to search extensively here to discover that one single fan switch operates either the heater fan or AC fan. One of those things people seem to not want to come right out and tell you.

If I disable the vacuum to the system, nothing works, no compressor, no AC-on light, nothing.

[katja]

Originally Posted by TylerW

If I disable the vacuum to the system, nothing works, no compressor, no AC-on light, nothing.

So is it possible to manually switch over to the lower fans for the air conditioning without a working vacuum system? In my car, none of the vacuum-operated accessories are functional. I don't know of the A/C blower even works. I don't really need or want the A/C anyway but it would be nice to be able to circulate air when the car is stationary.

[tram]

Originally Posted by Palolo

I'm tackling W108 A/C soon.

Reading that makes me want to kiss my car.

It's really not a bad system once you understand how it works. A heater blower in either car is equally bad, though.

[tram]

Originally Posted by katja

So is it possible to manually switch over to the lower fans for the air conditioning without a working vacuum system? In my car, none of the vacuum-operated accessories are functional. I don't know of the A/C blower even works. I don't really need or want the A/C anyway but it would be nice to be able to circulate air when the car is stationary.

What vacuum accessories do you have? Locks too? What sort of brake pedal do you have?

On a 240D, the yellow lines will be the door lock system, green will be the HVAC system, and black will be vac supply to to the accessories and the vacuum tank. All will branch off your brake booster line. The booster line has a one- way valve in it- These can fail and cause all sorts of issues.

[ScooterABC]

You turn the temp knob to cold. That switches the fans. If you turn it FAR to the left, it switches to the heater blower. You should hear some vacuum actuator move something also when you do this (even if the car is off)