Air Conditioning

[michael silverman]

I have a '74 Coupe with factory air. This spring I had the hoses changed (they leaked) then charged the system with R 12 that I had squirreled away.
I rotated the "on" switch and nothing happened until I was at the end of the rotation. Following a click, the fan came on full power and the air conditioning worked. After about fifteen minutes, the 16A fuse blew. I replaced it with a 24A fuse and it blew again.
My question is: Is it the switch? Is there a known fault?
Help please. This is the last item on my list to make the car very drivable.
Mike

 

[m_thompson]

Nice to see that you still have your coupe. There should still be five here in Rhode Island.

The A/C switch turns on both the fan and the compressor. You should hear a click from the compressor clutch when you first rotate the switch. The fan should run at a slow speed when you first rotate the switch because it is connected through a resistor in the switch. As you rotate the switch more the fan goes faster. At the end of the rotation, when you feel the click, the switch has turned on a relay that sends power directly to the fan so it goes full speed.

If the fan motor is drawing excessive current it could have damaged the resistor in the switch. This would explain the lack of slow speed and the fuse blowing.

You could disconnect the compressor clutch and replace the fuse with an Amp meter. Turn the switch just a little to see if there is any current draw. If not, then the resistor on the switch is probably toast. Continue turning the switch until you feel the click and see how much current the fan draws. It should be less than the normal fuse rating. Don't leave the fan on for more than a second if it is drawing too much power or you will cook the wiring.

If the current draw is normal with only the fan connected then there may be a problem with the compressor clutch. Reconnect the compressor clutch and measure the current again. This should also be less than the fuse rating.

 

[michael silverman]

thank you for your input. If the switch is toast, can it be replaced with a similar or factory part?

 

[61porsche]

A/C Blown Fuse and Fan Rheostat

Some observations:

1) A/C relay; located near the coil. If recent work was performed in that area check that the relay is fully grounded. It has a bracket screwed on the realy that grounds and completes the circuit. Mine was causing the same effect of blown fuses. Removed, repaired with new rivets, and problem was solved. Second item to check is the fuse to the auxillary fan, located behind the right headlights and accessed through the cover under the hood- corrosion built up on mine. Replaced the fuse holder and new fuse.

2) Fan switch- reference the manual- the switch design is such that the connection is at the bottom. If turned agressively, the connection can come partially loose and cause the same effect on the variable low speed side. The normal position from start is around 9:00 o'clock for reference. The switch doesn't appear keyed to the opening in the console keeping it stable.Once I got the switch in the proper orientation and the connection back- it works perfectly.

These things worked for me.

On the subject of a new fan switch- I tried the usual sources for new; no longer made/ available. Used or slightly different configuration on the connection.

Hope this helps.

 

[jmackro]

thank you for your input. If the switch is toast, can it be replaced with a similar or factory part?

I needed to replace one of those switches last year. No, they aren't available new - you need to find a good, used one.

The resistor dissipates a LOT of power when the blower is running at mid-speed, and the switches get HOT. Consequently, these switches will fail from heat-related issues.

 

[michael silverman]

Thank you for your input. I feel ready to address the problem now.
Mike

 

[Stevehose]

Does anyone know the value of this resistor? I have this symptom of no fan until last click and I am going to operate on it and would like to get the resistor before yanking it out.

 

[sfdon]

Keep in mind that double o2 salvage has a big box full of used ones for sale.

 

[Stevehose]

If I can't repair with a new resistor I will contact them, thanks. Would prefer to solder a new resistor in there if anyone knows the value.

Keep in mind that double o2 salvage has a big box full of used ones for sale.

 

[Stevehose]

Update: Fixed by cleaning!!

My fan switch would turn on the compressor but not the fan - until it got all the way clockwise, a symptom I have seen a few times with others on this board. It has been suggested that the resistor can be replaced, after I pulled it I saw on mine this is not so, it is a large ceramic coil riveted onto the bottom of the switch. Before giving up I looked at the coil and saw it was filled with crud and the metal contacts were corroded and rusty. I sprayed lots of electronic cleaner into the coil and onto the metal parts then turned the switch back and forth several times. Crud gone and now nothing in between each coil winding. Then I put some sandpaper inbetween the coil and the contact and turned the switch again to clean the metal contact piece that glides on the coil. I also sanded the other metal contact at the end of the coil which is stationary. I then put a few drops of fine oil on the shaft and other moving parts, it now operated smoothly with no friction noise. I hooked up the ohm meter and saw that I was getting variable resistance readings so back it went into the dash and it works perfectly. I believe the crud was negating the effect of the coil. Now my fan kicks on low and increases in speed as the knob is rotated until the last click where it goes to full blast. Attached are pics of what the resistor looks like after cleaning. The switch does get very warm when in use so it's not surprising that it fouls up. So before you replace your switch give this cleaning a try, mine's good as new.

 

[Simufly]

aircon pipes.

as a matter of intrest where did you get the new aircon pipes from? Mine are old and hard and will at some stage leak!

 

[pmansson]

Fan working but not the compressor

The guys at the local Bosch workshop says the power to the compressor is fed by a medium thick, black wire from behind the Colder and Power knobs above the radio. I can see it connected to the resistor behind the grille.
They measured the wire, with the switch on, and there is no current going forward.
When they took power from the glove box light, the compressor started.

What has happened here, please?
I cannot see a fuse anywhere in the engine bay, on the way to the compressor.
None of the normal fuses are blown.

 

[HB Chris]

Peder

Could the issue be the relay mounted under the ignition coil? All U.S. coupes have this round can style relay to power the compressor. Perhaps the lead from the switch to the relay is broken and that is why a jumper will work.

Chris

 

[61porsche]

a/c

The heater; if on or there is a seperate cutout relay if bad will prevent the a/c from coming on.

The relay under the coil- most common problem; bad ground. There are two. A short wire screwed on one side. The second is the mounting screws themselves on the relay. If the ground isn't working the compressor nor the aux fan will come on. The aux fan is directly powered from the battery and the fuse holder is normally corroded since its behind the center grille/ right headlight area.

The compressor has a seperate ground; hard to get to because it's behind the mount. If you've converted; it may be missing.

There is a wiring diagram for just the a/c in the bluebook under a/c retrofit.