Lopez

So, today my fan has stopped working when I turn on the A/C or vent setting, but works fine when I flip over to defrost (I am assuming the same fan blows for defrost and fresh air).

The A/C compressor is engaging as it should, just no fan regardless of where I set the switch (unless it's on Defrost). Is this likely a relay thing, or should I look elsewhere? Everything worked as expected yesterday, this is a new problem today...

Jim M.

Sounds like "Magic Blower Syndrom". The relay under the windshield wipe shroud is a known failure point. Call Roger for a replacement. It's not difficult to change unless you have big hands.

Lopez

Cool - thanks.

ammonman

It is possible it is the Fresh Air Blower relay, but on the 1979 wiring diagram the Fresh Air Blower Relay has to be energized to power the A/C Compressor clutch circuit. If you are sure the A/C clutch is energizing I think it's more likely you have a problem with the blower resistor pack or a wiring fault between the control unit and the resistor pack or blower.
Mike

Alan

It actually does not sound like the magic blower issue... this indeed sounds like a fuse or relay issue.

Check the fuse for the blower/HVAC or the Blower Relay...

Alan

ammonman

If this is the '79 in your sig, then I don't think a bad blower fuse would cause this. If the blower fuse is a bad there would not be power to the blower regardless of heater control position or blower speed switch position. It could be a bad Fresh Air Blower relay, but only if the A/C clutch is not energizing as the Fresh Air Blower relay provides power to both the blower resistor and the A/C clutch circuit.

Mike

aaddpp

Sorry to take the discussion back a bit, but what is "magic blower"?

Landseer

When the resistor mounted in the air duct fails, the blower may sometimes cycle full-on, off, full-on, off.

WallyP

On the '79, if the blower relay (Relay XXI) fails, the blower will not run on any speed EXCEPT when the condition lever is on Defrost. On Defrost, Relay XXII feeds the blower directly, and it will run at top speed. If Fuse #17 (Blower Fuse) fails or has a faulty connection, the blower will not run in any case, nor will the A/C compressor engage. (Thanks to Mike for the correction!)

If the blower resistor unit fails, the blower will usually run on top speed only, and the A/C compressor clutch will work normally.

On the earlier cars, if the blower motor draws too much current (usually dry bearings), or the air flow is restricted, or the thermal switch gets too sensitive, the blower will start randomly switching from low speed to high speed and back. There is a thermal switch built into the blower resistor pack that bypasses the resistors to increase airflow if the pack gets too hot. It can often be adjusted. This has been referred to as "Magic Blower Syndrome". On the later cars, the switch just cuts the blower off instead, and back on when it cools.

Lopez

OK - I'll check all these things - I am pretty sure the AC compressor is running but not 100% sure, when I switch it to AC I could feel and hear what sure seemed like the kick you get when the clutch engages. But I'll poke around under the hood and see if it is really engaging.

ammonman

Wally, is there a known error in the wiring drawings for the '79? Just curious as I hope to have my own OB money pit someday to go along with my S4. The drawing I have (from Jim M's excellent CD's) shows fuse #17 sending power to both XXI-Fresh Air Blower relay and XXII-Defroster Relay. Is there another source I am not seeing on the drawings? (My eyes are just starting to go so I don't always remember to get my glasses) I think all the built-in redundancy of these cars is so cool. How many mfg's would set up a system to ensure that you can at least get the defroster to work and keep the windshield clear if the speed control for the blower fails.

Mike

WallyP

Mike,

Nope, not an error in the wiring, just my error. You are correct - if Fuse #17 blows (or just has a faulty connection, which is just as common), the blower won't work in any case.

Thanks for the correction!

Alan

Yes you are correct both off the same fuse - I had in my mind that here was a seperate fuse supply on later models - but all years are the same. However the relays between regular HVAC/Blower & DEF are quite seperate paths.

Porsche actually designed in lots of redundancy here. In the early cars if the blower resistor pack overheats it switches to full speed mode (gives you fan and cools itself). Even if the blower resistor pack is removed you can still select max speed on the switch, and in DEFogger mode the blower will run even with a faulty blower relay.

DEFogger mode will clear interior misting on the windshield fastest, Defrosting of ice on the outside of the windshield is accomplished fastest in windshield vent slider position (console vents closed) max blower and max heat.(on later models the recirc position works too). It is a substantial difference (because the AC is always on in DEF mode).

Alan