question on the rear defrog button/switch

rnln · 04-22-2007, 01:26 PM

The rear defrog switch is not used in my car, I plan to use it for my rear condensor fan. Someone told that I need to have at least a 20A fuse inline for this purpose. Does that mean the switch also need to stand at least 20A?
Does anyone know if the rear defrog switch can stand this rating?
Thank you

fred cook · fred cook's Garage

Why not use the rear defrost relay as well. Then you will only have a small amount of amperage going thru the switch. The first click on the pull switch heats the center part of the defrost grid and the second click turns on the entire grid. On the second click you would be pulling about 25 amps thru the grid (but not the switch because of the relay). The quick and easy way to get power to the condenser fan would be to find the two power wires going to the rear defrost grid. Just tape off one and use the other for your power wire. Should be a "piece of cake" to do!

Good luck!

rnln · 04-24-2007, 11:29 AM

... then I still have another "click" of the rear defroster switch for something else...
Good to know.
May I ask where to find my rear defrost relay? I mean location, where in my car I should look?
Thanks fred.

fred cook · fred cook's Garage

The defrost relay is located in the engine compartment, left side, behind the plastic cover. It is about as big as a pack of cigarettes and should be silver in color. It should be just to the left of the Bosch CDI box. Here is a picture. Keep in mind that when you pull the switch out to the second position, both circuits will be "hot".

fred cook · fred cook's Garage

Oops, just noticed that your car is a Carrera and does not have a Bosch CDI. The relay should still be in the same place.

rnln · 04-24-2007, 12:38 PM

I have 2 square silver relays there. I can just pull one at a time to make sure which one.
I don't have the rouded relay as you have.
Do you know what are those fuses, right below, for? I have 3 fuses banks but there are only 2 present. The funny thing is that when I pull off the fuse and measure the voltage, both end of the fuse are 12+v.

fred cook · fred cook's Garage

Box #4 is probably the voltage regulator. The box marked #2 is probably the rear defrost relay. I would leave them alone and find the wires where they go into the rear window seal. You may have to remove the rear window upholstery to find the wires. Sorry, but I don't know what color they will be. The ground wires will be brown of course.

Good luck!

rnln · 04-24-2007, 07:13 PM

I think I wasted your time. I went out and pull the whole thing off to measure each wire and think my wiring is messed up at that area. All wires (connect to that relay) measures to be no voltage, there is no power sources to there. When the defroster turn on, there are 2 wires turn on with the same color. That relay has 6 prongs. I don't think I should re-wire to use that relay since I don't want to mess think up worse.

I think I will add a normal modern square relay (easier to find) there and reuse the litle wire which getting signal from the defroster switch. The problem is that I can't find power source (only when key are ON possition). All the red wires there are having 12+ even when the ign key is not in, so look like they are connect directly from the battery. Even the 3 fuses below there are having 12v when key is off. Is this normal to older 911 or something is wrong with my car?

rnln · 05-03-2007, 12:54 PM

Fred,
Thanks for staying with me through. Finally, my fan is is in place and is operating now. The only one more step I need to do this is to swap the ignition (ON) wire to the rear defroster button instead of the batery wire. Right now, you can turn it on any time, even when the key is off the car.
Thanks.