electrical querry

dave egan · 11-25-2008, 01:35 PM

Hey , guys , I'm retrofitting a ''maual on/off switch'' , in conjunction with the OEM automatically controlled ''front passenger-side-oil-cooler-fan-motor''.switch. The model = 1988 930 coupe. The ''circuit'' claims 25amp, all I can find in 12V electrical switches are 20amp. I am simply going to ''cut-in'' inside of front trunk where wires off fan motor come in to car. I will be ''cutting-in'' to a ground wire , thus the retrofitted switch in cabin that I am adding will be a ''ground two pole'' switch , simply delivering a ''ground'' pole to the constantly ''hot '' fan motor, and allowing it to energize at the flick ,of the new switch. At least that is how I am reading the electrical diagram off a Bently manual for a 1988' 911 coupe. Now that I will be ''switching" a ground wire,

in this situation, Do I still need a "25 amp" switch to comply with the 25amp circuit? or would a 20amp switch suffice , since its a "ground wire" and not a ''switched hot wire"? thanks ; dave

Porsche_monkey · 11-25-2008, 03:14 PM

The 20 amp is likely fine. Switches are typically rated for breaking current, not for the current carrying capacity. Additionally, you do not likely pull 25 Amps.

gshase · 11-25-2008, 04:35 PM

I just looked up and most 12 volt cooling fans moving 200-300 cfm do not draw near 25 amps, more like 8 to 12 amps. I would test the fan with a 50% air flow restriction and see how many amps it pulls. Watch your fingers.

dave egan · 11-26-2008, 05:02 PM

THANKS fellas'!!!!................will do...

old man neri · 11-26-2008, 05:20 PM

Electric motors usually suck back a lot more on start up.

You could always just use a relay and then get whatever switch you want in whatever style. It will also save you from having to run heavy guage wire all the way to the switch and back.

rick-l · 05:33 PM

Does the turbo have a oil cooler fan relay? If so you could ground pin 85 in parallel and not worry about it (<1 amp).

EDIT: Ooops... I read your post again and it sounds like that is what you are proposing i.e. energizing the relay coil.

rick-l · 11-26-2008, 05:31 PM

Quote:

Originally Posted by old man neri

Electric motors usually suck back a lot more on start up.

Permanent magnet DC motors are usually big inductors so interrupting the current is usually the hard part on the switch.

autobonrun · autobonrun's Garage

A few comments.

It's ok to switch the negative on a low voltage DC circuit but don't use that method on 110vac.

The current you will see is the same on the hot side as the ground side. It is circuit current.

The current will be limited by the fuse in the circuit regardless of what the full load rating is of the fan. Check your fuse, if it is 20 amp, your switch is a match.

If uncertain, turn the car off then use the switch. I assume the fan is off when the car is off so at that point the current is zero.