a/c switch question

[dw1]

On my '87 3.2, I'm looking into replacing the resistance-based a/c fan speed switching with a PWM circuit and I see that the this switch also includes contacts that "turn on" the a/c.

Since the PWM adaptation would require either the OEM switch (undesirable) or a substitute (preferred) to turn on the a/c, does anyone know the current rating for "a/c turn on" part of this switch (switch part no. 91161324300)?

[david.avrahami@]

It is not an answer to your question....but...you might want to purchase a VARIABLE speed kit from Charlie of Griffith a/c for Porsche.
it includes the ac start , instructions and you will love it

[dw1]

I have great respect for Charlie Griffith - he (and his products) have helped me quite a bit.

I seriously considered his variable speed controller, but it is not currently available and $300 is a bit pricey, especially compared to what PWM controllers go for these days, so after reading the Pelican tech article on this subject, I am taking a good look at engineering my own approach - thus my question on the current rating of the "a/c on-off" portion of the switch.

[kuehl]

Look at the schematic.

If you want one switch that does it all, besides PWM function, it needs to pass through
the thermostat enough amperage to power the ac comp clutch and
trip the front condenser blower relay (which is nominal).
AC comp clutch coils are 3 ohms nominal.
Plus you have the voltage drop between the thermostat switch
and the clutch coil.

[wazzz]

Yes clutch draws about 3.5 or 4 amps. That's about what I have measured under 13 V. But you also need to pass the evap blower motor current. This one is this highest of all currents running through this switch. I would think about 10 A on highest speed. Probably more if you get the hurricane. On the stock Bosch motor, I have measured as much as 15 amps, but that was with the blower out of its box. The wheel was free to run and that is when it draws the most amps. One would think it is the other way around.

[junkman300SD]

Per a friend of mine that did electrical design in the aerospace industry: fuses always are selected to protect the wiring.

Make sure the wires will handle whatever current you end up with.

[kuehl]

Originally Posted by wazzz

But you also need to pass the evap blower motor current.

You not passing the evap motor present current demand (12-18 amps) through a switch for PMW. PMW uses less current. And, it all depends on whether the PMW circuit has a separate relay.

[wazzz]

Originally Posted by kuehl

You not passing the evap motor present current demand (12-18 amps) through a switch for PMW. PMW uses less current. And, it all depends on whether the PMW circuit has a separate relay.

That's correct. I was referring to original design and schematic for a 1987 3.2.

In the PWM design the switch is deleted anyway. And no relay is needed with Mosfet drivers.

[kuehl]

Sounds interesting. Post your new PMW system circuit, and do you have a working model for us to try out?