On Mar 26, 11:53*am, "tm" wrote:
"Bimmer Owner" wrote in message

...

On Tue, 26 Mar 2013 09:10:56 -0400, Scott Dorsey wrote:

BUT, if you want to replace the device with a retrofit one, you need to
know what that mysterious input signal is.

All I know of the HVAC input signal is that it's a 2.0 to 7.8 VDC
signal from the HVAC controller, presumably to correspond to the
various levels of the fan blower motor speed.

But is that a clean DC level or a pulse width modulated signal? If it was
measured with a multimeter, you won't know.

The other possibility is that it's a digital command signal of
some kind. I don't know how they typically do that, but if a
0 is 0 volts and a 1 is 12V, for example, looking at it with a
volt meter, you would see a range like that given.
They do have that 16 pin chip there, doing something.
It might receive the command and then output the appropriate
PWM for rest of the circuit.




Has anyone probed around the module with a scope? Is that possible?

That and the question about the TO-220 devices. I think it would be possible
to make a better replacement if those questions were answered.

tm