This is a 1983 harness. I was intending to use DPI1 for the AC request. From reading the schematic, it looks like we go from the blower motor to the blower motor resistor to the fan switch which grounds at the switch to complete the circuit. The AC switch has a backlight that is provided voltage from a blue wire with a black stripe (LB). when the AC switch is engaged I should be sending voltage through the (LG) wire. I guess I was expecting to send a 12v signal from the (LG) wire to DPI1 as the AC request. With the little bulb lighting up to indicate AC function.

So here is where things have gone weird. Bench testing this setup before I connect my 750, I was getting 12v from the (YG) wire when the (LB) wire had 12v applied to power the bulb. Bulb did not illuminate as it was grounded through the closing of the contact on the switch. When I engaged the AC switch the voltage dropped or at least the digital multimeter started giving drifting readings.

My intentions were to run AVI1 for the frost sensor and the pressure switch is ran independently as well, although I don't recall that assignment.

I have included a modified schematic that represents what I think I am doing. Not shown is the AC output request which I believe would connect to the (YL) on the relay. Oh and of course (LG) going to the AC input request.

Are there holes in my plan so far? does anyone see a better plan. should I add a diode on the (LB) ?

Thank you for the fresh eyes.

 

It seems like you only need to interrupt the y/l and give it whatever is required to trigger it. Does it trigger when you ground it?

 

currently the Y/G reads as the following. if the ignition is on but the HVAC fan is off I get 12v. If the blower is on I get 12v. when I engage the AC button it drops down to about a millivolt. As I understand I can set the AVI to be "active" under a determined voltage and "inactive" over a determined voltage. With the DPI as I also understand it does not have that level of control as it is a binary input. yes or no. So would that millivolt throw off the DPI?

 

We couldn't get mine to work with a DPI, AVI worked well for the reason you mentioned.

 

I was trying to set ESP AVI to be powered off with 12v but it seemed to be limited to 5v,

 

If you set it to on under 2 volts and off over 3 volts it should work fine.

 

12v would not be over voltage then. Thank you.