7730 Pin E8 Question for Fan 1 Relay

**EagleMark** · 09-22-2013

I'm just going through 7727/30 $8D $A1 and a 8708 working on this harness and seems both have 2 fans. Fan 2 seems to always come on with AC, but not fan 1...

The part showing that these signals are 12 volt is very confusing when trying to wire relays! When in fact it is ground that completes the circuit to turn on...

**84Elky** · 09-22-2013

Originally Posted by EagleMark

I'm just going through 7727/30 $8D $A1 and a 8708 working on this harness and seems both have 2 fans. Fan 2 seems to always come on with AC, but not fan 1...

The part showing that these signals are 12 volt is very confusing when trying to wire relays! When in fact it is ground that completes the circuit to turn on...

Yep, ground it is. But strangely enough, Fan 1 comes on with A/C request and also in Diag mode with A & B shorted, and can't find that Fan 1 is set on in that area of the code like Fan 2. Just something else to look at when Ihave time.

**EagleMark** · 09-22-2013

In the 8708 I found a Sense/Fan request wire as well! Is there any sense wire in 7730 $8D ? Shows same pin D21.

**84Elky** · 09-23-2013

Originally Posted by EagleMark

In the 8708 I found a Sense/Fan request wire as well! Is there any sense wire in 7730 $8D ? Shows same pin D21.

Not sure what you mean by "Sense/Fan request wire", but it appears Pin D21 on the '8708 serves the same purpose as Pin D12 (note "12") on the '7730. D21 appears to go to a A/C Pressure transducer on the 8708. On the '7730, D12 is designed to be connected to a grounded (normally closed) High Pressure Switch. Most of these switches open at about 430# pressure and that is a signal to turn on Fan 2 (I believe by supplying ground to Pin F8).