84Elky

This is somewhat complicated so please bear with me. There has been much discussion here about using dual fans with the 1227730 ECM. No question that Fan 2 can be controlled by the ECM using Pin F8. Done that. But would appreciate clarification on another item that has also been discussed, but seems contradictory -- and that is:The last statement seems to contradict the wiring diagram for Pin D12 attached below. If reading that diagram correctly, it appears that Fan 2 can only be turned on via Pin D12 if an installed High Pressure Switch or Temperature switch is tripped and thus would not be activated just because A/C was requested. Because I'm an electrical idiot, maybe I'm reading the diagram incorrectly. If not, then some questions:

• What exactly does Pin D12 do (beyond the description on the ECM pinout)?
• How is it activated?
• When activated, does it receive or send a signal, and is that signal positive or negative?

Also anyone familiar with the code know what actually occurs? I've looked at JP's AUJP hack but can't figure it out.

RBob

iTrader: (1)

The fan code in $8D can be confusing. On the f-body calibrations pin D8 is pulled low when the secondary fan turns on. This occurs when either the A/C pressure is high (attached schematic of that switch is incorrect), or the head temperature switch closes.

All this does in the ECM is to tell it to turn on the primary fan. Maybe an IAC bump too, can't recall. Pin D12 is an input to the ECM, basically, secondary fan status.

Placing the secondary fan under ECM control by tying F8 to D12 is legit. The ECM will still turn on the primary when the A/C pressure increases. Along with controlling the secondary fan according to the calibration temperatures.

Note that the primary fan typically has lower turn on points for the fan temperature. So when the secondary fan comes on due to coolant temperature, the primary is already running.

RBob.

84Elky

Thanks RBob – I omitted something that’s likely important. My car is a G-body (El Camino) with a ‘7730 (added with addition of TPI). The car does not have a wire to Pin D12 because it doesn’t have a Fan A/C Pressure Switch or a Heavy Duty Coolant Fan Switch, both of which appear to be tied to D12 in the Service Manuals with a ‘7730.

So, given that and taking what you said:

it would seem that in my case, putting a wired pin in D12 and jumping it to F8 would do no good to turn on Fan 2 with A/C request since D12 will never get any input (no pressure, no temp). Is that right?

Right now, Fan 2 works as it should tied to F8 with ECM-set temps. But what I’m trying to add is for Fan 2 to operate both with ECM set temps or when AC is on (not on with high pressure or temp), and naturally turning off with AC off.

So with all that, I guess the real question is – what type of input is D12 expecting, +12v, +5v or ground? Once that is known, maybe I’ll be able to figure something out.

Sorry for all the detail, but I don’t have an ECM bench and I don’t want to fry anything.

Thanks in advance for your perspective.

RBob

iTrader: (1)

I suggest that you use the switches that GM used on the either the y-body or f-body. Otherwise how would you expect to get the fans working as they should be?

At a minimum, the ECM needs to know that the A/C is active. And, secondarily, that the engine CTS is high enough to require radiator cooling.

RBob.

utilityguy2

iTrader: (1)

Thanks RBob for that clear explanation. I've often wondered if D12 was an input. Now it all makes sense. Without knowing the code, the hardware is sometimes hard to figure out.