Controlling fans

I have a fan problem, one fan is always running. It happens just after start up.
PCM is set to turn on Fan 1 at 185 degrees f, today I checked coolant temp using OBDII scanner and it was 178 and a fan was running.

Fan 1 is also supposed to start at 32 dgrees f when AC is on.

AC request line to PCM is disconnected, seems the PCM in AC mode? Why? How?

Porsche AC system is isolated from the PCM, GM compressor is connected to PCM, AC system is not charged yet.

Are you running the fans off of the Porsche relay or an independent three relay control? If it was the Porsche relay, it may be fried. Three relay could be wired wrong. The PCM could have been programmed wrong.

I'm using SSP's set up, two fan relays, stock 944 fans isolated from the Porsche wiring and relays.
PCM controls two relays Fan 1, Fan 2 both wired to thier respective fans. Fan 1 is on all the time.
It seems like the PCM is in AC mode and turning fan 1 on all the time.

Porsche AC system totally isolated from PCM.

fliermike45 wrote:I'm using SSP's set up, two fan relays, stock 944 fans isolated from the Porsche wiring and relays.

I thought the GM ecu's need three relays? I got this diagram from one of 944-LT1's post's... I wired mine up like his diagram and they work....
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That's another way to skin a cat, if you look at the PCM schematic there are two fan trigger outputs.
Mine are suposedly wired one to each relay (and fan) and will react to the coolant temperature as per the attached schedule.
The three relay system puts two fans in series for fan trigger 1 (both run at lower speed) and both fans run fast when commanded by Fan trigger 2

Again looking at the fan schedule (in the PCM), Fan 1 will run at 32 degrees f if AC is commanded.
I believe my PCM may be in an AC mode.

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I think A/C mode is both fans on all the time when using a three relay system. Look at post #35 in this link. [You must be registered and logged in to see this link.]
It may be the way the two relay system is wired.

I agree that the three relay system can be used, it has the benefit of running two fans in series at a lower speed. That is basically Fan 1 trigger.
Fan 2 trigger runs both fans in paralell at high speed. It needs three relays to achieve this switching.
Neither fan should run below the trigger threshold unless AC is commanded.

My system uses Fan 1 trigger to run one fan and Fan 2 trigger to run the second fan. Independant of each other. (But both via thier own relay)

The table I published in my last post are the actual fan trigger temps that are programmed in MY PCM, it's part of a screen shot from my PCM tuner.

I still believe that the PCM is someway in AC mode that commands Fan 1 to run above 32 degrees f
If I pull the relay out the fan stops. I checked the coolant temp with my OBDII scanner and it had not reached the Fan 1 (Coolant temperature) trigger level.

I really don't need a tutorial on how to wire my fans guys, I need to know how the PCM appears to be in AC mode when my AC request line is disconnected.

Maybe you should send the ecu back and get them program out the ac request then if that's what you think it is.... Is there no way of seeing that with your scanner? Did you try a new relay?

Has anyone experienced the two PCM triggers that drive the cooling fans becoming locked to ground?

When my engine runs from cold, both triggers go to ground. With a cold engine they should be open and only go to ground when the PCM reaches the cooling fan turn on temperature(s) in the schedule.

PCM has had the security, rear O2, emissions removed.

Any PCM experts???

Is it throwing any codes? I think when they throw codes they trigger the fans as a safety precaution.

No codes are showing, but I believe that it's possible to disable codes.
A comment on LS1 tech is that the turn on temps are maybe set really low.

Still searching for a solution.
I have noticed that with the engine cold, fans start running when engine starts, after approx 35 seconds the fans shut of momentarily and the engine idle speed reduces (it may be missing).
The fans re-start and engine idle increases. This happens in the blink of an eye. It repeats at an interval of about 35 seconds.
My tuner tells me that the AC control is removed from the PCM. Really scratching my head on this one.
OBDII scanner indicates that the coolant temp sensor is working fine, I see the temp rising as engine warms up.

Can anyone recommend a source of reading to learn how the PCM software interfaces with the hardware?

After much head scratching I narrowed the problem down to the fact that the fans were being forced on. Only Temp sensor, VSS and AC Pressure switch could do this.
As I don't have an AC pressure switch sensor, temp sensor is new and vehicle is stationery it must be the AC Pressure sensor who's signal goes to Red Pin 14. So in desperation I cut this wire and Bingo....fans start up at correct temperature.
Not sure why my modified harness is doing this but there seems to be a varying DC voltage between 0.9 and 3 volts on this line with the engine running.
Eureka!

Yay!