In my 69 I have TPI fuel injection (speed density, 7730 ECM) and dual C4 fans. Currently, the ECM controls 1 fan and a high temperature switch in the head controls the other. Currently the ECM has no high-pressure switch so that pin is open and any time there is an a/c signal the primary fan runs continuoiusly. I have purchased a high-pressure switch so that I can send a ground to the ECM except when the high side a/c pressure goes over 300 psi. This way the fan will not run needlessly at speed with the a/c on.

Here is my question. Is there any sort of economical time-delay relay or circuit I could rig up so that if the head pressure goes high I could also turn on the second fan and keep it running 20 seconds or so after the pressure comes down? Is there an Integrated Circuit that has a 15 to 40 second timer that has enough current capacity to trip a relay? Some kind of wiper time delay maybe? I want to keep the second fan so it comes on at high temperature, but would like it to run to keep the head pressure down. Without a timer, I am afraid it will cycle too much. The ECM has some timer built in that does roughly the same thing with the "primary" fan. Appreciate any help.

 

You really don't have to run the fans thru the ecm.
It is always advantageous for the a/c operation to have as much air flow thru the condenser as possible, so having fans on always with the a/c is best.
Not only will it translate into better heat exchange, but also longer a/c parts longevity.

The DCC controller made by a forum member will handle 2 fans plus the a/c function.

 

I agree with noonie, but having said that....the chip you need for the timer is a 555 timer, and circuits to set it up with a 2n2222 transistor are found at most any radio shack.....just read their writeups....

 

I do want max airflow through the condensor, that is why I am trying to do this. At speed there is no reason for the fan(s) to run because there will be sufficient airflow through the radiator and condensor. I bought a Murray 35830 high pressure switch. It opens at 300 psi and closes again at 200. Application was mid-70s Kenworth or Autocar. It has a 3/16" schrader fitting like the '82 high-pressure a/c line I am using. I will use this switch to control the ECM just like a factory C4 setup.

My question really centers on how to run the second fan. I want to keep the "staged" fans so that the ecm controls the primary fan and the temp switch controls the other. That way both fans do not automatically come on to reduce temperature. Currently on the secondary fan, I have a relay that has ignition power to 1 side and the high temp switch provides a ground. I want to find a way to use the signal from the new high pressure switch to also run this fan. I would like to have it run the fan for at least 20 seconds after the pressure comes back down-like the ECM does for the primary fan. If I use the ECM signal for both, then both fans will run for both high pressure and (lower) temperature control, defeating my "staged" temperature reduction thought. So I need to construct a device that grounds (or powers) a relay as long as the ground supply is an open circuit and remains powered for at least 20 seconds after the ground signal is restored.

 

I will investigate that, thanks!

 

I'll go get my 730 ECM print and see what it does.???

 

The ecm should turn off the a/c at WOT too.

A couple of questions.

What expansion device is the a/c using?
What thermostat are you using, is it hot out to the rad or cool in to the waterpump?

Sounds interesting.

 

ftp://ftp.diy-efi.org/pub/gmecm/manu...730v8tpi-4.jpg

and according to my tunerpro program, Primary Fan turn off at 35 and the Secondary fan turns off at 50 mph....I'm not sure where I got that BIN; it's for 1990 L-98 manual, either Camaro or Firebird, suspect Camaro.

Looking at the simplified schematic, it would appear that the Secondary Cooling fan will turn off at 50 mph(in the BIN that I'm looking at), UNLESS A/C System pressure is too high, or Engine Temp is too high.....and then I gather it just wants the fan to run anyway. Kinda makes sense.

If you burn your own chips, Tunerpro also lets you determine how much time you want the ecm to run the fan after going to "OFF".

I believe that's the info you were looking for.

 

I am using an orfice tube a/c system from an 78-82 (Nippendenso compressor from a '90 C4)
I am using a 195 t-stat, it is a standard flow (not an LT1) system.

I have had no cooling problems with the current setup, in fact I don't think I have ever had the secondary fan run even on the hotest days. That is why I want to keep the staged nature of the cooling portion. I just think that the a/c might work even better at low speed with more airflow over the condensor.

 

I will look into this much deeper. I did not realize a 7730 could control 2 fans. I am using VSS so I will have all the inputs now to make it work correctly. If this is true, I have everything I need already. Thanks very much

 

After carefully looking at the diagram, it appears that this is basically how I have it hooked up:
-They show BC9 as an a/c input (input, 12v+). They show it only when the compressor is activated, I have it powered whenever the a/c is on but it should work either way.
-They show GE8 as the primary fan relay trigger (output, ground)
-They show BD12 as fan request signal (input, ground). This is what I thought was only connected to the high pressure switch; n/c, opens at 190psi. They also show it connected to the high temp switch in the head and triggering the secondary fan relay. This tells me that my high pressure switch is too high if it opens at 300 psi. Also if you connect according to the diagram, the secondary fan can cycle as the pressure drops. I can hook it up this way and use the timer Mrvette suggested and I think I can have the best of both worlds.

I do not see how the ECM controls the secondary fan in that diagram. I wish it would. Thanks for the diagram.

 

but the BIN definitely has 4 different ENABLE and DISABLE temperature setpoints.

My suspicion is that GE8 AND BD12 use the engine temperature sensor, monitored by the ECM, to control fan(s) start/stop....that's why I suspect it is the sensor which is monitored by the ECM. The ECM would then complete a ground path and get the fan to start at whatever temperature is in the BIN.

Anyway, it would look like temp sensor shown in diagram is a "separate and independent" sensor, whose only purpose is to start the secondary fan on hi-temperature.

If I could paste an image of the tunerpro screen I would.

Of course, I'm not sure about any of this.

 

You are saying BD12 is an output not an input? From the diagram it looks like BD12 is an input for high coolant temp and high a/c pressure and the same wire also triggers (grounds) the secondary fan relay. I can try it that way and see what it does with a lamp before I change the wiring.

 

when I spliced/wired mine in was that it completed paths to ground to actuate functions.

In this case; it would seem that there are 3 parallel paths to ground to actuate the secondary fan (actually to energize the secondary relay to operate the secondary fan).

These are the 3 parallel paths to ground:
One via the a/c pressure sensor.
Another via the secondary cooling fan temperature sensor.
And a third via the ECM at BD12.

Any one of these completing a path to ground will actuate the secondary relay. Note how the pickup coil in the relay is powered with 12vdc from the ignition at Terminal "D"; the coil now has 12 vdc available and any one of the above functions, completing a path to ground, will cause the coil to energize....when the coil energizes, it moves the contact (directly below the coil) and connects Terminals "E" and "A"....these terminals closing provides a path for the 12 vdc from the Battery (line 440 ORN) to complete a path, via the seondary fan motor, to ground, thereby energizing the secondary fan.

I'm not meaning to insult nor offend anyone, so if the tone of this message sounds innappropriate, please accept my apologies in advance. Also, this is just my understanding of how this system works...I could be all wet!

 

I got a copy of a C4 service manual page and I bought a 91 F-body shop manual on Ebay. I stand corrected, there is a secondary fan output. All the previous diagrams I had seen did not show this. Now this is not as hard as I was making it. Thanks!

 

I build 555 circuits all the time. These will actually drive a relay directly (up to 200 milliamps) without the need for a transistor. With one capacitor and two resistors, you can set the time duration from milliseconds to weeks.

http://en.wikipedia.org/wiki/555_timer_IC