More Fan 2 Wiring Problems and '7730 Pin F8

[84Elky]

If I'm not mistaken $8D (I assume you're using $8D because of the diagram you posted), does not actually use the "fan 2" output from the ECM, it is triggered from a head mounted sender, if memory serves.

If you look in the diagram you posted there is no fan 2 trigger to the fan 2 relay.

Six_Shooter, thanks for replying. Yes -- it's $8D & you're absolutely right about Fan 2 not being temperature controlled on a '7730 (at least not for AUJP). Fan 2 is tied to a switch that only turns it on under high AC pressure. Further, the pinout diagrams for the 90-92 '7730 shows F8 as "not used". But AUJP contains all the code to run 2 fans under temp control, and Pin F8 will cause that to happen just like Fan 1 on E8. I've been doing that successfully now for about a year now.

But where my problems arose is when I violated the fundamental rule for all cars: "If it ain't broke, don't fix it!". What I wanted to do is have Fan 2 be temperature controlled and also come on with AC. And that's when the problems started. But woke up at 3am this morning with an idea. Will hopefully be able to test it this afternoon or tomorrow. Would appreciate feedback on it below.

Currently, looking at the wiring diagram, Pin C9 is apparently grounded anytime AC is off because relay coil pins 85 & 86 are normally closed. Then when Fan 2 is turned on via temperature (not with AC) F8 is grounded and C9 remains grounded. Could it be that C9 is not happy with seeing ground when F8 is grounded by the ECM (or maybe not happy anytime)? Sure would appreciate comments from anyone who understands the innerworkings of a '7730! Further to that thinking, it is clear looking at the GM wiring diagram that Pin C9 is designed to only receive +12v indicating that the AC clutch is on.

Thoughts, comments, direction??? I need it all. Thanks
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[84Elky]

Weellllllllll -- Stupidy is a great thing to observe, except when you are the one being stupid!

Whoda thought it. The wiring diagram in this post is 100% correct and works like a charm --- when the proper calibration parameters are provided to the ECM. After days of thinking I had fried 2 ECMs, I just learned that a FAN DISABLE TEMPERATURE in the '7730 ($8D mask) MUST BE LESS THAN THE ENABLE TEMPERATURE. If not, when the coolant temperature is between the enable temperature and the disable temperature, a '7730 fan will rapidly cycle on and immediately off. This has something to do with the code logic and the Output Fan Timer value, but have not had time to fully find the cause. Just glad to have found the issue. So when testing fans, don't just set a low enable temp and leave a high disable temp in place.

This situation cannot be detected without watching Data Stream fan bits, or if you don't have a fan-on indicator light installed, because the fan will appear to be running properly. But the relay will be repidly clicking and the fan-on idicator light will be flashing.

Also, there are posts on other forums stating that Fan 1 must be set to higher temps than Fan 2. Maybe something has been overlooked in my testing (very possible), but have not found that to be true. The fans appear to be truly independent both in actual operation and tracing code..

Hope this helps someone who is considering such testing. The good news after all this is that I have 2 spare ECMs!

[RobertISaar]

fun stuff. fan 1 and fan 2 can be run in either method of one being on before the other. simpler to do 1 on before 2 though, especially if the code only commands one of them on for specific events.