More Fan 2 Wiring Problems and '7730 Pin F8

Really need some help from the problem solvers and electronic brain trusts in this forum. Have just fried another ECM and have a severe brain cramp trying to solve the problem with my Fan 2 wiring. Surely I’m overlooking something or have done something stupid. Post is a bit long but necessary to include all the details.

The attachment “My Wiring.jpg” is a diagram of how my fans are wired. If Fan 2 is enabled via the calibration turn-on temperature, the voltage on Pin F8 is all over the map, oscillating from 13+v to 0 and the relay is clicking 3-4 times a second (initially posted here: http://www.gearhead-efi.com/Fuel-Inj...in-F8-Question). As noted in the above post, this situation is apparently the result of a fried quad driver which I now know has to be caused by the existing wiring setup (more below).

So decided to approach the problem analytically looking at each piece of the puzzle. Would really appreciate any ideas as to what the problem could be or what I’m missing, leaving out or needs to be tested.

Resolved that the problem can only arise from the following:
- Bad relay
- Faulty F8 wire from relay to ECM
- Bad fan
- Faulty ground to fan
- Faulty positive wiring to fan
- Faulty positive wiring to the relay
- Relays wired wrong
Any other checks to be made are welcomed, please!

Addressing each one:
- Bad relay (No) – Have tried multiple relays and all the same. All test OK = 85-86 continuity of 60-90 ohms and 30-87 OK with 12v across 85-86
- Faulty F8 wire from relay to ECM (No) – Unplugged wire from ECM harness and grounded it. Fan 2 on OK. Continuity of wire from ECM to connection at relay OK, so the wire is good (and because the fan runs, the relay wiring would appear to be OK)
- Bad fan (No) – Runs properly when F8 wire is removed from ECM harness and grounded or when AC turned on (see attached diagram)
- Faulty ground to Fan (No) – Continuity OK and Fan runs as it should when grounded and when AC turned on (see attached diagram)
- Faulty positive wiring to Fan (No) – All tests Ok & fan runs
- Faulty positive wiring to the relay (No) - All tests OK and fan runs
- Relays wired wrong - Appear to be in accordance with GM wiring and Fan runs when F8 pulled from ECM harness and grounded (see Attachment “GM Fan & AC Wiring.jpg”). Also, Fan 1 relay wired in same manner as Fan 2 relay and no problem with Fan 1 at all.

With all that, now one more detail. Thought I had fried my first ECM because +12v was passed directly to Pin F8 for 4-5 seconds due to an incorrect relay wiring during some testing. But now don’t believe that to be the case because of frying a 2^nd ECM without doing anything. Iinstalled a 2^nd ECM and loaded up my test Bin that turns Fan 2 on at 65*F (it’s 85*F ambient here). Fan 2 immediately on at startup and see the oscillating voltage when probing and grounding F8 just outside the ECM. Grrr!

So pull F8 wire from ECM harness and installed a stub wire for testing. Start engine and have pulsing on the stub F8 wire with 2^nd ECM. Really pissed now because 2^nd ECM has to be fried – F8 stub wire is not hooked to anything.

So install a 3^rd ECM with the F8 stub wire and start engine. F8 appears to performs as it should with engine running -- only a few mV when grounded and perfect continuity.

So now have 2 fried ECMs and am clueless as to what the issue is. Based on the above tests and with the use of the F8 stub wire, the problem has to be with the other 3 wires on the Fan 2 relay, the relay itself, or the overall wiring. But then how does the Fan run properly when F8 is grounded??? I’ve traced and probed every wire multiple times and believe everything to be as shown in the attached wiring diagram.

Any and all help greatly appreciated!!!!! Thanks

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.

Quote:

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Originally Posted by Six_Shooter

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.

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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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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!

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.