OK, so I modified the cooling fan enable and disable temperatures to correspond with the 180 degree t-stat I have.

I was refilling the coolant after replacing a faulty T-stat and noticed when the engine was really hot (240 degrees F) only one fan was on. So I rechecked my on and off temperatures and they were OK. So I thought maybe a faulty relay, bad wiring, or bad fan motor..... A quick check to see if the relay was working (and also the wiring) is to place the vehicle computer into diagnostic mode (ie place a paper clip into A and B terminals of the ALDL)... and.... both fans came on??!!??!!??

So, knowing the fans were able to be commanded on via diagnostic mode, that means that the ECM can command both fans on. So I re-rechecked my fan temp settings and they are primary on at 190 F and off at 180 F, secondary on at 200F and off at 190 F.

1. .....so what am I missing? Why weren't both fans on at 240F??

2. Via diagnostic mode, the the fans are commanded on via the ECM, correct??

3. could the ECM be bad? if so, why would diagnostic mode turn both fans on ... if the ECM controls them?



I am running $8D in a 7730 ECM (1990 Corvette)

 

The secondary fan that's controlled by the 7730 ECM is very rare. Your secondary fan is controlled by a temp switch located in the passanger side head. Here is an old post:
https://www.thirdgen.org/forums/diy-...nd+fan+control
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Here's how to control the secondary fan with the 7730 ECM.
https://www.thirdgen.org/forums/dfi-...n-control.html
https://www.thirdgen.org/forums/diy-...ry+cooling+fan

 

OK, you got me, I am using a 7727 ECM. Even if I was not, does the 730 ECM actuate both cooling fas when in diagnostics mode? I do not see how it could if it is not wired to the ECM. If so, how?

Knowing that my cooling fans both come on when the ECM goes into diagnostic mode indicates to me that the ECM can control both fans. It is the secondary fan that is not coming on when in normal driving mode......

 

here is the fan wiring diagram for my 1990 Corvette

 

if you swapped to a LS1, the PCM controls both fans with both temp and speed variables.

 

Why would you think I swapped to an LS1?

 

That I'm not sure about. I would guess that it's just a safety precaution that enables both fans through the relays. I think that the 7727 has the ability to control the secondary fan, just like the 7730, however, I'm not 100%.

 

From my notes on this:
Fan 2 request
727 730
"D21" "D12" pin

Fan 2 output
727 730
"A3" "F8"

A/C request switch
727 730
"B15" "D3"

Turned on/off by 4004, bit 1 (Port 41)

 

Could it be that your CTS is faulty and telling your ECM the engine is cooler than it really is. Anyway just a thought. I have my 91 730 wired like the vette and it works fine. Is there a min speed enabled or just the max speed disabled?

HTH,