Radiator Fans only working with A/C on

[MrsMcNastys]

Hi Guys,

You guys have been a great help on maintaining 2 low mile ZR-1's with fixing the secondaries, injectors, thermostat etc.

Need some help on this one - 1990 ZR-1, 15,000 mile original car in the shop with customer complaint of overheating at idle/traffic.

Thermostat appears to be working correctly, with A/C off reached operating temp and no fans activate, temp hovered at mid gauge range at idle then slowly crept up higher.

Turn on A/C - both fans kick on, quickly brings temp down below half way, cycles on and off - can idle for hours no problem.

Turn A/C off - no fans, let the temp climb quite a bit one time to see if fan activation was delayed - still nothing.

Have verified both fans operating with A/C, relays good etc.

I am thinking ECM temp sensor? But, this same sensor is telling the fans to cycle on/off with A/C on also?

Found the ECM pins here:

http://www.zr1.net/forum/showthread.php?t=12004&page=2

Identified:

C2 A/C Request

C6 Primary Fan

C8 Fan Request

D11 Secondary Fan

So C6 and D11 would be the ECM grounds to activate relays?

C8 would be to the ECM temp Sensor, does the A/C request just activate the ECM grounds based on compressor pressure? It would seem to use the temp sensor to cycle.

In any case I cannot identify the other pin for the ECM temp sensor as it is a two-prong item. Once identified can I ohm it out before I pull the plenum?

We are a old-school speed shop and primarily work on pre-72 muscle cars. The same customer has a 375 HP 69 Nova SS he bought new and he was so happy with our work on that he brings us everything now.

Any suggestions would be greatly appreciated.

Ed