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

[Franke]

The primary fan turns on when:
AC request signal indicated yes, or Engine speed has been < 3500 rpm for 12 secs and Engine oil temp > 226 degree F, or Engine speed has been > 3500 rpm for 12 secs and engine oil temp is > 217 degrees F, or AC request indicated yes and vehicle speed < 12 mph and coolant temp > 226 degrees or AC request not indicated and vehicle speed >12 mph and coolant temp > 234 Degrees. So it would appear to me that the request for fans on AC are independent of the CTS or can be a combination of vehicle speed, oil temp, coolant temp and AC request. The secondary fan will not turn on if the primary doesn't run for at least 2 seconds. I would check the Coolant temp sensor first. Do you have a scan tool?

[c4koh]

Originally Posted by Franke

The primary fan turns on when:
AC request signal indicated yes, or Engine speed has been < 3500 rpm for 12 secs and Engine oil temp > 226 degree F, or Engine speed has been > 3500 rpm for 12 secs and engine oil temp is > 217 degrees F, or AC request indicated yes and vehicle speed < 12 mph and coolant temp > 226 degrees or AC request not indicated and vehicle speed >12 mph and coolant temp > 234 Degrees. So it would appear to me that the request for fans on AC are independent of the CTS or can be a combination of vehicle speed, oil temp, coolant temp and AC request. The secondary fan will not turn on if the primary doesn't run for at least 2 seconds. I would check the Coolant temp sensor first. Do you have a scan tool?

Interesting to read that.
I had a situation a few days ago where the engine was hot (very hot) and both the analog gauge and the digital gauge read hot (260), albeit for only a few minutes. Neither primary nor secondary fan would come on when I was stopped. Car was at idle.

In the various scenarios above, I didn't see what would happen if coolant temp > 234, AC request off, vehicle speed <12 mph (in my case, stopped). Is it only if oil temp >226F will the fans go on at vehicle stopped and AC is off?

In any case, what I did learn is that in case I see this again, switching the AC on will put the fans on if coolant is hot, and car is slow (<12 mph).

Learning all the time

Steve

[Franke]

I would check the CTS for proper resistance since it is a thermistor. It could go out of calibration. Interestingly the chart in the FSM indicates the max temp for the CTS at 212 degrees F and the low temp at -40 degrees F. Resistance at 212 degrees is 177 ohms.

[XfireZ51]

If the fans are coming on w A/C request then it's probably not the relays or the ECM. It's why maintaining good grounds is important to proper engine controls. ECM turns things on and off by grounding circuits.

[Jim Nolan]

I'll add on to the importance of the grounds. When I went to Gordon's school many years ago he placed a heavy emphasis on grounds by grinding them into the frame and not repacing the star washer with flat ones.

Jim

[MrsMcNastys]

We have an Actron CP9145 with an OBDII connector. Would that support with the correct connector?

Ditto on the grounding, we redundantly ground all builds especially our LS1 swaps.

Does the FSM list a procedure for checking the CTS? We only have the supplement.

Ed

[LGAFF]

First statement in FSM ....if the engine is actually overheating and the gage indicates overheating, but the cooling fan is not coming on the coolant temp sensor has fell out of calibration and should be replaced.

[LGAFF]

[MrsMcNastys]

Thanks LGAFF,

Ordered the necessities from Jerry on Sunday and they arrived today so we are pulling the plenum!

Ed