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General question about A/c and cooling fans

Boomer_106 on Wed May 27, 2009 3:07 PM User is offline

Year: 88
Make: Chevy
Model: Iroc
Engine Size: 5.7

I have the service manual for 88 camaros. The book says this about cooling fans. My questions are at the end.

Circuit Operation..

V6 Vin S, V8 Vin F, and V8 Vin 8

The coolant fan is controlled by the ECM. In the V6 Vin S, the coolant fan is also controlled by the coolant fan switch. When the ECM grounds the 335 circuit, the coolant fan relay is energized and battery voltage is applied to the coolant fan. If the ECM fails(V6 Vin S), the coolant fan switch will ground the 335 circuit and energize the coolant fan relay. The ECM will ground the coolant fan relay when the coolant temperature sensor indicates the coolant temperature is greater than 106*C (222* F) or when the A/C head pressure is greater than 233 psi and the vehicle speed is less than 40 mph.

When it says the relay is grounded when A/C head pressure is greater than 233 psi, does this mean the fan won't come on if the system is badly undercharged or empty? Does the amount of refrigerant have anything to do with A/C head pressure? Finally, how would you test A/C head pressure? Is that something a DIY should even be concerned about?

I just want to understand more about how these things work.

GM Tech on Wed May 27, 2009 3:52 PM User is offline

Head pressure is the same as discharge pressure- you use an a/c gage set- that monitors head/discharge pressure and suction pressure.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Boomer_106 on Wed May 27, 2009 4:31 PM User is offline

Originally posted by: GM Tech
Head pressure is the same as discharge pressure- you use an a/c gage set- that monitors head/discharge pressure and suction pressure.

Ok so, head pressure is the high side pressure. Got it. Do you have any answers to my other questions?

Suncr on Wed May 27, 2009 8:34 PM User is offline

The ECM for your Camaro drives or grounds the Electric Coolant Fan based on one of 2 signals: the Coolant Temp Sensor and/or the Fan Switch.

The Coolant Temp Sensor is a thermistor screwed into a coolant passageway, usually in the Intake, and it's resistance decreases as the Coolant warms up. It's getting a 5 volt reference from the ECM and as the resistance drops, more of that voltage reaches ground. The ECM then uses the voltage drop to calculate the Coolant Temp and when it hits 220 degrees, it drives the Fan Relay. This Sensor signal is also used for fuel delivery - the colder the temp, the longer the injectors are held open.

A/c operation has little to do with coolant temp (until it mucks up), so a normally closed, Pressure Sensitive Switch is screwed into the High Side. The ECM sends it battery voltage and when the switch is closed, the ECM senses 0 volts. When it's open, the ECM sees battery voltage and with that, it drives/grounds the fan relay. Mid to late '80's GM's usually open at around 230psi and reclose somewhere around 190 psi. In the overall scheme of things, GM has designed the system to maintain average head pressures (or the high side) at 200 psi and the fan (anytime it's less than 85 or so at the condenser), should be cycling on and off. Diagnosing the ECM is fairly simple: disconnect the harness at the Switch to simulate the open and if the ECM is working, the fan will come on.

Diagnosing the switch takes a Manifold Gage Set and since system pressures will vary based on ambient air temps, a Pressure/Temp chart for the gas you're using is helpful. At lower outside air temps - below 50 degrees, the high side may not reach the threshold for the switch to open (without some engine heat) and the fan may be off for some time or not come on until the Coolant Temp Sensor has signaled the threshold for fan operation.

Yes, a low charge can effect the High Side - but it also effects the Low where another Pressure Sensitive Switch cuts power to the Compressor Clutch. It's threshold is 25 psi and with a low charge, it may be/should be opening and closing rapidly causing the compressor to cycle on and off. With the compressor off, it's not going to make enough pressure to open the fan switch. Proper diagnosis requires a manifold gage set.

If your Camaro is overheating, you'll need to scan it so that you can see the Coolant Temp Sensor signal. Generally, a clean and fully charged a/c system will reduce non a/c Coolant Temps by 8 to 10 degrees. Once the compressor engages, pressure is built instantaneously, so the fan is coming on sooner and staying on longer with those models that use an electric cooling fan controlled by the ECM. It also needs a functioning thermostat to maintain proper coolant temps.

Boomer_106 on Thu May 28, 2009 10:31 PM User is offline

Thanks suncr. My primary fan works like it should. It comes on at around 230* Right now the secondary fan isn't coming on. I replaced the heavy duty coolant fan switch with no change. I swapped the relays of the primary and secondary with no change. I am low on charge as my compressor cycles on and off constantly. Like on for a sec or two then off for like 3 seconds. I took pressure readings with my gauge set. It's all in this thread.

Suncr on Fri May 29, 2009 12:46 PM User is offline

The secondary fan, assuming it's controlled by a temperature switch threaded into one of the Heads, is pretty much eye candy. With both fans cranking, the a/c on max, the CS130 alternator is easily overwhelmed at idle and when the volts start to drop, the fans slow down and the coolant and vent temps go up. Those temp switches close/ground at around 228 degrees and it really isn't needed if everything is clean. You could start by simply grounding the wire at the switch which should make the fan run. If not, and since you've switched relays, there's either an open on the Power side/Ground - check fusible links, or the motor is shot - jumper it directly to the battery. You can also block air flow and with the a/c off, it should reach the threshold for the switch to ground the relay. I would use a scanner to observe the Coolant Temp Sensor as it tends to be a lot more accurate than what your Dash shows. Otherwise, a basic cooling system test is to observe the Main Fan (a/c off), the temperature it comes on at, and for a clean system, it should drop Coolant Temps before cycling off. If it doesn't, something is usually plugged up.

A/c relies on the Main Fan controlled by the ECM via the Pressure Sensitive Switch I described and with a clean cooling system (and functional a/c) the main fan is all it needs. Looking at your other post, it's low, but I would also suspect a plugged up orifice as the low/high doesn't seem to be equalizing after shutdown or when you've checked it cold. If it's all original, I'd at least pull the orifice and swap out the Accumulator. I'd also pull the Resistor Pod on the Blower Housing and stick your detector next to the Coil.

Boomer_106 on Fri May 29, 2009 11:05 PM User is offline

I did pull the plug off that switch and ground it. The fan runs fine. It also runs fine if I jumper the leads on the relay going to it. I replaced that coolant switch after the fan ran fine because the service manual seemed to indicate that if you ground the wire and the fan runs, replace the coolant switch. I did and it made no difference. $40 spent for nothing. I think it might be like you say, it's just not needed. The main fan comes on around the temp it should and cools the car down. Now it's not real hot here yet so, we'll see when it gets that way.

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