A/C and radiator fans

[secondchance]

When the coolant is in operating temperture (my primary fan turns on at 208 and turns off at 205) and the AC is turned on, secondary fan turns on but the primary turns on at 208 and turns off at 205. Is this normal for 1994?
The reason I am asking is because the compressor is short cycling (about 6 seconds on and 6 seconds off) but I am told it has plenty of 134a.

[VetteMed]

Short cycling typically is either due to low ambient (outside) temps, or insufficient charge. A faulty pressure sensor also could lead to short cycling, I suppose, though usually this leads to a complete failure of the compressor to engage.

When you were told that there was plenty of 134a, did they connect a set of gauges to the system while it was running? What were the pressures doing when the compressor would disengage during the short cycling?

A three degree differential between fan on to fan off sounds too small to me, but I'm not certain if that's within spec or not.

Andrew

[secondchance]

Originally Posted by VetteMed

Short cycling typically is either due to low ambient (outside) temps, or insufficient charge. A faulty pressure sensor also could lead to short cycling, I suppose, though usually this leads to a complete failure of the compressor to engage.

When you were told that there was plenty of 134a, did they connect a set of gauges to the system while it was running? What were the pressures doing when the compressor would disengage during the short cycling?

A three degree differential between fan on to fan off sounds too small to me, but I'm not certain if that's within spec or not.

Andrew

He did have a pressure gauge but I can't remember what the pressure reading was when the compressor turned on. I can find out.

As for thefan on-off temp, this is Marc's chip and his website says 205 on 200 off. However, my coolant temp says 208 when it turns on.

Over the winter I had the system evacuated, replaced the condensor because of cosmetic damage - crunched bottom from an accident before I bought the car, and then had the same shop recharge the system. Could the system have been overcharged due to cold weather when serviced? Would an over charge result in short cycling?

[VetteMed]

If the shop did the procedure properly, then the system was filled by weight. Therefore the ambient temp should not have mattered, and an overcharge wouldn't occur just because it was too cool outside. temperature affects pressure, but not the weight.

I'm honestly a little suspicious that perhaps the system is indeed low on refrigerant, since that's the most common explanation for short cycling. With the gauges connected, the low side would draw down while the compressor is running, until it hits the point where the low pressure switch shuts down the compressor. This pressure is in the 28psi area in an R12 system, and I think it's a little higher (34 or so) in an R134 system. If you see the low side drop to that area, and then the compressor kicks off, until the pressure rises again, allowing the compressor to kick on again, then chances are there's a low charge.

If you don't see that behavior in the low side pressure gauge, also check the high side guage, as if the high side is increasing to the high pressure cutoff (around 400psi or so) and that's the cause for the cycling, then there may be a restriction or obstruction in the system.

If the gauges are fairly constant, and the cycling is not obviously in sync with any of the above behaviors of the gauges, then I guess I'd be more suspicious of a faulty sensor.

[secondchance]

Originally Posted by VetteMed

If the shop did the procedure properly, then the system was filled by weight. Therefore the ambient temp should not have mattered, and an overcharge wouldn't occur just because it was too cool outside. temperature affects pressure, but not the weight.

I'm honestly a little suspicious that perhaps the system is indeed low on refrigerant, since that's the most common explanation for short cycling. With the gauges connected, the low side would draw down while the compressor is running, until it hits the point where the low pressure switch shuts down the compressor. This pressure is in the 28psi area in an R12 system, and I think it's a little higher (34 or so) in an R134 system. If you see the low side drop to that area, and then the compressor kicks off, until the pressure rises again, allowing the compressor to kick on again, then chances are there's a low charge.

If you don't see that behavior in the low side pressure gauge, also check the high side guage, as if the high side is increasing to the high pressure cutoff (around 400psi or so) and that's the cause for the cycling, then there may be a restriction or obstruction in the system.

If the gauges are fairly constant, and the cycling is not obviously in sync with any of the above behaviors of the gauges, then I guess I'd be more suspicious of a faulty sensor.

Thank you for such a detailed explanation. I will take the Z back and have a further discussion. He did mention it was a high pressure that was shutting down the system and thought that was caused by only one fan running (secondary fan) that was causing a high pressure.

[VetteMed]

Originally Posted by secondchance

Thank you for such a detailed explanation. I will take the Z back and have a further discussion. He did mention it was a high pressure that was shutting down the system and thought that was caused by only one fan running (secondary fan) that was causing a high pressure.

Yes, if the condenser is not receiving adequate airflow, pressures will rise, and the system will shut down... again, this will occur at around 400psi. Poor cooling will be noted well prior to the high side reaching 400psi.

If you spray water on the condenser with a garden hose (through the area under the nose where the cooling stack receives its air), while the system is running, this will allow the condenser to shed all the heat it needs to, so you can see if the system is still cycling.

[secondchance]

Originally Posted by VetteMed

Yes, if the condenser is not receiving adequate airflow, pressures will rise, and the system will shut down... again, this will occur at around 400psi. Poor cooling will be noted well prior to the high side reaching 400psi.

If you spray water on the condenser with a garden hose (through the area under the nose where the cooling stack receives its air), while the system is running, this will allow the condenser to shed all the heat it needs to, so you can see if the system is still cycling.

Huh! A great suggestion. I will try the hose trick.
Do you know if the primary fan on-off is only coolant temp dependant or should turn on regardless of coolant temp when the AC is turned on?

[VetteMed]

I believe the primary fan is strictly temperature dependent, and the secondary fan is either triggered by coolant temperature or refrigerant pressure. I could be wrong about this, but I don't think the fan is triggered just by turning on the AC. I think the fan only is activated when a certain pressure is reached in the high side of the system.