Relationship Between A/C High Side Pressure and Cooling Fan

[HousTEN]

The performance of the A/C in my truck has always been mediocre when I'm not rolling down the road getting a bit of air through the grille. I put my gauges on the system this weekend, and noticed that, at idle, the cooling fan cycles up and down keeping the high side pressure between 260 psi and 305 psi. Seems a bit high to me, and the shop manual indicates the pressure transducer should cycle the fan on at 240 psi. Has anyone, by chance, observed the same behavior and pressures on their trucks?

 

[HousTEN]

Anyone??

 

[BigRed460]

I have not put gauges on the truck yet, but 410a will run 400psi+ head pressure at 120 degrees (on a hot highway), and still work fine. Head pressure in gerneral is a poor gauge of system porformance and refergant level. What is the suction side at? 30-35 psi is about normal, and it should never change from that to much.

...our a/c's dont cycle on pressure, the fan is controlled by temp. There are two pressure switches release the clutch on the compressor (high@ 450 and low at 25 I think)

 

[HousTEN]

I have not put gauges on the truck yet, but 410a will run 400psi+ head pressure at 120 degrees (on a hot highway), and still work fine. Head pressure in gerneral is a poor gauge of system porformance and refergant level. What is the suction side at? 30-35 psi is about normal, and it should never change from that to much.

...our a/c's dont cycle on pressure, the fan is controlled by temp. There are two pressure switches release the clutch on the compressor (high@ 450 and low at 25 I think)

Yep...suction was 35-40 psi...went down to 30 psi if I brought the engine speed up a bit.

According to the shop manual, there's one pressure transducer which varies its resistance based on high side pressure. The voltage of the transducer is fed into the PCM which determines what to do with the compressor clutch and the fan.

If, as you say, there are two pressure switches for high and low cutouts then there is a HUGE misprint and the shop manual. Moreover, that would imply that the fan controller on my truck is MAGIC because the fan cycled on at ~305 psi and off at ~275 psi like clockwork the whole time I was watching pressures.

 

[Voodoo]

is your ac kicking out on the high side or low side???
i have my low side switch jumped so it wont kick off, just have to be careful that in mild climate u dont slug the pump and ruin it.

my black truck is set up that way and at idle it will pump 26 deg air out the vent. awful nice when it is 128 deg outside

 

[BigRed460]

Yep...suction was 35-40 psi...went down to 30 psi if I brought the engine speed up a bit.

According to the shop manual, there's one pressure transducer which varies its resistance based on high side pressure. The voltage of the transducer is fed into the PCM which determines what to do with the compressor clutch and the fan.

If, as you say, there are two pressure switches for high and low cutouts then there is a HUGE misprint and the shop manual. Moreover, that would imply that the fan controller on my truck is MAGIC because the fan cycled on at ~305 psi and off at ~275 psi like clockwork the whole time I was watching pressures.

I'll look at it afternoon but none of the other dodge trucks we have have anything like that.

You'll regret jumping out the low side switch if you ever have a freon leak, not to mention the wear on it when it doesn't shut off at 6,000rpm.

 

[HousTEN]

is your ac kicking out on the high side or low side???
i have my low side switch jumped so it wont kick off, just have to be careful that in mild climate u dont slug the pump and ruin it.

my black truck is set up that way and at idle it will pump 26 deg air out the vent. awful nice when it is 128 deg outside

When I had the gauges on it, the compressor never cycled off. The fan on the other hand was being modulated to keep the head pressure below ~305 psi. It would seem to me that if the fan was a bit more generous in its airflow across the condenser then it would improve system performance at idle. Maybe not.

These 26 degree vent temps you refer to...is this at idle, or on the highway? I'm perfectly satisfied with performance on the highway, but at idle and at low speeds in traffic, performance is less than stellar.

 

[eddie102870]

i pulled a vacuum on mine 2 weeks after i got it. been working fine ever since. had to be a little low in freon. there were some threads about it on here in the beginning i think.

 

[HousTEN]

i pulled a vacuum on mine 2 weeks after i got it. been working fine ever since. had to be a little low in freon. there were some threads about it on here in the beginning i think.

I took mine to the dealer several months ago to have them look at this. They evacuated and recharged it to factory specs. They indicated that it was a few ounces low from the factory. Their evac and recharge didn't help performance any.

Was yours doing the same thing? Good on the highway, crappy at idle?