Adding high pressure switch to older unit help.

[Vetteman61]

Hello,

I have a 1971 Pontiac Grand Safari wagon. I would like to add a high pressure switch in the A/C hose that will control dual electric pusher fans I'm adding in front of the condenser (The condenser has been upgraded to a parallel flow. The rest of the system is stock). I have changed over to 134A.

I'm curious at what PSI I should set the switch to turn on the pusher fans. This system has a POA valve and the compressor clutch stays engaged at all times. Here is some information I got form the shop manual (original R12).

Pressure relief valve is set to 440psi. Shop manual says when charging, cut off engine if pressure reaches 535 psi.

It doesn't give a specific psi of what the normal operating range is for the high side, but there is a chart that gives the following:

When the ambient outside temp is 90 degrees: compressor head pressure - 263 to 287. 110 degrees 373 to 397 psi.
When the ambient outside temp is 90 degrees: POA Valve pressure - 30 to 32 psi. 110 degrees 37.5 to 34.5 psi.

It also states that the PSI of R12 at ambient temperature of 90 degrees = 99.6 psi, 110 degrees = 136 psi, 140 degrees = 204.5 psi


At what PSI on the high side should I have the switch begin to turn the condenser pusher fans on? I know the maximum pressures the system can handle, but of course the fans will need to come on much sooner than that.

Thank you

[JohnHere]

Do you plan to keep the stock fan? If not, you'll need to install a coolant temperature sensor to turn-on the fans at about 190°F, in addition to the HP fan switch. Opinions vary, but setting the high (fans on) limit to around 250 PSI and the low (fans off) limit to around 190 PSI seems reasonable. Some prefer to run the fans whenever the compressor is running. If so, you could wire them that way as well.

[tbirdtbird]

"Some prefer to run the fans whenever the compressor is running. "

I am such a person. I find the performance is better (cabin temps run lower) when done this way. Get rid of that heat!!!

[bohica2xo]

Is this system running the original A6 compressor?

Is the original mechanical engine fan and shroud still in place?

[Vetteman61]

Yes, the rest of the system is completely stock and the original mechanical fan is in place. I will upgrade to an HD clutch for the fan.

I see lots of trinary switches for the high side to turn the compressor on and off at high and low pressures, and also to control the electric fans at a different temp. Is there such a switch that is only for fan control?

[bohica2xo]

I usually just use a little snap disc switch and a relay. Usually the compressor discharge line near the compressor is a place for mounting.

https://www.mouser.com/datasheet/2/459/ ... 308879.pdf

Usually by the time the discharge gets to 140f or so it is time to add fans.

[GM Tech]

Seems kind of silly to have pusher fans running when driving 60 mph down the road- Also if wiring into the compressor circuit, the spinning fan(s) will back feed the compressor clutch- providing current when the compressor is not running because they generate current by free-wheeling in the wind. There can be as much as 2-6 volts into the clutch circuit which would cause the clutch to partially engage. There needs to be s a separate relay or diode in the circuit to avoid this. I've seen it on OEM vehicles. I'd go with an inline pressure switch in front of the condenser and relay to control a fan that comes on only when needed.

[bohica2xo]

Good to see you GM Tech. I totally agree with you on the relay for any fan.

As for as spinning them at 60 mph, they are spinning already, and draw almost nothing.

Maybe you know the answer to this. With the wide assortment of switches GM used in the head of the A6, is there anything that would work for the OP's fans?

[GM Tech]

I can remember on a 1991-94 GM Pick-up truck with the R-4 compressor a high-pressure switch mounted in the back of the old R-4 compressor that activated the recirc door automatically at about 250 psi pressure. Most all complaints were that the fan goes into a High-High speed on its own, and owners could not understand that the airflow path was less restricted and made the fan noise sound louder every time this happened. They wanted me to fix the fan! So if you search for that switch- it may work since all the switch ports were the same size. Just make sure to use a relay.

[bohica2xo]

Nice. Your GM in depth knowledge is awesome.

I think one reason we see these fan switch threads over & over is because there is not a surplus of sensor ports available on the average system

Trinary switches are the usual request, because the poster wants to add function without opening the system. Adding a port means crimping a splice in a line, and not everybody has a crimping tool in their toolbox.

That is why I asked about the A6 / R4 head mounted switches. An unused port in many systems.