Dave Stribling
September 13, 2016

Many early Mustang owners are still using a traditional air conditioning unit with an upgraded R-134 Freon system; mainly because the look of the original unit really enhances the car, even if it isn’t 100 percent correct under the hood. The biggest problem with the original system is not that the Sanborn compressor looks out of place, but that the A/C system does not have a safety cutout switch in case the pressures get too high. I discovered first hand all about this little problem, and found a way around it.

Pressure safety switches didn’t really come into play until the 1980s, so none of the early systems were originally equipped with one. A customer’s car was no different in that it had a 1965-style under-dash unit and a Sanborn compressor tied into the original unit. This customer was having overheating issues in parades, so we installed an electric fan to help with the cooling issues while idling along the parade route for two miles. I installed a Derale adjustable thermostat and I thought everything was operating fine. I then was on my way to deliver the car to the customer (with the top down) when the A/C stopped working. It would work for just a bit then go away again, so I knew I had a problem.

I changed the expansion valve and that solved some of the problems, but I noticed that while I had my gauge set on the system, the high-pressure side was racing up to around 350-400 psi! I had just remembered that with the new electric fan, the fan doesn’t come on until the temperature of the coolant gets high enough; not when the Freon has heated up. With the A/C running and the electric fan not running, the heat building up has to go somewhere, and it is turned into extra pressure in the system. If you have ever experienced a compressor failure or seen what high pressure does to copper pipes soldered together, you know how dangerous this can be.

I called the company that sold us the A/C system, and they no idea how to install a safety switch into the original style lines as the original system uses flared fittings, where the safety switch and fittings are #6 O-ring fittings. I came up with the following fix, and it only required a modification to the existing system to make the A/C safer for daily use.

Vintage Air (www.vintageair.com, 800-862-6658), sells a trinary switch which turns off the compressor if the pressure falls below 30 psi or rises above 406 psi. Additionally, it has a second circuit that can activate the electric fan when the pressure reaches 256 psi. The part number on the switch is 11076-VUS and the adapter is 34098-VUG. As mentioned, this switch and hardware are designed for O-ring fittings, so I found a company, Air Parts in Ocala FL. (www.coldhose.com) that makes conversion fittings to convert O-ring to #6 flare fittings. I also found that the fittings located at the liquid and aluminum lines are #6 flare. The switch needs to be mounted on the high-pressure side of the system, so I was in luck.

Wiring the fitting is easy: pull the wire from the compressor that connects to the clutch and run it to one of the black wires on the switch. Run the other black line back to the compressor clutch connection. The electric fan switch can be powered by ignition 12V or to ground, depending on your electric fan controller. The Derale controller utilizes 12V ignition power for the override, so I ran the power from the now-remove override switch to the new trinary switch and then out to the Derale controller.

Now with the engine cold and the A/C on, the electric fan will automatically kick on at about 260psi, then draw down to around 200psi, right in the range you want to be.

You should never run your A/C system without a safety switch, even if you have a mechanical cooling fan. Unless you are running an all-original show car, make this modification. Most aftermarket style systems have the switch built into them already. If you have a better way of doing this, Let us know. Stay cool!

1. I used the following parts to install a safety switch in an original 1965 style Mustang high-pressure line. On top is a trinary switch from Vintage Air, part number 1106-VUS; the middle is the line adapter made for #6 O-ring fittings part number 34098-VUG. The outer two pieces are #6 flare to #6 O-ring fittings from Air Parts, parts number 6240 and 2606.

2. I took the car to my A/C guys and had them evacuate the A/C system, then we installed the fittings and switch right between the liquid line and the aluminum line on this original-style system. The switch works off of pressure so direction isn’t a problem. Kind of makeshift, but it does work. We then refilled the A/C with 21 ounces of R-134 as per the original manufacturer’s recommendation and checked for leaks.

3. After wiring the switch per Vintage Air recommendations, we installed our gauge set and watched as the electric fan automatically cut in at around 260 psi. We saw the pressure fall immediately once the fan started to run. I do not recommend it, but you could check the high-pressure cutout by unhooking the electric fan and allowing the pressure to run up to 406 psi. But this is dangerous and again, I don’t recommend it. If the fan system is working properly and the expansion valve doesn’t clog you should never need it.