What is the purpose of this diode & resistor on the A/C clutch circuit? It does not appear to have any function in regards to operation of the clutch.
Thanks,
Larry M

 

The diode is shorted across the supply for the clutch coil. A diode only flows current in one direction. So when it is mounted across the wires to the coil, it is installed backwards in reference to the + and - of the power feeding the clutch. So the power supply feeding the clutch actually does not "see" the diode.

When the supply is turned off to the clutch, then that is when the diode is needed. A magnetic field is built up in the coil when power is applied to it. When the power is removed, this magnetic field collapses. When it does this, it sends a powerful reverse voltage back up the supply wires. The key here is it's reverse polarity. The reverse polarity of this voltage spike will "see" the diode as a short across the supply wires, and "short out" the spike. This protects the relay and switches that control the clutch.

 

Thanks, I kind of figured something like that as the way it is in the circuit it will not have any effect on the operation of the clutch.

 

Hi I am new to the forum. Found this while looking for answers.
I am working on a 1955 pickup with a 2000 V-10. I will be using the stock A C comp. While removing the wiring for it, I came across the diode, and also some factory splices using different color wires in the middle. The wires will be going to a Vintage Air unit. Their wiring has only one wire going to the compressor. My guess is that one of the two factory wires is a ground? guessing the black? the other is black with a yellow stripe. Any help will be greatly appreciated!
Truck is chopped, tilt forward hood, one piece side windows, 2000 V-10 and 4R100W trans. Ron Francis wiring kit, The Detail Zone Telorvek wiring and computer. Should be nice when done!

 

Yes, one of the wires is ground. You should still have the diode wired in the harness. If you can see the diode, it should have a single band or stripe on it. The band side will be the positive wire, and the plain side of the diode will be the negative or ground side.

If you happen to hook it up backwards, you will smoke the diode, but you can get replacements from Radio Shack.

 

Thanks Dave, hard to tell which wire has the diode,both are inside a piece of heat shrink. The all black ended up at a different bulkhead than the black with yellow. Just curious, why did Ford have splices, and change color in a 5' length? Any thoughts?

 

Don't try to figure out why the OEM's do many of the things they do. Just remember they are trying to put these vehicle together as quickly and cheaply as they possibly can.

One way to check the diode polarity is to use a meter on the ohms scale. Unplug the compressor clutch from the wiring. Put the meter on rx1 scale. Put the red lead to one wire and the black lead to the other. You should read a low ohm reading, or you should get a very high or no reading. You can swap the red and black leads around and get one or the other.

When you have the leads to where you are getting the very high or no reading, then the wire that is on the red lead is + and the wire the black lead is on is the -.

 

With regard to the relay in the circuit, does this affect the clutch cycling off? I have a F250 in which clutch does not cycle off. See separate post under Forum Misc> Air Conditioning etc.

 

The pressure switch controls the clutch going on and off when the dash control is in A/C mode(this would be in "max", "normal" and also "defrost" positions). When the dash control is in one of those positions, it sends voltage out to the pressure switch. Depending on the pressure on the low side of the system, it will send power to the clutch. As the evap coil inside the duct gets colder and colder, the pressure on the low side gets lower and lower till it hits a setpoint, and turns the clutch off. As the evap coil warms up, the pressure rises till it cuts the compressor on again. This is how it controls the cooling of the system.

Some systems do have a relay in the above circuit. It's usually hooked in series with the pressure switch, and is controlled by the computer. All it does is give you more power when you mash the gas pedal to the floor. It's sometimes called a WOT relay.(Wide Open Throttle). That's it's only job, to cut the compressor off during a high power demand to the engine.

The clutch will not cut off on a very hot day, especially if you keep the control on "normal". In that mode, it pulls in outside hot air into the coil. Also if you keep the blower fan on high on a hot day, it will also move lots of warm air over the coil, keeping it warm enough that the compressor will stay on. If you put the control on "max", this will recirculate cooler inside air across the coil. That along with reducing the blower output, will usually make the compressor start cycling, unless it's a very very hot day.

 

By around 99, most Fords went to a relay-controlled clutch as opposed to the clutch being directly controlled by the cycling switch. The switch provides an "A/C Clutch Request" signal to the PCM. The PCM, based on current operating parameters, either activates the A/C clutch relay or defers the operation. Such a deferment would typically be the WOT condition.

 

Thanks for that info.