I need some help…I am trying to get my air conditioner back going after being out of the car for 20 years. The wiring diagram shows the power from the ignition to the blower switch and then it goes to the blower motor via the low, med and high position on blower switch and then at the blower motor depending on the switch position the blower speed is reduced for low and med thru resistors and when the switch is on high no resistor to get high speed blower operation. My question is shouldn’t there be 12 volts at all the speed positions on the blower switch as well as the green wire that goes to the AC relay before going to the AC compressor? The voltage at the blower switch is reduced on low and medium and full voltage only on high. These same voltages are also present at the green wire at the blower switch. Shouldn’t the voltage be 12 volts at all the blower speed positions as well as the green wire to the AC relay? The way it is now the only way that the AC compressor gets 12 volts is with the blower switch in the high position. What’s going on here? Is the blower switch bad? Help…
The Blower switch is where power comes from for the blower motor. 12 volts goes in on wire # 257, which leads to the AC circuit breaker mounted to the windshield motor bracket above the fuse panel. The breaker gets 12 volts from the yellow switched harness under dash, which is accessory power from the ignition switch.
Wire #348A (green) leads from the AC compressor relay to the icing switch.
Wire#348 (green) goes from the blower selector to the AC compressor relay.
There is always 12 volts on the blower switch regardless of which speed is selected. The AC compressor always gets 12 volts to the icing switch at any fan speed. The icing switch decides whether or not the compressor runs by placing 12 volts on wire #348B (green).
Thanks! That’s what I was understanding the power to be at the switch as well. Why am I getting different voltage at the different switch selector positions? Is the blower switch bad? The blower does operate at all 3 different speeds.
Typically, Ford designed the heater speed selectors to be the grounding point, so what you’re seeing as voltage is what’s left over for the blower motor itself.
OK… I have a mechanic friend coming over tomorrow so we can figure this out. What is the best way to check out the 12 volts going to the AC compressor? I don’t want to power this until I can insure there is 12 volts there…Sounds like the blower switch is OK? To verify what is the best way to check? I really appreciate your help.
Thanks!
There is a resistor block inside of the heater plenum. The resistors drop the voltage so that the blower will spin slower at the lower settings.
Thanks for all the help from all! I still need to know what is the best way to check for 12 volts going the wire that feeds the AC compressor? This is where I am stuck…Help!
Thanks!
The wire to the compressor clutch goes through the firewall and has a single wire connector inside the engine compartment to the left (your left) of the master cylinder. You can unplug the wire and measure for 12v to ground with the switch on MAX, COLD, and fan on. There will also be another single plug at the compressor that can be separated and checked. If you have repainted the compressor and the clutch assembly you may not be getting a good ground to energize the clutch.
What you are saying is true if this was a heater only car. In A/C cars, it is the opposite.
What activates the AC relay? Is this a mechanical process? Is the relay physically closed or does it close with wire?
If you select any fan speed other than off it puts 12 volts on the AC relay, which closes then puts 12 volts to the icing switch. If the Icing switch is above 32 degrees F it puts 12 volts on the compressor clutch wire. All the wires in this circuit are green except the compressor clutch wire under the hood which is black.
I’m still puzzled on this! The green wire going to the AC compressor simulates what’s going on with the blower motor speeds. 3 1/2 volts on low, 8 volts on med and 12 volts on high. These readings are taken in the engine compartment. What’s going in here? I can’t run the compressor like this…Help guys!
I’m stuck here…I’m getting different voltage readings on the green wire in the engine compartment. 3.6 volts on low blower speed, 8 volts on medium an 12"volts on high. What’s going on here? Is the blower switch bad or what? I can’t run the AC like this with variable voltage? Needs some help here…
With the engine running?
Yes…I’m thinking that the blower switch is bad? The green wire from the switch also reads like this…
I have not looked at the circuit, but if the wire to the compressor is not attached to a load, you may be seeing the voltage going to the only load that it hooked up. Attach the lead to the compressor (with the AC switched to the on position) and then check the voltage.
The compressor can only get the same voltage as the battery. The voltage is coming from the relay. Perhaps you have some wires swapped or spliced improperly under the dash? The circuit to the compressor is getting power through the relay.
Like Bill said, check the voltage with the compressor connected and the engine running.
So I replaced the blower switch thinking that I was causing the voltage drop issue to the AC compressor issue. Well I got the same results of different voltages on low an medium blower speeds and full voltage at high blower speed. So at that point I decided to attach the wire to the AC compressor and see wha t happens. The compressor clutch would not engage until the blower switch was in the high position. So now I’m thinking the system is designed to operate the AC only when the blower is on high. Well to my surprise, the AC clutch stayed engaged on medium and low blower speeds and disengaged at the off position. So now I’m thinking that once the AC wire gets the full 12 volts on blower high position it keeps the 12 volts on medium and low blower positions but when I put a test light into the connector at the AC compressor the voltage was only 12 volts on the high position and lower on medium and low. How is this compressor operating on lower voltages? In addition the radiator has electric fan that are also operating when the AC compressor is operating and does not change speed as the blower speed is reduced. What’s making this system work like this?
The blower switch has 2 circuits in it. The first one is the stepper for the blower. The second is a separate contact that comes off the ignition and powers the compressor. If you get the same voltage drop with both switches I’d say your wiring is borked. You need to see where it’s crossed.
Then why does the compressor and the electric fans work after the blower switch is on high and both work on low and medium? The radiator fan relay that is attached to the compressor lead comes on when it sees the 12 volts and holds it until the blower fan is off just like the compressor clutch operates off it’s on in line relay. Are the relays for both the AC and the radiator keeping everything running until the blower switch is in the off position?