A/C Clutch and Coil Problem

I replaced the compressor and clutch with a used assembly on my 2001 PT Cruiser. I bench tested the coil with a OHM meter and was around 5 Ohms. 2-5 is a good coil. Hooked up 12 volts to the clutch and coil and the clutch pulled in just fine. Put compressor /clutch assembly back in car and clutch would not engage with the ac switch. Checked OHMs on the coil and it was close to .4 OHMs. Bad Coils. Applied 12 volts straight to the clutch coil and still nothing happened other than wires getting hot because of a short. Could it be a fluke that the clutch coil went bad or is something in the wiring caused it.? All my wiring checks out fine with the wiring diagram. Checked for continuity and power to connectors and nothing is bad.

I do have a question that is puzzling to me. In the line from AC relay to the Clutch coil, there is a diode. Symbol shows a Transient Voltage Suppression diode. I know what a regular diode is for but I have no idea what this diode does. Could this be a problem why it went bad? I'd hate to get another clutch and it burn up also.

The TVSD is designed to protect the clutch from electrical system weirdness (usually overvoltage) by shunting the circuit to ground. Just to clarify, when you re-checked the ohms on the clutch, it was disconnected, just like on your bench test?

It's possible the TVSD has failed, but that's rare unless there was an electrical system problem (sometimes a faulty alternator can destroy the TVSD).

cap269 wrote: The TVSD is designed to protect the clutch from electrical system weirdness (usually overvoltage) by shunting the circuit to ground. Just to clarify, when you re-checked the ohms on the clutch, it was disconnected, just like on your bench test?

It's possible the TVSD has failed, but that's rare unless there was an electrical system problem (sometimes a faulty alternator can destroy the TVSD).

I did re-checked the ohms on the clutch on the car, it was disconnected, just like on your bench test. Used a multi-meter and got .4 Ohms (2-5 is good). Originally, on the bench it had 5 Ohms. Also applied voltage to the two leads at the connector to see if it would engage.

Just went to Pull a Part and bought another A/C compressor with clutch. Bench tested the coil and got 4 ohms. This should be good according to specifications. I applied 12 volts to the coil and did not get the clutch to engage. Now the coil reads .2 ohms. Now bad. Is it bad now from creating a load to the used coil? Is there a diode in the ac clutch coil in a 2001 2.4L PT Cruiser? Would the coil short out if you place a 12 volt positive source to the negative line of the coil and a the negative source line to positive of the coil?

This is really getting frustrating.

The clutch coil is an electromagnet. It doesn't matter which way polarity is hooked up.

I have an idea the the bench testing is what is killing the clutch. I'll bet you didn't re-check the resistance after you put voltage to it. The diode in the diagram may be there to limit the amount of current flowing through the clutch, so without that, the clutch pulls an unrestricted amount of current and the coil wire melts, shorting it out.

cap269 wrote: The clutch coil is an electromagnet. It doesn't matter which way polarity is hooked up.

I have an idea the the bench testing is what is killing the clutch. I'll bet you didn't re-check the resistance after you put voltage to it. The diode in the diagram may be there to limit the amount of current flowing through the clutch, so without that, the clutch pulls an unrestricted amount of current and the coil wire melts, shorting it out.

Good to hear that polarity doesn't matter. So I didn't burn it up from switching positive and negative voltage leads. Here is what I did exactly. Compressor and Clutch out of vehicle disconnected. I measured the Ohms at .4 before voltage testing the clutch. Battery I used had close to 12 volts. Hooked up positive to one line in connector and negative to other line. Rechecked resistance after putting voltage to coil and it measures .5 ohms. Bad coil.

First compressor and clutch I tested, clutch engaged perfectly with voltage supplied. I put this compressor completely back into the vehicle and turned the ac switch to air. Nothing happened. Disconnected connector at the and measured resistance, which was .3 ohms. Could it be possible that both used coils were weak and could take the 12 volts?

Therefore this is a big puzzle to me. So I'm burning up the coil in the vehicle and also on the work bench.

I do not have any power to Low Pressure Switch (DB/OR wire at pin #2}. Best I can guess is it coming from Blower wiring diagram? Am I wrong?

The DB/OR wire is a switched ground, not a power feed. 12v should be on the LG/BK wire. The computer switches the DB/OR to ground when, and only when, all the programmed conditions for safe operation have been met (pressures ok, ambient air temp ok, evap core temp ok, blower on, radiator (condenser) fan running, etc.)

EDIT: I misunderstood which DB/OR wire. I see the other one now, at the pressure switch. This is also a switched ground, controlled by the mode switch. Notice how it connects to the same ground as the blower motor (trace the lines on the diagram back to the G203, and S200 for the motor). The only 12v going to that control unit appears to be for the illumination.

cap269 wrote: The DB/OR wire is a switched ground, not a power feed. 12v should be on the LG/BK wire. The computer switches the DB/OR to ground when, and only when, all the programmed conditions for safe operation have been met (pressures ok, ambient air temp ok, evap core temp ok, blower on, radiator (condenser) fan running, etc.)

EDIT: I misunderstood which DB/OR wire. I see the other one now, at the pressure switch. This is also a switched ground, controlled by the mode switch. Notice how it connects to the same ground as the blower motor (trace the lines on the diagram back to the G203, and S200 for the motor). The only 12v going to that control unit appears to be for the illumination.

So you are saying, G203 of the Blower diagram is the connected switched ground that the "AC Switch Sense" (AC wiring diagram top left corner)? Is the "AC Switch Sense" (AC wiring diagram bottom left corner) a ground also and derived from 12 volt parameters? I'm looking at the basic ac wiring diagram.

I didn't know that circuits could be switch grounded. After doing a little research, I see how is works. I found a basic ac wiring diagram. Actually, it was a you tube learning video.



So you are saying that many things (pressures ok, ambient air temp ok, evap core temp ok, blower on, radiator (condenser) fan running, etc.) have to be met before the coil to the relay is energized, therefore pulling in the switch to the coil? The wiring diagram I have of my 2001 PT Cruiser is just a stripped down version of what other parameters of the program have to be met? How does someone find what other component parameters have to be met? Basically for testing.

I do know one thing. The fan to the radiator is not coming on. I'm a little perplexed. What do you suggest checking in the PT Cruiser wiring diagram first?

lmarkie wrote: So you are saying, G203 of the Blower diagram is the connected switched ground that the "AC Switch Sense" (AC wiring diagram top left corner)?

No, G203 is not a switched ground, it is the ground "source" (always a continuous ground).The switching is done by the knobs on the dash.

Is the "AC Switch Sense" (AC wiring diagram bottom left corner) a ground also and derived from 12 volt parameters? I'm looking at the basic ac wiring diagram.

No, the AC Switch Sense is a computer input (also a ground, and has nothing to do with +12v). When the AC is selected on the dash, this wire becomes grounded. This "pulls down" a transistor inside the computer. The computer is programmed to recognized the state of this transistor as knowing the on-off status of the A/C selection. The computer looks at the other parameters it is programmed for, and if all conditions are met, it then activates the output that switches on the compressor relay. Inputs, outputs, pull-ups, and pull-downs are essentially how computers operate.

I didn't know that circuits could be switch grounded.

Ground-side switching is probably the most common type of switching you will find in modern cars. It is safer than voltage-side-switching, as short circuits become less hazardous. A ground shorting to ground doesn't blow fuses and cause fires. Voltage shorting to ground does. In automotive circuits you will often find the 12v is fused and wired directly to the component while the switching occurs on the ground. This switching can be manually controlled through mechanical means or knobs/buttons on the dash, or digitally controlled by the computer.

So you are saying that many things (pressures ok, ambient air temp ok, evap core temp ok, blower on, radiator (condenser) fan running, etc.) have to be met before the coil to the relay is energized, therefore pulling in the switch to the coil?

Correct. The actual number of devices and programmed conditions varies from one vehicle to another, but the idea is the same. There are conditions that must be met. When those conditions are met, the system operates. If the conditions are not met, the system won't. This is to protect expensive parts like the compressor. These system controls are in place to shutdown or prevent operation in order to protect them. A complete circuit diagram of just the A/C system, rather than a general diagram of the whole vehicle, will show all the various components involved. Once you are familiar with reading and interpreting wiring diagrams then it becomes fairly obvious which components are used as controls (generally any time a component is connected to a computer or a control module, especially if connected to a port with 'sense' in its name).

I do know one thing. The fan to the radiator is not coming on.

This could be the part of your problem. Does the fan not come on at all, even when the engine is warmed up? That needs to be repaired first, as engine damage could result from overheating.