This 2000 Toyota Camry came in with the complaint that the a/c compressor would cycle on for a second or two, the a/c light would start flashing and the compressor would cycle off. This would happen once during a key cycle. The most common cause of this general complaint would be the a/c magnetic clutch relay. However if the relay was truly at fault the compressor would not engage for a split second and then disengage. It simply would not turn on at all at some point.
In order to test the basic system functions a jumper wire is placed between the two terminals that go to the relay contacts. In this case they are the two larger terminals that run parallel to each other at one end of the relay. Caution: Jumping the wrong terminals can permanently damage the engine computer. With the engine running the a/c compressor should be on and the a/c light should not be flashing. This would indicate that either the relay or the control wire circuit from the computer to the relay is at fault. In this case the light was flashing while the relay was bypassed. The a/c compressor was on, the pressures were normal and the vent temperature was cold. Caution: Do not leave the jumper wire in place for longer than a few minutes even if the previous conditions are met. Even less time if those previous conditions were not met. Operating the compressor in this manner is for test purposes only. Compressor damage will occur if this wire is left in place. The flashing a/c light under this test condition can indicate that there is a problem with the compressor speed sensor.
In testing the compressor speed sensor circuits it is easiest to begin at the compressor harness connector. It is a four cavity square connector with three wires installed. The black/white wire is the command circuit from the relay to the compressor clutch coil. Since the compressor worked when the relay was jumped in the earlier test I knew that it was okay. I moved my focus to the speed sensor circuit. In the next picture I am showing my test leads connected to the speed sensor terminals inside of the connector body at the compressor. They are the two lower ones.
I had a reading of 99.2 ohms. I could not find the specifications for a good sensor but it is within a range that I would expect to see from experience. My assumption at this point was that the sensor was probably not the issue.
I checked for resistance on the wires that attach to the speed sensor. The white/blue wire is routed back to the engine computer and it would be the wire that carries the signal from the compressor speed sensor back to the engine computer. There was a reading of 10.23 k ohms between that terminal and battery ground. Again there was no specifications for this, but it would not be an unexpected reading from a computer circuit.
The brown wire on the other hand showed an open circuit to battery ground. When I looked at the wiring diagram it clearly states that this wire is attached to ground. I checked the brown wire a few inches above the connector and found the ground signal present.
A gentle tug on the wire pulled it out of the connector body. I removed the terminal from the connector body in order to make a repair.
I used this tool to release the plastic clip that held the terminal in place. There was also a white positive assurance clip on the end of the connector that I had to remove first.
I looked through my inventory of connectors and terminals and could not find an exact match. So what I did was used the terminal ears that would normally hold the seal in place and crimped a new wire onto the terminal at that point. Afterwards I slid the seal over the crimped connection and placed it all back into the connector body. I also made sure that the seal was fully inserted into the connector body by gently pushing on it with a pocket screwdriver until it was set. But not so far as to expose the revised crimp location.
I spliced my new wire to the original brown wire at a convenient location.
I rechecked my circuit and now had a proper reading of .5 ohms.
I installed split loom over the harness and secured it as it had originally been.
After reassembling everything, the a/c system now worked correctly.