What should rotation sensor output look like?

[m32825]

I've got a 1987 Toyota Camry. The A/C comes on for a couple seconds then the light on the button starts blinking. Guages show that pressure is reasonable. It actually starts to cool, so I think everything is working, it just won't keep running. Belt is good, tried swapping out the clutch relay, no change. Today I looked at signals going into the amplifier under the dash on the passenger side with a scope. The signal from the ignitor looks good and tracks engine rpms. The rotation sensor input is putting out something that looks like ground with a little noise on it occasionally, looks the same whether the compressor is turning or not. Anyone know what rotation sensors are supposed to put out? Thanks!

-- Carl

[GM Tech]

The few I have had- it was always the amp under the dash- replace it..and be cool...scrapyard ones are cheap and work.

[mk378]

I think the sensor is a reluctor like the pickup in a distributor, puts out a small AC voltage, frequency proportiional to rpm. This type of sensor is quite trouble-free and easy to check by coil resistance. Like GM Tech said, the amplifier is almost certainly the culprit. If you have some experience with electronics you could take your amplifier apart, inspect for bad solder joints and swelled / leaky electrolytic can capacitors, those are by far the most common problems with automotive modules.

[m32825]

Good suggestions, guys. The rotation sensor resistance is in range. I'll drop by the junk yard and pick up a couple amplifiers to try out. Thanks!

-- Carl

[m32825]

Update: haven't made it to the junk yard for a new amplifier yet, but I spent some time studying the schematic and experimented a bit. I located the transistor that activates the magnetic clutch when rotation is good and grounded it's output, then went for a test drive. Everything seems to work fine. That's great because it's painful not to have A/C in Florida this time of year!

-- Carl