Found very little info about this online so thought I would contribute what I was able to figure out myself.

Couple days ago on a very hot day I realized my A/C in my 99 Camry LE (5S-FE without engine immobilizer) had suddenly stopped working with the A/C indicidator light blinking. I measured the resistance of the compressor lock sensor and saw over 1 megaohm when the value should be approx 100 ohm. I jumped the mag clutch relay and with the engine on, the A/C blew cold. I replaced the mag clutch relay with the horn relay, which didn't solve the problem. After these diagnostics, I was about 98% sure it was the lock sensor. Not wanting to drain the system to pull the compressor and replace the sensor, I looked for another way.

After reading someone mention in another forum that a shop offered to bypass their lock sensor, I started thinking about how this could be done. After pouring over the wiring section of the original service manual, I convinced myself that I could jump the ECM tachometer output directly to the ECM LOCK IN input that would normally come from the lock sensor. Even if it didn't work it probably would do any further damage.

So, I pulled my glovebox, found the ECM, and stuck wires into the TACH output (pin 8* of connector E7) and LOCK IN input (pin 18* of connector E9) to just test this out temporarily. I connected the two wires through a 4700 ohm resistor with alligator clips. Amazingly, this worked. The A/C button still works, and the compressor still cycles, and most importantly it blows cold. I'm going to run with this temporary setup for a couple days and if everything is still good after a couple days I'll make it permanent with some wire vampire taps and report back. Attached is a quick snapshot. Doesn't show much but it does show you can try this without cutting or modifying any original ECU wires.

I did encounter one strange thing during this process. The wiring diagram shows that the lock sensor is actually a reed switch, and should be closed when near a magnet and be the open when taken away from a magnet. One side of the reed switch is connected to ground, the other is connected to the LOCK IN pin of the ECM. I would imagine the system works like this: the ECM puts 12V on the LOCK IN pin and watches for it to be pulled down to ground, which is done when a rotating magnet passes the reed switch inside the compressor, causing the reed switch to close and short the LOCK IN pin to ground. But when I measured the voltage on the LOCK IN pin, it read as 0V. So one of two things is true:
-The ECM is damaged and isn't able to put 12V on the LOCK IN pin. The event that caused this damage might also have destroyed the lock sensor itself-The lock sensor isn't actually a reed switch, but is an inductive coil pickup, like the ABS sensors. If the sensor is inductive, the LOCK IN pin doesn't need 12V on it because the rotating magnet would create a voltage on the pin itself. In this case the manual would be wrong, which I hesitate to say because the manual has never been wrong before, in my experience.
Any thoughts on which is more likely?

*Connector pin numbers will vary according to engine model and presence of engine immobilizer.

 

If you get hung up, for a fee Toyota offers access to service information at the site below. Two days for $20.

https://techinfo.toyota.com

Blinking light can mean issues with the A/C amplifier. Lock sensor input goes to the amplifier while the other wire goes to ground. Ebay sells used amplifiers.

Did you check for low Freon? Can use the sight glass on the dryer to get an indication of Freon level.

Other things to check.

AC and heater fuses

AC pressure switch

Heater main relay

 

According to the service manual, the 5S-FE models don't have a separate AC amplifier. The functionality is built into the ECM. This also agrees which what I saw under my own dash.

Appreciate the extra diagnostic info but as far as I'm concerned I know the problem is just the lock sensor, since like I said, giving the LOCK IN pin the TACH signal made everything go back to normal. I did check my pressure switch when I was first diagnosing, and it was fine.

 

You are correct about the 4 cylinder not having a separate amplifier.

For later years of Camry Toyota refers to the sensor as being able to detect RPM. This is in line with the computer being able to determine if the compressor RPM is an acceptable range when compared to the engine RPM.. Here an on/ioff switch would not work.

 

Hello, I have an 02 Camry XLE V6 with digital climate control. I have had AC Compressor issues for years and have not been able to solve the issue. I ended up manually wiring w/ a switch my ac compressor. So it comes on and off with the flip of a switch. It works fine when you are moving, but if you idle too long in traffic it will get too high in pressure and purge. I have replaced nearly everything under the hood. The fail condition shows when my AC button starts blinking.

Recently I have experienced random hard starts, where some days it starts right up, and others itll take 3 or 4 longer cranks before it finally starts up. I was wondering something this week, if my Crankshaft Sensor or Camshaft Sensor was failing, part of those sensors job is to report back RPM to the ECU, if that signal is not correct would that put a variance between what the AC Compressor Sensor reports to the A/C Amplifier and what the ECU is reporting to the A/C Amplifier correct? Anything beyond 200 rpm, the condition is to turn off the compressor correct? Could this be my issue?

I have also replaced my actual climate control unit inside of the car. However I have not replaced the AC Amplifier yet... What is the location of that thing under the dash? Thank you for any help you can provide!

 

I When I looked up 4700 ohm resistor they had different watts. What wattage should I use?