[chrissteffen599]

Please see post #37 ...

Any other avenues to take before I send it to a shop? I haven't inspected the wiring harness from the tachometer plug on the AC compressor (mainly because I'm not sure where it leads). I assume the compressor harness is fine if it's operating normally when forced on.

 

[FJtest]

If your AC light is flashing at 0.5 second intervals (1/2 second on, 1/2 second off) then the AC 'belt slip detector' HAS BEEN TRIGGERED. Again, this can be triggered by a slipping belt, a damaged wiring harness between AC compressor tach and the AC Amplifier module, a defective tach inside the compressor, or a defective AC amplifier.

If you've replaced the belt and verified that the belt tensioner is maintaining correct tension, then the next step is to check the wiring harness between compressor and AC Amplifier.

If there is an open circuit in the wires from the compressor's tach, or one of the wires is shorted to ground, the AC Amplifier module won't get the signal it requires and will shut off the compressor and trigger the flashing AC light.

If you have an oscilloscope you could confirm that the compressor's tach was providing the correct AC sine wave output, but not everyone has an oscilloscope in their garage.

Here's what the output of the compressor's tach should look like:

 

[Alchemus]

I have a 2008 FJ with 117k miles, and the AC light started blinking 2 weeks ago. I changed the relay first, with no luck. Since then I’ve checked pressures with manifold gauges, but can’t get a stabilized reading because the clutch won’t stay engaged for more 5-10 seconds. Any ideas what to do next? I don’t think it’s a leak/low refrigerant because it’s ice cold for those 5-10 seconds it does engage and work!
Thanks everyone!

Sounds like low pressure as suggested. The frequency is when high side builds pressure it removes it from low side causing sensor to cut out. Then pressure equalizes and starts the circuit again. Above responce is accurate diagnostic.

 

[chrissteffen599]

Sounds like low pressure as suggested. The frequency is when high side builds pressure it removes it from low side causing sensor to cut out. Then pressure equalizes and starts the circuit again. Above responce is accurate diagnostic.

im not sure what you mean by this? Low pressure switch or low refrigerant?

 

[FJtest]

Sounds like low pressure as suggested. The frequency is when high side builds pressure it removes it from low side causing sensor to cut out. Then pressure equalizes and starts the circuit again. Above responce is accurate diagnostic.

No.
Because the pressure switch is on the HIGH side (outlet side) of the compressor, if the static refringent pressure is too LOW, the compressor will never be allowed to start, and there will never be any 'movement' of refrigerant whatsoever.

Actually the pressure switch has two sets of contacts inside it, and the contacts will open if the refrigerant pressure is too low, or if it is too high.

The key observation here is the 'blinking' AC light ... that means the AC Amplifier, which controls the compressor clutch, has disabled the clutch for one reason or another.

 

[chrissteffen599]

No.
Because the pressure switch is on the HIGH side (outlet side) of the compressor, if the static refringent pressure is too LOW, the compressor will never be allowed to start, and there will never be any 'movement' of refrigerant whatsoever.

Actually the pressure switch has two sets of contacts inside it, and the contacts will open if the refrigerant pressure is too low, or if it is too high.

The key observation here is the 'blinking' AC light ... that means the AC Amplifier, which controls the compressor clutch, has disabled the clutch for one reason or another.

so I just pulled out the ole’ multimeter. No voltage showing at all coming from either ac compressor plug when tested. I had the key in the ON position but truck was not running. This would indicate no power to the wiring? But confuses me because the compressor does click on for 1 second before shutting down.

 

[FJtest]

so I just pulled out the ole’ multimeter. No voltage showing at all coming from either ac compressor plug when tested. I had the key in the ON position but truck was not running. This would indicate no power to the wiring? But confuses me because the compressor does click on for 1 second before shutting down.

The compressor clutch is disabled when the ignition switch is ON, but the engine is not running.
The clutch is also disabled when the engine is being cranked.
The clutch is enabled AFTER the engine starts IF the AC controls are turned on AND the AC Amplifier is receiving the tach signals ('compressor lock sensor') from the compressor.

So, if you were making your voltage measurements with the engine not running or the compressor tach not connected, you won't see any voltage at the AC clutch power connector on the wiring harness.

And of course you won't see any voltage at the wiring harness side of the compressor tach connector because it doesn't supply voltage, it receives voltage signal from the tach.

Two easy, additional tests you can do to check the compressor tach:
1. With the wiring harness tach connector disconnected from the compressor, measure the resistance of the compressor tach coil. Resistance should be no more than 100 ohms. Anything more that that indicates a defect.

2. Measure the AC voltage that the tach provides when the compressor is running. Set your multimeter on a low-voltage AC scale, back-probe the tach connector while it is connected to the compressor, start the engine and see if there is an AC signal being provided from the compressor tach. Obviously you'll have to 'jump' the Mag Clutch relay again to 'trick' the AC Amplifier into allowing the compressor to run. You should see the waveform and voltage level shown in post #42.

UPDATE: Here is a writeup on a Camry forum describing EXACTLY the same problems and symptoms you are seeing. The writer confirmed that the failure of the clutch to stay engaged was because of a fault with the compressor tach. He came up with a way to bypass the tach signal and immediately got his AC working, but while this gave him cool air it also disabled Toyota's belt safety scheme.

I'm just providing this link to confirm that the FLASHING AC light means that the AC Amplifier has intentionally disabled the AC clutch because it was not receiving the tach signal.

How I bypassed my failed 99 A/C Compressor Lock Sensor - Camry Forums - Toyota Camry Forum

 

[chrissteffen599]

The compressor clutch is disabled when the ignition switch is ON, but the engine is not running.
The clutch is also disabled when the engine is being cranked.
The clutch is enabled AFTER the engine starts IF the AC controls are turned on AND the AC Amplifier is receiving the tach signals ('compressor lock sensor') from the compressor.

So, if you were making your voltage measurements with the engine not running or the compressor tach not connected, you won't see any voltage at the AC clutch power connector on the wiring harness.

And of course you won't see any voltage at the wiring harness side of the compressor tach connector because it doesn't supply voltage, it receives voltage signal from the tach.

Two easy, additional tests you can do to check the compressor tach:
1. With the wiring harness tach connector disconnected from the compressor, measure the resistance of the compressor tach coil. Resistance should be no more than 100 ohms. Anything more that that indicates a defect.

2. Measure the AC voltage that the tach provides when the compressor is running. Set your multimeter on a low-voltage AC scale, back-probe the tach connector while it is connected to the compressor, start the engine and see if there is an AC signal being provided from the compressor tach. Obviously you'll have to 'jump' the Mag Clutch relay again to 'trick' the AC Amplifier into allowing the compressor to run. You should see the waveform and voltage level shown in post #42.

UPDATE: Here is a writeup on a Camry forum describing EXACTLY the same problems and symptoms you are seeing. The writer confirmed that the failure of the clutch to stay engaged was because of a fault with the compressor tach. He came up with a way to bypass the tach signal and immediately got his AC working, but while this gave him cool air it also disabled Toyota's belt safety scheme.

I'm just providing this link to confirm that the FLASHING AC light means that the AC Amplifier has intentionally disabled the AC clutch because it was not receiving the tach signal.

How I bypassed my failed 99 A/C Compressor Lock Sensor - Camry Forums - Toyota Camry Forum

For anyone else it may help, it was the tachometer in the compressor. New compressor, and she’s back to cold!

 

[FJtest]

For anyone else it may help, it was the tachometer in the compressor. New compressor, and she’s back to cold!

Exactly what I was telling you since post #12, with additional details in posts #23 and #42 highlighting Toyota's 'compressor lock' detection scheme based on the AC compressor's tach signal. No tach signal = no AC compressor turn-on.

Glad you got it straightened out ... just in time for the weather to start cooling off, right?