[FJtest]

The AC compressor has an integral tachometer ("lock sensor") that measures compressor shaft speed. If that value doesn't correlate with the engine crankshaft speed, the AC Amplifier circuit disables the compressor's magnetic clutch to prevent a seized AC compressor (or a badly slipping belt) from destroying the serpentine belt and disabling the engine.

If you've already replaced the AC relay, it's possible that a slipping belt is the cause of your problem, as that will cause a mismatch between the compressor speed and engine crankshaft speed.

Check the condition of your serpentine belt and belt tensioner. A badly worn and glazed belt may allow slight slippage, and a bad belt tensioner seems to be common after ~150K miles.

 

[FJtest]

Sparky sounds correct. Compressor kicks on starts pumping it down , low pressure kicks it off. Small can of gas R-134 is cheap and will keep it running for the time until it leaks down again while you look for the leak and repair. This should work as long as the system has never went negative.

If the static refrigerant pressure in the system is so low that the AC pressure switch is open, the compressor clutch will never be allowed to engage.

Note that the refrigerant pressure switch has both low pressure and high pressure cutoff points to protect from both excessively low and excessively high pressure.

 

[FJtest]

Static pressure can be sufficient for pressure switch to be made, then when the compressor is called for it comes on, the metering device is what will cause the pressure differential.

No, that's not the way the FJ's AC system works.

The pressure switch is located on the high side of the compressor, so if the refrigerant static pressure is too low and the switch is open, the compressor's magnetic clutch is totally prevented from engaging, even for an instant.

The system will not allow the compressor to run until enough refrigerant is added to allow the switch to stay closed when the compressor is not running. The metering device doesn't have any effect at this point because it cannot generate any pressure differential if the compressor is never allowed to turn on.

The pressure at the pressure switch can only increase when the compressor is allowed to run.

The Toyota pressure switch is a little unusual in that it actually has two sensing mechanisms, one that detects system pressure that is too low, and one that detects when system pressure is too high. In both cases an open pressure switch prevents the compressor from turning on.

Later Toyota AC systems used an actual pressure sensor that provided a DC output voltage proportional to system pressure. That allowed the AC Amplifier to provide a diagnostic code output that indicated specifically if system pressure was too low or too high.

From the factory service manual:

Switch opens at pressures below 28 SPI and above 455 PSI

 

[FJtest]

With your assistance and knowledge of this system we might as well just roll the window down and live without it.

Or, after gaining an understanding of how the system actually works, one could verify that the pressure switch was open, add sufficient refrigerant to allow the compressor to turn on and build some high-side pressure, find where the refrigerant was leaking from and fix it, fully recharge the system, and enjoy ice cold air conditioning for another 14 years.

 

[FJtest]

Glad you agree with my first post.

Nope ...

Relative to the OP's problem, it's not low refrigerant pressure, because if the pressure switch was open, the compressor would be prevented from starting.

OP states that the compressor starts and runs for a few seconds, then drops out.
If the compressor starts, the AC relay is good.
If the compressor starts, the AC pressure switch is closed.

The remaining probable causes of the compressor dropping out after several seconds are:
1. The AC amplifier has detected a mismatch between compressor speed and engine speed, and if that mismatch persists for more than a few seconds, the amplifier disengages the compressor clutch.
2. There is some damage to the wiring harness (CAN bus, etc.) between compressor tach and AC amplifier so the amplifier is not receiving the compressor's tach signal, or receiving the engine speed signal from the engine ECM.

Let's see what happens when the OP verifies there is no damage to the compressor wiring harness and replaces the belt and/or the belt tensioner ... hopefully the OP will update this thread when he gets it fixed.

 

[FJtest]

If the pressures are just low enough to open the switch when system is activated there is a problem. When the system is dormant the pressures inside the system will equalize and can rise and close the switch. “but can’t get a stabilized reading because the clutch won’t stay engaged for more 5-10 seconds.” You activate the system, compressor starts pumping, because switch is closed, because pressure has equalized, pressures start to drop, switch opens.

Nope.

The pressure switch is on the OUTLET (high side) of the compressor. The pressure on the high side of the compressor can only INCREASE from whatever the static pressure is as soon as the compressor starts pumping.

If the pressure switch was on the INLET (low side) of the compressor, it would be possible for the static pressure to be high enough to allow the compressor to start, but within a few seconds as the compressor started to pump the pressure would drop enough to allow the switch to open and then the mag clutch would be commanded to disengage. This is the situation you describe in post #13.

The scenario you describe in post #13 would be true IF the pressure switch was on the INLET side of the compressor, but that's not where it is ... go look at your FJ, the pressure switch is mounted on the condenser outlet plumbing, obviously the HIGH SIDE of the system.

 

[FJtest]

Also, are the low and high pressure switches the same plug that is near the grill (near the expansion valve)? I tried jumping those but not sure I did it correctly.

Yes, later models have a single pressure switch with two sets of contacts inside. Does your system actually have two separate pressure switches?
That's not the expansion valve, that's just an aluminum block that provides a mounting point for the high-side test port and the pressure switch. The expansion valve (or expansion orifice) is located at the inlet to the evaporator.

 

[FJtest]

It does not matter where the pressure switch is located when at static pressure. The internal system pressure is equalized, high side, low side , there is no difference in pressure. Static pressure only increases or decreases with ambient/system temperature, thats why its called static. The ac systems high and low sides do not have different pressures when the system is off. the pressure will equalize always.

pop -
All your statements above are 100% correct, but you are drawing the wrong conclusion as to what is allowing the OPs compressor to start, but to consistently drop out after a few seconds of operation.

Please reply where you think this analysis is faulty, or if you take issue with any of the statements:

1. OP states that when AC is turned on, compressor will always engage and run for a few seconds, then drop out.
2. The Toyota FSM tells us that refrigerant pressure switch will open if refrigerant pressure is less than 28 PSI, or greater than 455 PSI.
3. If the pressure switch is open, the AC Amplifier will prevent the compressor from ever starting, even for a second.

If statements #1-3 are all TRUE, we know that the system has 28 PSI or more of refrigerant pressure in the static condition, and the compressor will always start when the engine is running and the AC is turned on.

4. The static pressure (compressor not operating) in the system is equal at both the low side and high side of the compressor.
5. As soon as the compressor starts pumping refrigerant, the pressure at the compressor low side will decrease, and the refrigerant pressure at the high side will increase.
6. The refrigerant pressure switch is located in the high side of the system.

If statements #4-6 are all true, the refrigerant pressure at the switch can only increase when the compressor starts.

7. When the compressor starts, the pressure at the switch cannot decrease and cause the pressure switch to open.
8. It cannot be the pressure switch opening from LOW pressure that is causing the compressor to drop out after several seconds of operation.
9. If the system is grossly overcharged with refrigerant or the expansion valve is restricted, the high-side pressure could conceivably exceed 455 PSI.

The remaining two factors that would allow the compressor to start, but then drop out are:
a) The belt-slip detection scheme (compressor RPM mismatched to engine RPM because of belt slip) is cutting power to the compressor's clutch;
b) The pressure switch is sensing an excessively HIGH high-side pressure (greater than 455 PSI), which is cutting power to the mag clutch after a few seconds of compressor operation.

Situation (a) can occur if the serpentine belt is badly worn or oil contaminated, the wrong length belt was installed, or the belt tensioner is defective.

Situation (b) can occur if there seems to be a cooling problem, and someone just starts adding refrigerant to the system without knowing the total refrigerant charge weight, or without monitoring high side and low side pressures plus ambient temperature.

 

[FJtest]

See post #8. Now we have a belt slippage issue See post #2 what Sparky said. Add Gas it is a 2008 and hvac systems leak , schrader valves, shaft seal ect. It worked start prior , start with the easiest check Add gas.

You're TOTALLY missing the point.

If the system was low on refrigerant, the pressure switch would be open, and the compressor would NEVER start. That's clearly NOT what's happening.

OP states that the compressor always starts, but drops out after several seconds of operation. That doesn't correlate with low refrigerant.

Not sure why you find the belt slippage scenario so funny, you ARE aware that Toyota has a belt slippage detector built into the AC system that can shut down the compressor if the belt is slipping, right ?? Kinda seems that you may not be familiar with this feature ...

Here's a description of Toyota's belt slippage detector system, and how it cuts power to the compressor clutch when belt slippage is detected. This circuit is used on almost all Toyota models that use a single serpentine belt:

Here's the FJ's schematic showing the belt slip detector ("lock sensor", actually a tachometer) built into the AC compressor:

 

[FJtest]

If anyone is actually following this thread, one might ask "why does Toyota build a belt slippage detector circuit into the AC system that can shut off the AC compressor clutch"?

The reason is to prevent the serpentine belt from being destroyed if the AC compressor seizes, and reduce the risk of engine damage from overheating due to lack of drive to the water pump if the belt is destroyed.

Earlier Toyota truck engines used three separate accessory drive belts without dynamic tensioners:
1. Alternator belt, which also drove the water pump;
2. Power steering belt, which also drove the water pump;
3. AC belt which only drove the AC compressor.

In this system, the water pump had redundant drive from two belts, and if the AC compressor seized, only the AC belt would be destroyed, and the critical water pump and alternator would continue to operate normally.

When Toyota went to the single serpentine belt, it was realized that a catastrophic belt failure was a serious problem ... the engine would continue to run, but without any water pump drive it would very quickly overheat and possibly suffer permanent damage.

Since Toyota apparently thought that the AC compressor was more prone to seizure than the other belt-driven accessories, and that a seized compressor could destroy the serp belt within minutes, a way was needed to disconnect the AC compressor from the belt if any sign of compressor seizure was detected.

Since these engines have a crankshaft position sensor, the engine RPM is accurately known. Simply adding a tachometer pickup to the AC compressor would allow the engine speed and compressor speed to be monitored. Since the pulley diameters are fixed, the ratio between crank speed and compressor speed should remain constant. If there is any evidence that the compressor was 'slowing down' relative to the engine RPM (e.g. the belt was starting to slip), the magnetic clutch could be disengaged and the belt saved from destruction.

This is exactly what Toyota implemented ... a tach in the AC compressor, and a circuit in the AC Amplifier that continuously monitors engine and AC compressor RPM. Beyond a certain amount of 'slip', the AC compressor clutch is disabled, and the engine continues to run normally except for no AC. Also added was a dynamic belt tensioner that tries to maintain a consistent belt tension despite belt wear, slight length variations between belts because of manufacturing tolerances, etc.

 

[FJtest]

No it has the one switch on that block and that's it.

OP, was any refrigerant added to the system during any of the preliminary atrempts to fix the AC?

And when you get this resolved please let us know what you found, and what the final 'fix' was.

 

[FJtest]

Post #21 states that the two conditions that could the symptoms the OP describes are the belt-slip detection scheme, or excessive high system pressure caused by too much refrigerant (or possibly a blocked/restricted expansion valve).

The OP didn't mention adding any refrigerant to the system.

If the over-pressure switch was being triggered, the compressor should re-start periodically as the high-side pressure bleeds down, unless the system is totally blocked.

 

[FJtest]

See post #8.

So pop - you've gone back and edited all your previous posts in this thread so they now just reflect your sarcastic comments in post #8, without adding any useful information that might help solve the OP's problem.

Based on your now-deleted comments, it appears that you probably weren't aware that Toyota's belt-slip detection scheme exists in the FJ's AC system.

The OP's AC problem shows up with some regularity on this forum, so I'm interested in finding out if the actual root cause of the OP's problem is a mag clutch relay problem, a wiring harness problem, a belt slippage problem, a refrigerant overcharge problem, a restricted expansion valve problem, or something else entirely.

I invited you to critique my belt-slip hypothesis that you apparently found ludicrous, but you never responded. I'd still like to hear your thoughts if you've got alternate ideas on what's going on. This is a forum for discussing and sharing ideas and information, right?

 

[FJtest]

What other fuses or relays other than the mag clutch relay is there? I’ll check voltage too.

You read the content of posts #23 & #24?

Everything you have described can be attributed to the belt slip detector circuit cutting off power to the mag clutch relay after it detects belt slippage ... this is exactly how this protective circuit is intended to operate.

What do we know with certainty?
1. We know that the mag clutch relay is functional.
2. We know the refrigerant pressure switch is closed.
3. We know the mag clutch and compressor are functional.
4. We know the freon quantity is sufficient to provide cooling as long as the compressor can be kept running.
5. We know that jumpering the mag clutch relay contacts allows the compressor to keep running.
6. We know that without jumpering the mag clutch relay, the compressor will run for ~10 seconds, but then power to the mag clutch relay coil will be cut, the relay contacts will open, the mag clutch will disengage, and the compressor will stop pumping.

What could be consistently cutting power to the mag clutch relay after 10 seconds of operation?

There could be some bizarre intermittent failure in the solid-state AC Amplifier module (which controls the mag clutch relay), some defect in the high-speed CAN bus or BEAN bus communications links between the engine ECU and the AC Amplifier, or it could be the belt slip detector circuit doing EXACTLY what it is designed to do ... disengage the compressor magnetic clutch when some belt slippage is detected.

Of these, the belt slip detector circuit seems the most likely cause.

 

[FJtest]

chrissteffen599 -

Was your AC issue ever resolved?
If so, what was the final solution?
There was lots of speculation on lots of potential causes, and more than one person would be very interested what the final outcome was.

 

[FJtest]

What other fuses or relays other than the mag clutch relay is there? I’ll check voltage too.

Please see post #37 ...

 

[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:

 

[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.

 

[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

 

[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?