A/C shuts off at higher elevations (1 Viewer)

[sl3807]

My air conditioner seems to die whenever I get to the mountains. First a little history:

- Bought the LX back in April, 137K miles.
- New Aisin Blue Hub Fan Clutch with 10,000 cst oil.
- New Belts
- New Radiator Cap, Hoses, PHH & FHH's. Coolant system flushed by dealer, filled with Toyota Red.

I noticed that the a/c was cutting off when I first took the truck to the mountains. Originally thought I had an overheating issue, which turned out to be the radiator cap. I now have a OBD2 scanner and have kept a close eye on temps.

Driving around town in stop and go traffic, 90+ deg, a/c blows cold and I never see the coolant temp break 196. Last weekend while wheeling, temps looked great, but A/C only blew warm air. As soon as we got back on the highway and made it down to a lower elevation, A/C started blowing cold again.

Any thoughts? My only idea is the expansion valve, but I know next to nothing about the A/C system, so it is just a guess.

 

[mlv289]

You just need to get keep driving higher. At some point you won't need it.

But actually, it probably is somewhat normal. I think R134a runs at higher pressures, therefore more heat is produced at the condenser coil. Thinner air at high altitude cannot cool the condenser coil as efficiently, therefore higher discharge temps. So, with the age of the system (is it mostly original?) you probably see more of this.

 

[Coolstream]

While I would guess the expansion block might be the problem, if the system hasn't been serviced, I'd do that first since it could just be low on refrigerant. If you're climbing extended mountain passes where you're deep into the throttle, that can also shut down the compressor.

 

[sl3807]

If you're climbing extended mountain passes where you're deep into the throttle, that can also shut down the compressor.

Happens at altitude regardless of speed or whether I'm ascending/descending .

I guess I will take it in next week and get the whole a/c system checked out.

 

[cruiserdan]

What engine temps (scanner) are you seeing when it cuts out?

 

[canman]

A quick check of the Electrical Wiring Diagram manual for the LX450 (1997) states the following:

If the a/c automatic amplifier detects the following conditions, it stops the air conditioning.
1. The temperature at the vents is low.
2. There is a marked difference between compressor speed and the engine speed.
3. The refrigerant pressure is abnormally high or abnormally low.
4. The engine speed decreases.
5. Rapid acceleration occurs.

I have 10 plus years experience designing vehicle HVAC systems, but it been a long time since doing so, but I can comment on some of the above points.

1. All mobile a/c systems have a means to disengage the compressor clutch when evap temps approach 32F in order to prevent condensate from freezing in the evap. This is likely not the cause in your case.
2. Never heard of this before, and I'd like to know how comp speed in relation to engine speed is determined. Maybe this is the problem with a loose belt even if you just installed them.
3. Again, all a/c systems typically have a low and high ref press switch to prevent damage to comp from excessively low or high ref press. When wheeling, I assume your going very very slow. This means low airflow through condenser which could mean high ref press if hot outside and warm inside truck. Then sys might disengage comp clutch. At same time, comp speed is low, and a/c capacity is proportional to comp speed given all other variables are equal. So sys could be "working", but you don't feel "cold" air because, there is insufficient cooling effect at condenser due to low airflow due to low engine speed as well as low comp speed/ capacity due to low engine speed.
4. Not sure what this is about and why.
5. This is designed to protect comp clutch from excessive slippage when engaged typically at Wide Open Throttle (WOT).

 

[western flyer]

A quick check of the Electrical Wiring Diagram manual for the LX450 (1997) states the following:

If the a/c automatic amplifier detects the following conditions, it stops the air conditioning.
1. The temperature at the vents is low.
2. There is a marked difference between compressor speed and the engine speed.
3. The refrigerant pressure is abnormally high or abnormally low.
4. The engine speed decreases.
5. Rapid acceleration occurs.

I have 10 plus years experience designing vehicle HVAC systems, but it been a long time since doing so, but I can comment on some of the above points.

1. All mobile a/c systems have a means to disengage the compressor clutch when evap temps approach 32F in order to prevent condensate from freezing in the evap. This is likely not the cause in your case.
2. Never heard of this before, and I'd like to know how comp speed in relation to engine speed is determined. Maybe this is the problem with a loose belt even if you just installed them.
3. Again, all a/c systems typically have a low and high ref press switch to prevent damage to comp from excessively low or high ref press. When wheeling, I assume your going very very slow. This means low airflow through condenser which could mean high ref press if hot outside and warm inside truck. Then sys might disengage comp clutch. At same time, comp speed is low, and a/c capacity is proportional to comp speed given all other variables are equal. So sys could be "working", but you don't feel "cold" air because, there is insufficient cooling effect at condenser due to low airflow due to low engine speed as well as low comp speed/ capacity due to low engine speed.
4. Not sure what this is about and why.
5. This is designed to protect comp clutch from excessive slippage when engaged typically at Wide Open Throttle (WOT).

And here I always thought #5 was to remove parasitic load (comp) so you could accelerate just that much faster.

 

[canman]

And here I always thought #5 was to remove parasitic load (comp) so you could accelerate just that much faster.

That's true too

 

[sl3807]

What engine temps (scanner) are you seeing when it cuts out?

Temps were in the 193-196 range. I did see the temps rise above that, but it was never for more than 30sec, never exceeded 203.

 

[cruiserdan]

Temps were in the 193-196 range. I did see the temps rise above that, but it was never for more than 30sec, never exceeded 203.

Not high enough to trigger compresser cut-off (~226 degrees) unless the cut-off temp sensor is bad.

 

[alia176]

Temps were in the 193-196 range. I did see the temps rise above that, but it was never for more than 30sec, never exceeded 203.

It'd be interesting to see if the clutch was engaged during one of these episodes when you're at high altitude and getting warm vent air. At least this way you can narrow down your troubleshooting steps.

 

[sl3807]

Quick Update:

I got another chance to take the truck to higher elevations 2 weekends ago. On the first day, we went from Denver up over Georgia Pass (~11,500ft), down into Breckenridge, back over Boreas Pass, and on to Buena Vista. We had no A/C after Georgia Pass.

One thing that I did notice, when I did try the A/C, very little air was coming from the vent, and the noise from the blower was substantially louder. This led me to believe that the evaporator my be completely frozen, restricting the air flow. If this is the case, what would cause this? Low pressure in the system? I still have not had time to take it to a shop and have the system checked, but it is on the to do list.

 

[Coolstream]

One possibility would be the thermistor, who's job it is to let the amplifier know that the evap core is freezing, then the amplifier should shut off the compressor.

 

[sl3807]

Another Update:

I have replaced the evaporator, expansion valve, drier, and any o-ring that I touched. Had the system charged and checked for leaks. Everything checked out.

Went up to Walden Sand Dunes over the weekend. We had cold air all the way up Poudre Canyon. We stopped at the dunes for a few hours then headed back home. Turned on the A/C and got some cool air at first, then just ambient temps. Tried driving with just fresh air and no A/C for about an hour, still no cold air when the AC was turned on.

When we got back to town, I stopped to check on the compressor. As far as I could tell, the compressor was engaging correctly. The weird thing, I shut the truck off for ~5 minutes, and on start up we had cold air. WTF???

My mechanic has suggested that it could be the amplifier, but I am hesitant to start throwing parts at it. My next step will be procuring a set of gauges, that way the next time this happens hours from home, I can through on a set of gauges and really see what is going on. I can not duplicate the problem driving around town.

 

[Qball]

3. The refrigerant pressure is abnormally high or abnormally low.

Maybe your refrigerant pressure is too high, when going higher elevation the outside air pressure drops and inside pressure raises. This definitely happens to tire pressure but not sure about refrigerant.

Just a thought, maybe it is a bit over charged?

 

[60wag]

You've got the auto climate control right? I'm not familiar with the exact Lexus setup but many cars use several sensors to provide input to the system. Maybe a malfunctioning or dirty sensor in the cabin is the problem.

 

[sl3807]

3. The refrigerant pressure is abnormally high or abnormally low.

Maybe your refrigerant pressure is too high, when going higher elevation the outside air pressure drops and inside pressure raises. This definitely happens to tire pressure but not sure about refrigerant.

Just a thought, maybe it is a bit over charged?

I think I am going to have to get my own set of gauges so I can check this. I trust my mechanic, he is the head tech at my local dealership, as well as the founder of our local club, but you never know...

You've got the auto climate control right? I'm not familiar with the exact Lexus setup but many cars use several sensors to provide input to the system. Maybe a malfunctioning or dirty sensor in the cabin is the problem.

Yes, the LX has the auto climate control. I am not sure if there are any different sensors compared to the LC.

 

[Flank]

3. The refrigerant pressure is abnormally high or abnormally low.

Maybe your refrigerant pressure is too high, when going higher elevation the outside air pressure drops and inside pressure raises. This definitely happens to tire pressure but not sure about refrigerant.

Just a thought, maybe it is a bit over charged?

x2 on this as a possiblilty - the system will have a high pressure cut off switch, the elevation change or excessive heat on the high pressure side could trip this, perhaps.