LSPilot

When a/c mode is selected, I have noticed that the compressor does not cycle. On all other cars I've owned, if a/c is selected, the compressor clutch engages and disengages to maintain a set coil temperature. In my '92 LS, the compressor never disengages and the low pressure tube that heads back from the evaporator gets frost on it after awhile.

From the maintenance records, I found that the evaporator, receiver/dryer, and compressor had all been replaced a short time ago. Could they have messed up installing it or maybe given it too much of a charge? Or maybe my expansion valve or coil temperature sensor is bad. Any input would be appreciated. The reason I'm asking is that I don't want the compressor to seize up someday because the evaporator became an iceblock.

-Josh

SRK

Sounds like a low refrigerant condition. Eventually the low pressure switch will prevent the AC clutch from engaging at all. Get an AC shop to put gauges on it to determine the level.
Probably a leak developed after all the work was done.

LSPilot

Thanks SRK, that's what I would have thought, too. But the A/C is really icy cold. If refrigerant pressure is low, wouldn't that mean decreased cooling capacity? Also, why would the return line be frosting up if the charge was low? I would think in a low charge situation, with the car idling, the cooling capacity would be lukewarm. Maybe I should just stop worrying about it altogether and that will solve the problem.

SRK

You may be right if it truly is the evaporator discharge that is frosting. Normally the when the inlet frosts, it means low refrigerant. But the only check is to put gauges on it. Find a good AC shop and let them at it.

hyperopt

First order of AC troubleshooting is measuring high/low AC pressures. You can also look into the eyeglass on or near the AC dryer to get an indication whether or not the system is fully charged. Other plausible culprits are bad on unplugged thermistor used to sense evaporator temperature, and/or bad AC module.

LSPilot

I talked to the owner of a local A/C shop here in SoCal. He said there are two main types of systems, I can't remember what they are called. One uses a coil temp sensor to cycle the compressor clutch on and off and maintains a range of temperatures. The other system uses a pressure regulating evaporator valve (or something like that) and maintains a constant pressure, which corresponds to a constant temperature? Anyway, he wasn't sure about Lexus because he doesn't work on foreign cars, but he said frost accumulation after the valve is normal on the second type of system.

Does anyone know anything about this?

LILEVO

iTrader: (1)

Ever figure this out?

My AC fans are on 100% of the time. They do not shut off.

The AC coming out of the vents is 33 degrees and the low pressure pipes are all covered in frost.

I looked in the cabin air filter area and neither the expansion valve or evap were covered in frost.

It's like the AC never shuts off.

Banshee365

There ARE 2 types as you mentioned LSPilot. One that most domestics use that utilize a orifice tube with a compressor cycling switch, usually mounted on top of the dryer. These type of systems cycle the compressor to keep the right pressures in the system.

Most foreign cars use an expansion valve instead of the orifice tube. The orifice tube is a very crude, cheap way to engineer an A/C system but works extremely well. Many think they have a problem when their compressor continuously cyclces but it is completely normal. The expansion valve does the work of keeping the pressures right so the compressor just stay's running. If you're compressor starts to cycle, then start thinking about looking into some possible problems. U'r good to go!

sam12345

iTrader: (1)

I noticed that on the ls the compressor always runs when the ac is on on all my other toyotas the compressor cycles. I assumed lexus did this to give you a more constant temperature in the cabin for better comfort.

Bill Dowd

I just talked to a tech at Lexus in Tustin California and he says they are supposed to cycle

sam12345

iTrader: (1)

Bill what about the ones that are R12. I have a 92

Bill Dowd

I also have a 1991 and I was told by the Lexus tech it was supposed to cycle but 2 local A/C shops said they don't cycle I am still running R12 and I don't think my cycles I also saw a note about when the electrical fans should come on ,,mine don't come on utill I am in traffic on a hot day I also replaced my relays for the fans but I all I did was waste money the old ones are still good. Since you are in Texas it gets hot like So Cal in the desert here is a good bit of advice given by an old A/C guy go to the $2.00 self car wash lift the hood and wash out the A/C condensor top and bottom and then go to motor area and wash out the radiator DO NOT WASH THE ENGINE IT WILL CAUSE BIG PROBLEMS DO THIS EVERY YEAR..Your car will run cooler and the A/C will cool better good luck. Also always run the A/C in the winter months for about 10 minutes a week to keep the seals in the compressor soft.

sam12345

iTrader: (1)

Even if I set the temperature to 85 the compressor is still running. I still think that lexus designed it that way so that the air is not humid and the air temperature is a constant temperature rather than the cycling variety that is somewhat noticeable in the temperature. Only way to turn it off is to hit the ac button. So yes I am running the ac nearly all the time unless I conciously hit the ac button. Winter, summer, fall and Spring.
Like I said before all my other toyotas cycle (camry, 4runner, avalon) My ac was retrofitted with R134 but it is an R12 system. Also it works very well

sam12345

iTrader: (1)

From an IS forum

"The compressor in our car is not simply "engaged" or "disengaged". Rather than cycling on and off, we have a variable displacement compressor that is able to vary it's capacity depending on load, engine demand, etc. In cool weather, the compressor will run with a very small displacement, dehumidifying without exacting a large fuel mileage penalty"

https://www.clublexus.com/forums/is-...-winter-2.html

I wonder if the early ls uses this method?

sam12345

iTrader: (1)

here is another one

The Compressor on my 92 SC300 doesn't cycle on and off... Call it what you want. Drive an SC and an ES or RX back to back and you'll see what I mean. Early LS400s (had one) are the same way, as is our GS400. It is a variable displacement design on the GS,LS,SC ---no clutch cycling. ES And RX have a standard setup that cycles on and off.

https://www.clublexus.com/forums/sc-...-concerns.html