A/C System Cycling

Perhaps you won't like to hear this; but as long as the inside air is cold enough, it is normal; It may not discuss it in the user manual but in many cases the ac clutch will kick out to keep the evaporator from freezing and to keep liquid refrigerant from cycling back to the compressor.
This is true on all toyota's I've owned, Sienna, Tacoma, Corolla, Highlaner and Rav4, except those with fully automatic air-conditioning. It it's DOESN'T Cycle off, alot of time there is a partial loss of charge, OR it's so hot/humid that it's working hard to keep up.
On 15/off 5 sounds like normal.

 

Just bought a 2013 that had 24,000 miles on it and now know why they sold it. The AC does exactly the same cycle thing and will not come close to keeping the cab cool if it is above 85ºF. The compressor stays on for less than 5 seconds and cycles off for another 12 seconds. It makes no difference where the temp is set nor what fan speed is selected. The charge and wiring have all been checked by other than a Toyota dealer and they say it is not normal but do not know how to fix it. The problem will put the truck back on the sellers block soon if there is no resolution. It is not fun in Texas summer heat.

 

There is probably an issue with the Thermistor at the evaporator coil, if the charge is correct and it is not low on refrigerant...

 

I agree ^^ but posting the gauge pressures would be helpful.

 

You are quite correct. Too many unknowns at this point.

 

Cycling is a sign of a GOOD A/C design. I have not seen a compressor run continuously in 25 years or so now. Much older vehicles did so, at the expense of fuel economy. We have three vehicles at the moment, my Tacoma, my wife's 2011 honda accord and my son's 2011 chevy Malibu. ALL cycle on and off as the high pressure side demands. If it doesn't cycle off, it is likely low on R134a.

 

"1987" says it all. First, that is using R12, not R134a. Second, in 1987 nobody was concerned about gas mileage so running the compressor non-stop was pretty normal. But it wastes power (and therefore gas). A home A/C doesn't cycle the compressor because it would cost a fortune to spin it up as opposed to letting it run continuously. But for a truck, energizing a coil is low-cost compared to running the compressor 100% duty cycle.

I have an 07 4.0 and it cycles, as it should. That's why your truck has a high-pressure cutoff switch. When the liquid line is "stuffed" it can continue to cool even with the compressor turned off. As the pressure drops, the compressor comes back on.

As far as freezing goes, if the suction side gets below 40 psi, R134a will boil at a temperature that will freeze water. Which you don't want. However, freezing a taco is hard since it uses a thermal expansion valve which uses a sensor on the suction line to measure evap core temp and keep it above the freezing point by cutting the flow of freon through the evap core off. Many vehicles (Ford for example) use an orifice tube and just spray freon non-stop. The A/C will still cycle on/off (the compressor, NOT the fan) but if the suction side gets too low, it will freeze in a heartbeat. Better home A/C systems use the Tx valve approach so that they don't freeze when the refrigerant (R22 or R410a) charge is low.

 

I'm having a hard time wrapping my head around this. Are you saying normal operation is cycling the comp with the HP switch? If so, it seems to me that on a hot day, with a hot cab, the AC system would never have a chance to cool down the cab. What pressure does the HP switch cut out at? I can find the 'normal' range, but not the 'cutout' pressure.

I'm not saying you are wrong, I just want to learn more. I've been in the biz a long time just like you, and these newer systems are a PITA.

 

That is correct. The Tx valve leaks R134a into the evap core, and monitors the outlet (suction) line temp where it exits the evap core, to (a) prevent too much freon from spraying which would drop the temp below freezing and cause problems and (b) prevent a low charge from causing the evap core to freeze by again monitoring the suction line temp to keep it above freezing. If the Tx valve says "no more freon" because you have the fan on low and the heat exchange is reduced, what do you do with the pressure build up? You shut the compressor off until the pressure drops. If you run with the fan on high, you might never trip that limit switch. But I generally run on the speed right above low (I live in Birmingham and it is HOT here) and it cycles regularly. You can raise the hood and watch the clutch engage and release. If you hook up gauges you can watch the high pressure raise and lower in sync with the compressor running and not running.

I will add that some A/C systems directly monitor the evap core temp and use that to directly stop the compressor. I've not seen any such sensor in my Taco, although I have not looked real hard. But the service manual doesn't show anything other than the high/low limit switches and the Tx valve which is purely mechanical in operation.

No idea what the pressure switch limit is, I'd guess somewhere in the 300psig range. but the cycling is 100% normal.

My main complaint with new systems is the 2x higher pressure compared to R12, which blows out evap cores and everything else much faster... An R12 system will produce freezing air at idle. R134a won't unless you park behind an air boat and let 'em run the prop up to 60-70mph...

 

The following is from the Service Manual and it does show a Thermistor to control the compressor:

Here is the Thermistor that controls the compressor clutch:




Here is the description of operation:

 

There you go, although there's no telling on which years this was done or not done. Does seem a bit redundant to have that thermistor AND the Tx valve hardware.

 

With out pressure readings every thing is a guess if the low side drops too low it's going to cycle on low pressure if it does not drop the low side to the cut off point then that is the way the system works.

 

This^^ I was going to copy this from the pdf I have saved that Jimmyh posted a while back (thanks Jimmyh!) The statement seems to say that the comp is only controlled by the pressure switch in an abnormal situation. There is no AC thermostat like the olden days . The only thing left to control the comp is the thermister. Cab temp is controlled by the blend door, right?

One question to the OP @Shadowmyst . Is the truck cooling properly or not? Or are you only concerned because the comp cycles? Make that 2 questions.

 

I know on my 88 Toyota Camry it used a Thermistor also.
The print I posted from is a 2005 Tacoma Service Schematic.

I just checked the prints for my 2012 Tacoma and it shows the same.

They are all operating off of the Thermistor control sensing the evaporator temperature. Except for abnormal conditions in which they use pressure.

Better?

CurtB: Yes, the actual temperature you feel at the vent is controlled by the blend door servo.

 

Ehrm, pardon me, but the thermistor is actually the sensor *component* of the thermostat.

 

Not in this case. It is simply a low temp cutoff switch. It is not connected to the temp control. The blend door controls the cab temp.

 

What do you think a thermostat is?

 

I think the two of you are quibbling about semantics. The thermistor simply shuts the compressor off if the evap core gets too cold. Has nothing to do with regulating cabin air temp so it is like a thermostat, but one that can not be adjusted whatsoever. To adjust cabin air temp, you blend hot and cold air together to get the desired temp. Not very efficient, but that's the way they work.

 

A thermostat need not be adjustable to be a thermostat.
A thermostat need not regulate CABIN temperature to be a thermostat.
It doesn't matter HOW you achieve the desired temperature in the CABIN, this has no relevance to understanding the function of the circuit to which the thermistor is attached.
This is not a matter of semantics at all, it is a matter of DEFINITION.

 

I think so too. In my earlier post about the 'old days' I was referring to an actual thermostat that controlled the compressor AND was adjustable by the user. Think 60s, 70s. One wire from the thermostat to the compressor. Troubleshooting was sooo much easier then.