AC Compressor not running

[bcossa2001]

Hello all..1997 Toytoa Avalon, 260K, manual air conditioning. I recently replaced the evaporator coil, expansion valve, and receiver/drier. Evacuated, tested, and charged with 24 oz. of R134A, which is slightly short of the required amount of 28. Compressor will not come one. I jumped it out from the relay and it will work, but will not come on automatically. I traced it to the lack of ground at the clutch relay.
Now the embarrassing part. I discovered after i reinstalled everything that i forgot to reinstall the thermistor. After doing some reading i thought that the thermistor was used only on the Auto AC, not the manual AC; however, looking at the wiring diagram, it looks like the thermistor "contributes" to the ground on the AC magnetic clutch and sensor.

I still somewhat suspect the pressure switch, as you are supposed to be able to jump out the pressure switch and the compressor turn on, and this is not happening (i.e. when i jump out the switch the compressor does not run).

Have checked the fuses (2 - 1 at the driver left side and 1 beneath the glove box) and the heater relay (beneath the glove box) and all checkout ok. Also traded out relays on the clutch relay with no luck.

#1 question: is the missing thermistor contributing to the problem?
#2 question: anyone know what the resistance readings should be across the thermistor when it is hot/cold??



thanks to all

 

[HerrStig]

Try Toyota Nation.com There's an Avalon specific thread.

 

[kschachn]

The thermistor provides a signal to an amplifier in the instrument panel which protects against evaporator icing by cycling the clutch. So yes, with a missing thermistor signal it will prevent the compressor from engaging.

Many times you can remove and replace the thermistor without removing the evaporator. On my BMW and the ECHO that is the case but I don't know for sure about the Avalon.

 

[mk378]

Yes you need the thermistor. These thermistors have a resistance that increases as temperature decreases. Thus open circuit = very cold = compressor should not run == so it doesn't.

You can just plug it in for a test, it does not need to be touching the evaporator to make the compressor start. Of course the evaporator will eventually freeze over without feedback through the thermistor.

Trouble with the amplifier (typically just cracked solder connections on the circuit board) is also rather common.

In the manually controlled system, the evaporator always tries to cool down to the amplifier's programmed shutoff point of about 40 degrees. If the driver sets the temperature dial toward the warmer side, some of the very cold air from the evaporator passes through the heater before reaching the vents.

 

[Trav]

Stupid question I know but did you change the relay?

 

[JMJNet]

"#1 question: is the missing thermistor contributing to the problem?"

"looking at the wiring diagram, it looks like the thermistor "contributes" to the ground on the AC magnetic clutch and sensor."

You realized that you inadvertently answering your own question.
Probably not on purpose that you read the manual???

 

[bcossa2001]

Yes--I tested it and also swapped it out with another relay with no positive results.

 

[bcossa2001]

Well, I guess I was holding out hope that the thermistor was not required and looking for validation from the forum. I'm afraid I'll have to pull the dash to access the connection and wanted to avoid that...

 

[kschachn]

On my ECHO I can get to it by pulling the passenger air bag. On the Sienna I can get to it from underneath after removing the glove compartment. Maybe your Avalon is somewhat similar?

 

[bcossa2001]

When I removed it the first time I labeled it " front of the blower" which is accessed from beneath by removing the glove box and access panels. Great suggestion to remove the airbag.....I'll look into it

 

[Trav]

The FSM has it be accessed by removing the lower panel and the blower. I don't know I never did one but from the description it doesn't sound too bad but we all know how that goes.

 

[Vikas]

I am confused. Were you not able to test if fully before putting the dash back together or you need the dash before you can test the A/C ?

 

[bcossa2001]

I previously pulled the dash and installed a new evaporator core and expansion valve and then re-installed the dash. After everything was back together i discovered that i had forgotten to re-install the thermistor. I've been trying to get the AC to work and it won't because the thermistor is missing. So i have to re-install the thermistor in order to test the AC again. My hope is that i won't have to pull the dash to reinstall the thermistor.

 

[Merkava_4]

How much different is a 1997 Avalon from a 1999 Camry ?

I have the FSM for a 1999 Camry if I can be of any assistance.

 

[kschachn]

Originally Posted By: Merkava_4
How much different is a 1997 Avalon from a 1999 Camry ?

I have the FSM for a 1999 Camry if I can be of any assistance.


It may be quite a bit different. It certainly is different than my 1999 Sienna since that doesn't require the removal of the instrument panel to replace the evaporator. What I saw online for the Avalon seemed to be a complete panel removal.

 

[bcossa2001]

Merkava that is a very kind offer. I am going to remove the glove box, etc. this weekend and see if I can install it.

I've checked it in ice water and the resistance increases as it gets colder but i have no way of knowing if the resistance values are correct. I'm wondering that if i have to go to the trouble of pulling the dash if i should buy a new thermistor since it is 20 years old...

 

[Merkava_4]

You're probably going to need service information if you're going to pull the dash. It'd be nice to know where all the fasteners are ahead of time. I'd suggest the DIY one year Alldata subscription. Gives you access to Toyota service information right out of the factory service manual.

 

[nthach]

Originally Posted By: Merkava_4
You're probably going to need service information if you're going to pull the dash. It'd be nice to know where all the fasteners are ahead of time. I'd suggest the DIY one year Alldata subscription. Gives you access to Toyota service information right out of the factory service manual.

Toyota TIS is also available to DIYers - $15 for a 2-day(starting at 12AM) subscription. Alldata sometimes has inaccurate/missing info.

 

[bcossa2001]

Well the good news is I pulled the glove box and associated panels and the thermistor connection was staring me in the face--i had even marked it when i removed it the first time.
Bad news is I reconnected it--still no compressor. I jumped out the connection--still no compressor.
AC gauges reading 75 on the low pressure side and about 100 on the high pressure side--no change from last time. I jumped out the compressor again and the low side kicked to about 50 and then slowly crept back up to 70.
I jumped out the pressure switch--all the radiator fans came on but still no compressor. I guess i still suspect the ECU is not sending ground to the relay....
AARGH....

 

[Trav]

Is there a separate coolant temp switch somewhere in the cooling system for the A/C? I ran into this with a VW some years ago and it was just a small 2 wire $15 sensor in the upper hose connector.
I will look at the wiring diagram later tonight.