What inside the cab tells the AC Compressor to engage.

[Eddie Haskell]

I have no AC and have done some research and it looks like everything after the under-hood relay is working correctly. That tells me something under the dash isn’t telling the clutch to engage and run the compressor. I knew I would have to look into this once the weather turned warmer and I know I have ran across several “things to try” suggestions on other posts regarding AC.Trying to go back and find them again can truly be a struggle. I did search because I hate posting what end up being redundant threads but…
Vehicle is in my sig
Thanks

 

[BlackWorksInc]

I mean the AC Amplifier/HVAC control panel are what command the clutch to cycle... but that can be affected by a couple things including the pressure sensor for the refrigerant lines. Are you sure there's an appropriate amount of refrigerant in the system?

Additionally, when you say "everything after the relay looks fine" do you mean that you removed the relay and manually jumped the pins to see the compressor engage? How did you test it?

Also I don't see your signature or vehicle listed... is this a V8 perchance? I have seen (and have made this mistake once or twice in my career) where the ground strap for the clutch circuit wasn't screwed down when doing a timing belt (little wire can run away and hide sometimes, I swear!) and that will prevent the clutch from engaging even when commanded.

 

[Eddie Haskell]

Yes, I jumpered across the relay with the ignition on, engine off and I hear and see the clutch engage. I left the jumper in place and drove and the AC did appear to be working but not very cold. I fully anticipate having two issues, one with refrigerant and one with the system itself mechanically.
The clutch for the compressor does not engage at all unless I bypass the under hood relay. I have tested the relay and it tests normal, I am 99.9% sure it isn’t the relay. In my opinion, whatever tells the system that the driver wants AC appears to be working properly but in reality the under hood equipment never gets the message.
Now I need to figure out why the system isn’t turning on, I have read about some under dash sensor that may be bad but like I say, I can’t find the posts I want to refer to.
I try to subscribe to threads I need but the ads on this site prevent me from accessing anything under “Thread Tools” in the tool bar, very frustrating at times.
Yes, on the model; V-8, 2004 Sport Edition

 

[Syr4RinNC]

Here was my experience

AC Amplifier replacement 4th gen 4runner

 

[Eddie Haskell]

Here was my experience

AC Amplifier replacement 4th gen 4runner

I read that, very good write up BTW. How did you diagnose the AC Amplifier as being the issue, was it the issue?

 

[BlackWorksInc]

Yes, I jumpered across the relay with the ignition on, engine off and I hear and see the clutch engage. I left the jumper in place and drove and the AC did appear to be working but not very cold. I fully anticipate having two issues, one with refrigerant and one with the system itself mechanically.
The clutch for the compressor does not engage at all unless I bypass the under hood relay. I have tested the relay and it tests normal, I am 99.9% sure it isn’t the relay. In my opinion, whatever tells the system that the driver wants AC appears to be working properly but in reality the under hood equipment never gets the message.
Now I need to figure out why the system isn’t turning on, I have read about some under dash sensor that may be bad but like I say, I can’t find the posts I want to refer to.
I try to subscribe to threads I need but the ads on this site prevent me from accessing anything under “Thread Tools” in the tool bar, very frustrating at times.
Yes, on the model; V-8, 2004 Sport Edition

I would first verify that there is enough pressure/refrigerant in the system, if the refrigerant amount is too low it will not bother to cycle the compressor at all because the pressure sensor/switch will not be active. Does the A/C light flash after you turn it on? That usually indicates a fault in the system and one of the first steps in the FSM is to verify refrigerant amount.

 

[Syr4RinNC]

I had eliminated all the other common causes and so I was left with the AC amplifier/ HVAC control module vs the speed sensor and the evaporator core. The HVAC control module was the easiest and cheapest next step so I went with that


- a few months ago, the AC light started blinking and system wouldnt come on. I identified a bad mag clutch and had the compressor replaced by an auto shop earlier this week. They said they charged it up, but couldnt get power to it.
- I took it home, figuring, Id try to handle it if it was something easy (like the relay)
- A new mag clutch relay didnt help and using the fog light relay doesnt work
- The low pressure refrigerant reading is 40 PSI
- The system works if I bypass the relay in the fuse box with a paperclip (clutch engages and blows cold air).
If I disconnect the pressure switch, the AC light stays on, but blows warm air. If i reconnect, it blinks and nothing
- So I have a new compressor (denso OEM part), the refrigerant is filled to adequate pressure, and I can get power if i jump the relay. This seem be the major components
- I checked all the electrical connections to the compressor and the pressure switch and all seem fine.

I thought I was left with the thermister (AC evaporator temperature sensor) or the coupler that ties the compressor to the engine RPMs

Heres another thread

AC light flashing, relay is good

 

[Eddie Haskell]

There are so many things that can potentially be wrong with this HVAC system! I am now reading and learning that the clutch will not engage the compressor if the refrigerant level is too low, just as you stated BlackWorksInc. I will find some gauges and go from there on that end. I did the HVAC diagnostics and it gave me a 23 Code which indicated that one of the possibilities is “Refrigerant Pipe Line” but there are others including AC amplifier and or a pressure switch. The refrigerant level will answer many questions. Thanks, I will keep updated

 

[Eddie Haskell]

BTW, the AC light is NOT flashing, I just have no AC and I am trying to figure out why and will start with checking refrigerant level.

 

[Eddie Haskell]

So can someone educate me? I am going to check/refill refrigerant (if needed). If my compressor won’t run as it is, am I to assume that if I connect a bottle and open the valve), somehow it will flow by itself or will I have to jumper across the relay to make things happen?
I intend to have the manifold pressure gauges and depending on what they tell me, act accordingly. I just don’t know how I would add refrigerant if the compressor isn’t running…

 

[AuSeeker]

So can someone educate me? I am going to check/refill refrigerant (if needed). If my compressor won’t run as it is, am I to assume that if I connect a bottle and open the valve), somehow it will flow by itself or will I have to jumper across the relay to make things happen?
I intend to have the manifold pressure gauges and depending on what they tell me, act accordingly. I just don’t know how I would add refrigerant if the compressor isn’t running…

The compressor needs to be running to add Freon.