Troubleshooting HVAC

I am trying to get my ac working. I just had the system charged but the tech told me there is an another issue stopping the compressor from engaging. I checked the magnetic clutch and it clicks when I hit it with 12v directly.

With the motor running and the fan on, the connector to the clutch does not get 12v when the dash button is on. The dash button does light up. The 10a fuse in the under hood box is good. Are there other conditions that must be met to fire the clutch?

When the system had a residual low charge in it the button would light, and then when it was drained for another repair it would not turn on so I assume the pressure switch is good.

 

Expansion valve possibly

look a few threads down
Another member has similar issue

 

This one

https://www.tacomaworld.com/threads/ac-failure-help-me-understand-gauge-pressures.858050/

 

Hey thank you for replying, I just replaced the EVAP core, expansion valve and the pressure switch.

 

Expansion valve won't prevent the compressor from running.

I need to look up the diagram but if I remember correctly the A/C Amplifier needs 4 inputs to be correct to turn the compressor on.

A/C button on.
High pressure cutout switch closed.
Evaporator thermister reading above 34F.
A/C enable signal from the ECM.

 

Ugh

did mechanic say anything about the gauge pressure readings?
(Neighbor did mine….im not expert on H and L gauge readings)

 

Dm93 with the knowledge!!!!!! Yes Sir!

 

Ok this is super helpful, thank you.

A/C button on. I can see this one.
High pressure cutout switch closed. I can check this with a multi meter for continuity that its closed.
Evaporator thermister reading above 34F. Not sure what to do with this other than assume its working.
A/C enable signal from the ECM Going to have to research how to test this one as well...

I read about an a/c relay behind the glove box. It that where these signals terminate to enable the clutch?

 

There is no A/C relay on 1st gens, on 03 the A/C Amplifier is built into the A/C Controls and controls the compressor clutch directly.

The Thermister is probably the easiest thing to check.

 

Would the a/c button turn on if the pressure switch was not working? I'm going to test it, I'm just curious.

 

That I don't know, 2nd gens do but I'm not sure about 1st gens.

 

ok thank you, reason I ask is I don't remember plugging the harness into the switch after I changed it. I don't see how I couldn't have. I may try and scope it just to verify.

Edit: oops, all I have to do is check for continuity with a charged system right? If there is more than 28 psi it should be closed. If its open, either the switch is bad or its not connected...

 

Correct.

 

Well dang. I checked both the thermistor AND the pressure switch for continuity and they are both open. Temp sensor read as .77 ohms at 80F so I'm guessing its shot. I checked the old pressure switch and it read as open sitting on the bench. AC-81 is a little confusing though...should it read as closed with greater than 28 and less than 455 psi? The test is supposed to be done with the system on and running, which I can't do.

 

Are you sure you read your scales correct for the thermister and it wasn't 0.77K ohms (770 ohms)?

The switch will be open with no pressure (below 28 psi) or if the pressure goes above 455psi, this is by design to protect the compressor.

Can you jump out the pressure switch and see if it turns on?