A/C quit, no blink light , compressor doesn't come on

About three weeks ago my 07 4WD, V6 Manual Trans Tacoma's AC went out.
Note - Although when I started debugging this issue I had no blinking A/C light on the dash , once I jumper-ed the AC mag clutch I now do have the blinking light (more on that below).
I put manifold gauges on the AC lines and saw both the low pressure and high pressure readings were at 25 PSI. I thought that was too low (given it was in the 90's F) so I decided to add a dye-charge (4 second blast of dye and R134a from a can I picked up at the local Auto-Zone store). This charge was added through the low-pressure side. I then added about 3 oz of pure R134a with the vehicle running thinking the system needed more refrigerant. This brought the pressure up higher than I really wanted (100 on both low and high) and still the compressor didn't kick-on. It does concern me that both high and low sides increased given the compressor didn't come on. I only had the low-pressure side gauge open. Maybe there is a stuck expansion valve? Does that dash have to be removed to get to that valve (assuming I have my mechanic evacuate the system of R134a) ?

So now I first am debugging the lack of compressor turn-on and thought I should start at the relay box.
(1) I pulled the MG CLT (mag clutch relay) and probed control pin 2 and load pin 5 with a test light hooked to battery ground and the ign. key in the normal run position. The A/C button on the climate control was engaged. Those pins both show power (test light lit).
(2) I then switched the test light so it was connected to battery positive and probed MG CLT /(load) pin 3 which again lit the test light. I believe that tells me I have connectivity from the pin 3 through the wiring harness and compressor coil back to ground (which is desirable). However, I would have thought I would hear the compressor clutch click but perhaps the test light itself is the low resistance path . Regardless the coil appears to be able to pass current.
(3) I then used a paper-clip (with the same truck interior setup but w/o the test light) to jumper pin 2 (effectively battery positive) to pin 3 (the load-side of the relay). Indeed the clutch click occurs so it at this point I am refocusing on the control circuit.

I'd like to now assess the inputs to the AC amplifier (module) which collects signals from the pressure switch (there is at least one on the low pressure side) and the evaporator sensor.
I believe I can just use a multimeter to check the pressure switch Ohms but the evaporator sensor is supposed to be checked via a scan tool. Is there a way to backprobe this reading elsewhere in the car/dash?
Beyond that what would be a reasonable next diagnostic step?

 

Sounds like the system has a leak since your pressure was very low, it's normal for the low and high side to be equal when the system isn't running.
The static pressures should be at the saturation pressure for a given temperature if the system has at least enough charge to turn the compressor on, for a 90 degree day that would be around 104 psi.

The system pressure has to be high enough to close the dual pressure switch (above about 30 psi) before the A/C Amplifier will command the MG CLT Relay on.

The fact that you don't have a flashing A/C light and the clutch is not turning on tells me the A/C Amplifier isn't seeing the correct conditions to turn it on.

Have you tried running it with everything connected since you added refrigerant?

 

If you have equalized pressure on the high and low side, it ain't compressing no matter how much refrigerant you put in it. It [compressor itself with the clutch engaged] may be spinning but it isn't doing anything past that. Or, the compressor still isn't spinning even though you think it is. FWIW, liquid doesn't compress.........

 

Apologies - I edited my original post near the very beginning text to state that once I jumper-ed my compressor I now do get the blinking A/C light. Unfortunately the WebMaster settings prevent me from editing the post's title to reflect that change..

When originally I added the three ounces of R134 (with the engine running) I saw the pressure increase to 100 psi for both the low and high pressure sides (from 25 psi each) . However the clutch on the compressor never engaged so I was surprised to see the high pressure side go up - I thought for that extra refrigerant to end up in the high-side the compressor would have had to suck it into the high-side. This is what is leading me to suspect the expansion valve is stuck open, i.e. it is bleeding backwards through the expansion valve to the high-side.

Regardless, now given there is 100 psi throughout the system I would thought that'd have satisfied the pressure switch (assuming the switch is good).
I have now put a multi-meter to that high-side pressure switch (with everything OFF and disconnected electrically from the rest of the truck) and found it has continuity between the pins but is not registering any resistance (i.e. is registering on my multi-meter no differently than just touching the test probes directly together which is 0.8 ohms). At this time this is what I could scrounge up for a 2004 Camry on a similar switch (though my switch is not dual purpose as it has only two pins/leads):

if it is a dual purpose switch, assuming the 28psig side is NO
(normally-open) and the 430psig side is NC (normally closed) then
look for continuity on 2 pairs of pins.
Place positive OHM meter lead on the #4 terminal(6 oclock) and
negative lead on the #1 (twelve oclock). There should be continuity
between 28 psig and 455 psig. Above or below those pressures, no continuity.
You should see ohms across 2 of the 4 pins, looking for
between 165 and 260 ohms otherwise the sensor has failed​

The write-up above is a bit confusing to me but seems to set the expectation there should be some resistance
with a pressure of 100psi. This makes me think the switch is bad ...
The switch isn't cheap ($140 new) so before i declare I need to replace it I'd like opinions...
​

Beyond that:
After all of the relay probing I re-installed the Starter relay and horn relay (which I had removed simply to make it easier to remove the mag clutch relay). I did not re-install the mag clutch relay and with the engine running the I again jumper-ed the mag clutch load circuit and this engaged the clutch.
Now with the compressor running the low and high side both rose together to 125 psi. As soon as I remove the jumper the mag clutch disengages and the pressure starts dropping back to 100. This behavior is not at all what I am used to on the AC of my Subaru. There is of usually a drop in pressure on the low-side when the compressor clutch engages as it is sucking on the low-side. Here I am seeing the low-side pressure increase.

But for now I am focusing on the mag clutch of the compressor not engaging. I am currently focused on the AC Amplifier inputs:

So my questions at this point are:

1. Does anyone know the specific resistance reading for a 2nd gen tacoma high-side ac pressure switch?
   
2. Given the evaporator sensor is supposed to be checked via a scan tool. Is there a way to backprobe this reading elsewhere in the car/dash (i.e. from a connector accessible from under the dash)?

As to the expansion valve:
Does the dash have to be removed to get to/remove the expansion valve (assuming I have my mechanic evacuate the system of R134a) ?

 

So now with everything connected the A/C light flashes when you try to turn it on but the clutch does not engage even for a bit?

Something isn't right there, even a bad expansion valve wouldn't cause a pressure increase on the low side. Are you sure you had your valves closed on your gauges when you ran it?

The switch should be closed (0 resistance) when the pressure is in range.

You can check it at the 2 pin connector behind the glovebox.

No I can be removed from the engine side through the firewall without removing the box.

 

Correct, clutch never engages with the MG CLT relay installed. I checked pin 1 (control-side) with the relay removed and never see even a brief presence of continuity to ground (I read the AC amplifier is supposed to ground pin 1 of the relay to energize the control-portion of the relay. There is positive battery voltage at pin 2.

I believe so. I set it up that way and then double and tripled checked the manifold high-side (red) gauge control was closed. Even so at this point when I jumpered the compressor to run I see both sides with a pressure increase which is wrong, in my opinion). Again measured with both knobs on the gauge set closed.

I have zero resistance (effectively). I checked the wire harness pigtail for the switch and it has 12 V's present.

Pin 2 of the AC Amplifier or another connector? I don't have great documentation for the 2nd gen Tacoma. The best I've found (for this problem is in the AC section of https://www.tacomaworld.com/threads/2nd-gen-information-diagnostics.784008/

 

Continuity is not a valid test on a live circuit, you need to be checking voltage or using a test light between battery positive and the relay control pin (pin 1).
If you still have nothing then we need to check at the A/C Amplifier Pin 21 (Green).

You should never see an increase in pressure on the low side when the compressor turns on, that's really weird and I can't say that I've ever seen that happen. I've seen them nearly equal with a failed compressor but not where they both increase.

Being you have a flashing light and not clutch engagement I'm not worried about the pressure switch or Evaporator Temp Sensor because it wouldn't be trying to turn the clutch on if those weren't in range.

 

When I checked pin 1 of the control-side (at the empty MG-CLT-relay position) I used my test light against pin 1 and the light hooked to battery positive - the light never came on.

I think at this point I will have to do some lower dash dis-assembly to expose the AC Amplifier and its connector. I can disconnect the connector and the connectors at the other wire harness branches that feed the amplifier. I will then perform some continuity tests for the wire harness(s). Perhaps there is a broken (or nibbled on) wire

I agree. I don't know much about expansion valves but I read this type of pressure reading can be the result of a valve that is stuck open or one with a leaking diaphragm. If I don't have another explanation for the unusual pressure readings I may have to pull the system apart and clean (or replace) the expansion valve. Sadly, not a trivial job...
I'll update the thread with my findings... hoping for something to jump out of the woodwork that says 'fix me...I'm broken...'
.

 

Not really any disassembly required to access the A/C Amplifier, just take the glovebox and maybe the cover behind the glovebox out. It's behind the glovebox straight up.
You can also check the control wire at Connector IA1 Pin 31 (Green) behind the driver's kick panel, I just went through this with someone else on an 07 a couple weeks ago.
https://www.tacomaworld.com/threads/2007-tacoma-a-c-blinking-light.834279/

Let's solve the flashing light/no clutch engagement first but I can't see any way for both pressures to rise at the same rate even if the expansion valve was stuck. With the location of the service ports I don't see any way for that to happen unless the line was plugged between the low side port and compressor which shouldn't be possible.
I'd really like to see a video of these gauge readings.

 

lol - I have been reading (and re-reading ) that post for this entire past week. It has helped me tremendously.

I can certainly get something posted pretty quick.

 

From the spreadsheet generously provided by Dm93, at the connector:
IA1 Terminal ID Wire Color Circuit Description
Pin
31 MGC Green A/C Relay Control from A/C Amplifier (A14 Pin 21) to MG CLT Relay Pin 1
Is there a layout/diagram for the pin assignments of the AC Amplifier or is this wire located solely by wire color?

I have located the A/C Amplifier just behind the passenger air bag (above the blower assembly visible once the glove box is removed).
It is awful to access unless your as twist-able as a Gumby doll.
Also I plan to make a video of the gauges today...in 100 deg heat!

 

Here's the pinout, the end pin number is also stamped into the connector body at both ends of the top and bottom rows if pins.

 

It took several days of fidgeting with the connector wires b/c it is so awkward to work on the A/C amplifier connector (upside-down with virtually no service loop in the cable harness). I've attached a photo of the connector to assure readers that it is the right one (at least by its location above the glove-box, it is a match to the diagram Dm93 supplied and wire colors).

• At this point something has changed (for the better) because when I began this whole enterprise the compressor wouldn't kick on. I previously wrote that at that time pressures were 25 psi on both the high and low side. After adding dye and 3+ ounces of R134 the pressure was now 100 psi (both low and high) yet the compressor would still not come on.
• At that point I could only get the compressor to come on by jumper'g it at the MG CLT relay (relay removed) but not when I let the truck try to do it itself (with the relay installed again). What has changed is that now the truck can turn on the compressor itself. I suspect that after I had added dye and R134a I might have had the pressure switch disconnected and failed to notice I hadn't re-connected it. Stupid me but at this point it is re-connected and so I think the controls are working aok.
• With everything (relays/connectors) attached I still have my backprobes attached to A14/pin-1 (Gnd), A14/pin-21 (MG-CLT) and A14/Pin-5 (PrSw, pressure switch) (A14 being the A/C amplifier connector). See photos.
  
• The other change is that the manifold readings are now different when the compressor runs than I first reported. The initial pressures after that additional R134a were 100/100 (lo/high). Today they are 90/100 so that is perhaps a leak. When the truck turns on the compressor the low-side pressure drops very quickly to 0 and the high-side pressure remains at 100. I don't run for very long as I don't like the high-side pressure staying at 100. it should increase. When I shut down the engine the low-side returns to 90 and the high side stays at 100. So something is still wrong. I tried to upload a video of the gauges but the site will not accept video uploads so I have a before-photo of the manifold gauge and one when the compressor was running (low pressure is zero).
  
• Is the expansion valve stuck closed instead of before this change when the pressures moved in unison (due to an expansion valve stuck wide open?). I suspect the expansion valve as that seems to be the point (at east by design) where the pressure differential is supposed to occur.

1- Prior to the backprobe tests I had A14/Pin 1 of the A/C Amplifier wire-harness back-probed (with the connector connected from the amplifier) and did a continuity check against chassis ground to assure myself that the subsequent tests had a that pin properly back-probed. Many of the measurements on the other A14 pins are tested against pin 1.
2 -I ran the following tests with the relays installed and all under-the-hood components connected:
2-1: MG_CLT (A14/Pin-21 (Green) tested against A14/Pin-1:
Key Off: 0.0 V
Engine Running, A/C enabled: 13.04V at onset , change at 5 seconds to 0.01 V which it remains at.
The change then causes the A/C clutch to engage and the low-side pressure to drop to 0.
When the engine is shut off the pressure returns almost instantly to 90

 

My take on this after reading all that is it was a low charge to begin with which prevented the compressor from coming on, in the process of trobleshooting the pressure sensor (and maybe other things) was left disconnected and we went down a rabbit hole to the A/C Amplifier.
The pressures staying equal on the first test I'm thinking may have been user error not having the low side gauge quick connect screwed down quite far enough to "connect" the gauge pressure to system pressure (I've done that myself a few times).

Now that we have everything connected correctly it looks like we're still low on charge (very low suction pressure and low discharge pressure).
At this point I'd add some refrigerant to it and see what happens.

 

Will do. Even though I probably did go down that rabbit hole it still served me well. I wanted a better understanding of how the A/C system is managed and I've learned a lot (with your help!). I'll continue to post on my progress..

 

Betting the orifice is plugged. Those pressure splits are whack if the compressor is running.