Please Help: AC diagnosis!

Can't quite get this figured out, but I'm sure someone here has seen this issue.

My Taco: 2010 reg cab 4x4 4 cyl man trans. 36K miles. All AC components original.

First problem: turned on AC, AC light blinks, no cold air (blower is working), compressor does not engage (no resulting drop in RPM's). My diagnosis was insufficient refrigerant in the system. I purchased an AC service manifold with hoses, a vacuum pump, and refrigerant (134A). Using the setup, I vacuumed the system for 45 mins, then turned off pump and left system for one hour - no vacuum 'leakdown' so system seems OK. Installed approx 20 oz's refrigerant and approx 5 oz PAG oil.

After finishing, the system starts normally and the AC is very cold, just like new. Stays that way for a week.

After a week, the compressor does not engage now when activating the AC. AC light stays lit, appears normal (no blinking). Compressor not engaging, no cold air, blower working.

With engine off I shorted 12v to compressor clutch magnet and it closes properly. With engine running & compressor power detached (AC button on) I short the magnet, clutch closes, RPM's drop and compressor engages, turning normally. Cold air begins to 'flow' from the AC vents.

So why is the clutch magnet not closing when the AC button is pressed & lit? I checked all associated fuses - all OK. With power to clutch disconnected & 12v shorted to clutch, the clutch closes, compressor turns & system cools. I'm guessing that A) 'some' refrigerant may have leaked out, causing a low refrigerant pressure signal to not turn on compressor; B) some 'sensor' or 'switch' is not telling the clutch to close. Because no light is blinking on AC button, I'm thinking that refrigerant pressure is close to / or at proper operating pressure (since low pressure activated a blinking AC light before).

If anyone has ideas please advise; I've already done 5 searches for this issue without any clear findings. Thanks in advance!

 

There is a relay in that circuit that may be intermittent

 

Thanks for the reply!

I did check the Denso relay marked MG Clt; I pulled it and applied 12v to both short leads; the relay 'clicked' so I assume that it's OK.

 

So I just found another test, the 'pressure switch' test. With engine running & AC 'on', I disconnected the pressure switch connector from the high pressure line. By shorting the connector, the magnet clutch comes on and the compressor begins turning. Un-short the connector, the magnet clutch disconnects, compressor stops turning. Conclusion: Pressure switch OK, likely low refrigerant pressure as the culprit. So I've got a leak in my system, I'll hafta drag out the gauges & check the pressure....AGAIN. Sound right?

The one thing I have noticed is that the engine no longer 'revs up' once the AC system has engaged. It used to be that when I turned on the AC system, I'd hear the compressor engage and the engine would take a hit, but would 'rev up' to recover. That doesn't happen anymore - any ideas?

 

Well, if that's the right one (I don't know) what you've proved is it worked in a test.

You're problem is more random.

If you have another relay put it in place (swap) and see if the AC works consistently. And whatever else you swapped with doesn't

 

Put a dye in the system that a black light will reveal a leak point. Cheaper than a sniffer, unless you have one you can borrow.

Don't forget under dash (unfortunately) where the evap/expansion valve are.

Under vacumn, around 30#, I typically let it sit overnight. Still there in the AM, system is tight.

I don't know where the 'idle up' sensor is on your vehicle, or if it just exists in the ECU. As in AC on, idle goes up.

Maybe @Bebop @Jimmyh or @Bishop84 can help with better info.

In the meantime, maybe this thread, and a link or two inside of it, might be helpful.

https://www.tacomaworld.com/threads...hy-a-c-light-is-flashing-sometimes-on.553985/

 

CBill, Thanks again for the reply. Since the system worked so well shortly after the refrigerant re-charge, I'm assuming that the major components are in working order. Can anyone speculate as to the most likely point for a leak? I'm thinking the compressor/hoses/condenser but I'm short on experience with AC systems so I'm trying to learn. I'd really like for knowledgeable members to comment on their diagnosis of the lack of 'idle-up', which I believe is associated with the primary problem. Even after I re-charged the system, the AC blew very cold but no 'idle-up' of the engine occurred as before.

 

I know nothing about AC systems but couldn’t soapy water help?

 

Not sure, but I think not. It was suggested above to use IR dye in the freon that can be detected with a blacklight. If it becomes necessary to re-charge the system (it's looking that way!) I'll install some dye.

 

I believe it has a rpm sensor on the compressor that probably has a feed back to the idle kick. Shorting the connector does not test the switch just the low pressure cut out circuit. Use an ohmmeter to check the switch to make sure it's closed you may have to short the connector to bring the pressure up at that point to make sure the switch is closing. I don't remember but I think the switch has a Schrader valve fitting so the refrigerant won't leak out.

 

Good point on the sensor on the side of the compressor, I did see something in these forums about a way to test that component. Apparently, it detects differences between the speed of the compressor and the speed of the engine and was found to be a cause for a intermittantly flashing AC light. Was going to hook up the multimeter but the batteries are dead!

 

Recommend refilling your system to proper levels with a scale. if full and operating properly, put soapy water on the schrader valves (caps off). I've seen those leak (and remain undetected while pulling a vacuum).

Also recommend pulling a vacuum overnight (the longer the better). If there are no indications of leaks, check your valves real good or jsut replace them... they are cheap. You will have to drain the system to do so.

I wouldn't troubleshoot the electrical components if the system was working as intended for a week after filling it. This has signs of a leak.

To know if your system is low, plug your gauges in, engine off, both readings should be close to ambient air temperature.

Out of curiosity, when you did the initial vacuum, where did you recover the refrigerant? I have a tank that I use and I put the tank on my scale and zero it out before starting the vacuum. When it's done sucking, I know exactly how much refrigerant I pulled out of the system.

 

TNT, obviously you've got some pretty decent experience with these types of systems. Thanks much for the excellent input. I plan to hook up the gauges and get some pressure readings first thing tomorrow morning, then I'll have 'completed' my troubleshooting. Do you have any tips for the 'speed sensor' device located on the side of the compressor? Could that be responsible for lack of idle up during compressor operation?

 

Once you fix the leak and everything is working fine, the speed increase will fix itself.

The reason its not increasing when you short the clutch switch is because the computer did not send the signal, thus the rpm increase relay did not engage.

I recommend picking up a schrader valve kit for both high/low side lines. vacuum the system to "0" then replace the valves. vacuum again for 30mins, 1 hour to rid the moisture in the system.

disconnect highside line from truck.

fill truck with r134... mine is 570 min 630max grams. Not sure what the 4cyl is rated at. There should be a sticker under ur hood.

for me thats 2 cans of the cheap gas at walmart. can says 340 grams of r134a. When i measured the last 10 cans ive used in multiple vehicles, true measurement was 338grams.( Yes... with hvac systems I like precision!) once i bled the line of air, my truck takes two cans to give me exactly 630 grams of r134 (minus can/hose weight empty).

my air coming out the vents is 32/33°f even at idle/stop lights. Clean cabin filter helps too.
if you run into problems, pm me. Ill give you my number. Im in hawaii so figure the time difference.

 

my scale and cheap r134a at walmart.

 

The idle up on the A/C is detected by the ECM and uses the A/C amplifier. The operating voltage of the A/C clutch is detected and input to TERMINAL AC1 of the engine control module. If it's not idling up it may be the ECM, wiring connection, or A/C amplifier. My guess is as stated above, if you shorted it to tell the clutch to engage, the amplifier is not sending the signal to the ECM, so it is likely not a problem for you

 

So thankful for the expert advice here! Feeling incredibly lucky to have an AC guy from Hawaii (you know they use AC there!) diagnosing my truck!!

So my subsequent pressure check was just about zero. Vacuum produces 25 (inch-abs of mercury?) which falls to 21 in one hour. If I shut coupler valves (disconnecting vacuum from system, only gauges) the vacuum holds steady. So the leak is in my system. Planning to acquire a schrader valve kit as soon as I find the best one, then install R134a with dye.

If that does not do it, I'm going to acquire new components (compressor/clutch, condenser/accumulator-drier/expansion valve, 2 hoses & one evaporate pipe w/pressure switch) and start new. I try to use OEM parts but in this case I may substitute quality aftermarket stuff if necessary. (i.e., not paying $1200 for a compressor, $450 for a condenser, etc.).

Once again, thanks to everyone that contributed here.

 

Unless you somehow lost most of the oil in your system, you now have about twice the amount needed. Not good.