Debugging non-working A/C

[jepherz]

Being that it's starting to get hot out, and the AC in one of my cars does not work, I'd like to take a stab at fixing it. As of right now, when you turn on the fan and push in the AC button, the light lights up, but the compressor doesn't kick on. What are all of my possible problem points? Besides the clutch being bad, I know there are some sensors too but I'm not sure if any of these sensors could render the compressor useless.

 

[solidviper89]

The sensor at the top of the thermostat housing. Make sure it works. That was the problem with my A/C not working

 

[iceman69510]

Is there refrigerant in the system? Low pressure switch will not allow the compressor to run without it.

You can also verify if there is 12 volts at the connector to the compressor when the a/c switch is on.

 

[jepherz]

Thanks for the replies, guys. No idea if there is refrigerant; I wanted to make sure the compressor works before I had to vacuum and fill it. I guess I'll check those other things and then fill it though instead.

 

[jepherz]

Quoting iceman69510:
Is there refrigerant in the system? Low pressure switch will not allow the compressor to run without it.

You can also verify if there is 12 volts at the connector to the compressor when the a/c switch is on.



Will there be 12 volts at this connector even if the low pressure switch isn't triggered? I'm guessing not.

 

[Barnes]

Quote:
Will there be 12 volts at this connector even if the low pressure switch isn't triggered? I'm guessing not.



You are correct. The next step would be, IMO, to check the reading on the low pressure switch. I can't remember how to do this, but the factory manual lists the values you need to read. This will confirm low refrigerant.

 

[jepherz]

What's the appropriate procedure for doing this? Do I need a special vacuum to do it properly? Seems like there is a big variance in some of the pumps on ebay:

This one looks like it needs an air compressor to work.

This one must not?


Anyone that has done this before, can you help me out?

 

[Barnes]

I wouldn't bother with buying vacuum equipment. A set of gauges, sure.

1)Start the car, turn on the A/C, measure the voltage at the A/C clutch connector. If it isn't getting 12volts you can probably assume the clutch is fine, and something else is tripping.

2)Check the temp sensor on the top of the t-stat housing. I think we already talked about this.

3)Check the low pressure switch to see what it reads. I'll try to dig through my manual tonight and see what the procedure is.

If you determine you have low refrigerant, I suggest you do the following:
Take the car to a shop that does A/C work. See how low it is on refrigerant. It might be worth it to just have it filled up again if it is not that low. If it is empty, time to do a r-134a conversion. At that point, you can have the shop vacuum down the system to get all the old r-12 out. Then do a r-134a conversion at home, and you should be set. Most people have good luck with it. Mine didn't work, but I think the reason is that I suspect my expansion valve is slightly clogged. Too bad I have to get my system vacuumed AGAIN after I replace the dumb thing.

 

Search for my posts on the A/C system, as I posted complete schematics and troubleshooting tips. When you push the A/C button, the 12 volts has to go from the A/C computer to the HI/LOW pressure switch, then to the thermoswitch on the thermostat housing, before it gets to the ECU. I believe that MMcD and TMO loggers both will show you if the ECU is ever seeing the +12 volts making it to the ECU. Then, the logger has another logged item that shows whether or not the ECU is activating the A/C compressor clutch. A logger makes it easier to know where to start looking. /ubbthreads/images/graemlins/wink.gif

 

[467]

I had a slow refrigerant leak in my Galant GS so had fluorescent dye added to the refrigerant. I found out it was the evaporator unit behind the glove box.

 

[jepherz]

Ok, tested out the clutch by hooking it directly to the battery, and it works. I also know there is a little bit of pressure in the high line, but not much. I think I'm just going to fill it up with r134 and see what happens.

 

[Hertz]

Bob had his A/C controller die (behind the glovebox)...

 

[gtluke]

my controller died as well.
anyone ever figure out what the pots do on the controller?

 

[jepherz]

What does the AC Controller do, and what would the symptoms be if it goes out?

Here's where I'm at. I got some r134 because I wanted to make sure some stuff worked before actually draining what was in there, out. Hooked up the r134 and almost immediately, the compressor started up. I could see bubbles flowing in the little site up by the front bumper. At idle, the pressure showed "good" on the gauge, but it would climb very rapidly with RPMs, so I didn't want to charge it any more.

One line off the compressor was hot to touch and the other was a little cool. However, it didn't translate to much inside the car. The other weird thing was that when I turned the AC on in the car, the rpms would raise a small amount as if the ECU was anticipating additional load. I could then hear the clutch click on, but there was never really any additional load on the engine.

Tips? Tricks?

 

[ktmrider]

Ok, so it was low on refrigerant ( reason the compressor started once you added some refrigerant ).
The pressure spike is somewhat normal with higher RPM. Did the gauge climb into the "Red" warning area? If so it's possible the expansion valve is bad ( or sluggish/stuck from lack of use ). See below for more.
The change in RPM when engaging the A/C is perfectly normal. ECU does this to compensate for the increased engine load at idle.
I would hook up gauges to both the high and low sides. They should read nominal ( green ) at idle and A/C on. Then bump the idle to 2000rpm and watch the gauges. Both reading Low you need more refrigerant. Both reading High you are overcharged. High going high and Low going low means the expansion valve is toast.
Note, if the refrigerant level is good you will always see the pressures change initially with RPM's. The Expansion valve is designed to modulate the pressure of the refrigerant to maintain optimal cooling and operation. Gauges can be bought for decent prices at all major parts stores and are the only true way to measure the health of the system.
GL, post up the results.

 

Quoting jepherz:
What does the AC Controller do, and what would the symptoms be if it goes out?



As I said, the A/C computer supplies the 12 volts that has to go through the HI/LOW pressure switch and thermoswitch. It looks at 3 different sensors, and the dash A/C switch of course, to decide whether or not to send the 12 volts down the line. So, if you never see 12 volts on one side of the HI/LOW switch that would be the first sign you need to start at the A/C computer.

 

[blacksheep]

"keydriver" ftw...

 

[jepherz]

So it's safe to say that if the clutch on the compressor is engaging, then my AC Controller is 100% good to go?

 

Yes, the clutch on the compressor will not engage if it doesn't receive the 12 volts in on pin #7.
There won't be voltage on pin #7 if the thermoswitch on the thermostat housing is open.
There won't be 12 volts present at the thermoswitch if the HI/LOW pressure switch is open.
And, there won't be 12 volts present at the HI/LOW pressure switch if the A/C computer is bad, or the fuse to it is blown, or the A/C switch in the dash isn't good.

 

[jepherz]

Well, I bought a can of 134 oil, 2 cans of r134, a $12 vacuum and a AC gauge set from harbor freight for $45. Vacuumed the system and it held vacuum, charged with the two cans and both gauges were optimal at idle and 2000 RPM. Blows COLD! About $100 in supplies and I ended up with some useful tools and working A/C.

Thanks for all of the help, guys.