Does this A/C troubleshooting check out? Plus, a bonus question at the end!

[6716]

Many thanks to Mooseman for the manuals (I had to laugh, though, that the "overview" section for how the HVAC system works was at the END of 450-odd pages of documentation).

So here's where I'm going with this:

I have a recent issue where, after a few hours of continuous operation, the A/C is still "cold" but not much air comes out of the vents. Given that the problem resolves itself overnight, the blower motor and resistor pack check out, the compressor is running, the clutch doesn't seem to be slipping, the hard lines and accumulator are cold, it is possible that the evaporator is icing over and blocking airflow.

What I'm thinking is that the compressor is not shutting off at any point (since I stood and watched it for 15 minutes last night (the neighbors already think I'm a nut)). The manual suggests that there are numerous (12, specifically) conditions which will cause the A/C compressor to shut down.

So if the compressor is still running
1. Ambient air temp is > 35*F
2. A/C compressor temperature switch contacts are closed
3. Throttle position is < 100%
4. A/C low pressure switch is closed
5. A/C high side pressure is not higher than it should be
6. A/C low side pressure is not lower than it should be
7. Engine coolant temperature is lower than 253*
8. Engine speed is less than 5500 RPM
9. Transmission is not shifting
10.PCM does not detect excessive torque load
11.PCM does not detect insufficient idle quality
12.PCM does not detect a hard launch condition

Since the compressor is still running, I think I would have to rule out everything BUT either #2 or #4 as the source of the lack of shut-down on the compressor.

Now, here is some FURTHER INFORMATION:

Last summer (maybe 13 months or so ago) my A/C was blowing hot and I took it in to Firestone (I get Destination A/Ts there, the last set lasted 70K, and with the full warranty package I get lifetime free rotation ... so I go in for oil changes and free rotation every three months) and asked for an A/C check. They said everything checked out, but I was low on refrigerant, so they topped it up. $50 for the check, $8 for the refrigerant, it blew cold, and I was happy. For four months.

Four months got me past my next oil change, and into September, so when it quit I just toughed it out since winter was coming anyway.

This May when it got hot again and I was due for an oil change I brought it back to Firestone and had them re-check it (which they did at no charge) and they still couldn't find a leak, but it was definitely low, so they topped it off (again, at no charge). I pressed the guy at the counter for an answer, and his only thought was that there was a leak, but it was under the dash where they couldn't see the dye.

So here I am today ... it's still making cold air, I'm due for an oil change in a week, if I have a four-month slow leak under the dash the issue won't appear for another month ...

and I have to wonder (here's the bonus question) ...

Are the possible system leak and the lack of compressor shut-down related? Or no, and each could and should be approached and resolved independently?

 

[The_Roadie]

The leak and the electrical issue are independent. Only a full evacuation and vacuum test will confirm the existence of the leak, and UV dye additive can help pinpoint the location, even if it's behind the dash.

The electrical troubleshooting and thought process are fine, but I'm even more suspicious than you. I suspect a welded ON AC relay, underhood #44. Do you have a meter and want to poke into the relay operation a bit? A trick I use to troubleshoot a relay when it's still plugged into the fuse block is take tiny stripped wires and wind pieces around the blades of the relay, then plug it back in. Be careful. Then you can touch the meter to each of the four legs on the relay and see what the control line is doing compared to the output line.

 

[6716]

Be careful.

Be careful not to break something, not to get electrocuted, or both?

Don't have a meter, but not opposed to getting one. What would you recommend?

By the way, the compressor does shut down when I push the A/C button on the dash to turn it off ... I don't know if that was clear from my post, and if that affects your recommendation.

If there weren't a leak, would the system be low? If so, then low refrigerant must be evidence of a leak .... right? And if there must be a leak and the shop I've been going to can't find it, then they must not have the technology to figure it out .... right?

Many thanks.

 

[Wooluf1952]

Not to hi-jack the thread, but, I'm confused.
If the freon is low or gone, does the compressor still run?
I thought that with no/low freon the clutch would not engage.

 

[6716]

Not to hi-jack the thread, but, I'm confused.
If the freon is low or gone, does the compressor still run?
I thought that with no/low freon the clutch would not engage.

You are correct (I believe) about lack of freon. I believe that if the low side pressure is too low (say from lack of refrigerant) then the clutch will either disengage or not engage.

The two problems have separate timing. At the moment, I have enough freon and pressure to produce cold air / run the compressor. I may have a slow leak, but right now I still have enough to keep on trucking. If I don't do anything about checking pressure, I am 90% certain that in another month the compressor won't run because the leak will have let enough out to lower the pressure sufficiently.

In what appears to be an entirely separate issue, when the A/C button is pushed, it looks as though the compressor won't shut off, which I noticed when I went looking for the reason that the airflow to the cabin was greatly reduced after significant A/C run time.