A/C compressor seems to turn off at idle

[Ntwrkguy1]

My 2001 Suburban is exhibiting some strange behavior recently.

 

The ABS light and brake light recently came on (getting some help on that on this thread: http://www.gm-trucks.com/forums/topic/152470-abs-light-and-brake-light-showing-up-at-once/?do=findComment&comment=1343182) and now my wife called me with the following news.

 

She was driving it this morning, and while she was driving, the a/c was blowing nice and cold. But when she stopped at the light, the fan continued to blow but no cold air was coming out. She also said that the idle seemed to drop just slightly, to around 500rpm (while she was in drive, with her foot on the brake). Once she started up again, the a/c resumed it's cold temp.

 

Some facts that might be relevant: It's a 1500 series with a 5.3 liter motor. The battery is 3 months old. I tested the alternator at the same time, and it tested out fine. The serpentine belt is under 3 years old, and appears to be in good shape. The vehicle has 187,000 miles on it, and has the original a/c compressor. Generally, all of the maintenance that needs to get done has gotten done over the last 9 years we've owned it, so it's been pretty well-maintained.

 

 

Any thoughts on this? Also, should the idle be that low in drive? With the a/c on, my wife said it felt like it was going to shut off.

 

Thanks in advance,

 

David

 

[txab]

From your description, I'd check the fan clutch for proper operation for starters

 

Might want to clean the MAF and throttle body for the idle situation

[Ntwrkguy1]

txab,

 

Is there another way -- other than visually seeing whether it is working or not -- to check the fan clutch? Thanks for the tip on the MAF and TB -- I think I have the cleaners for those at home, so I'll give them a cleaning.

[txab]

You can take some rolled up newspaper and slowly advance toward the fan blade, as you make contact if the blade stops or offers little resistance, it's toast. That's one way. Slow, no jabbing.

 

Fuel filter. Have you ever changed it?

[Ntwrkguy1]

Yes -- I forgot to mention that both the fuel pump and the fuel filter were replaced in April '12.

 

I'll try that trick with the newspaper. How does the engine fan impact the a/c system, though?

[Chris]

It pulls air across the condenser

 

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[ujtruck]

its handy to have Tools to fix AC issues

• UV leak set (black-light and special glasses) to check for leaking refrigerant
• Multi-meter capable of testing Ohm resistance and Voltage in Direct Current to test electrical
• Cans of RF134a to recharge the system
• A/C Pressure Tester

Check that the pressure switch is activated

1
Locate the pressure switch.
The pressure switch is inline with the A/C coolant pipeline.

2
Remove the cap and connector from the pressure switch.

3
Test the resistance to the pressure switch.

4
An "Open" reading:
Indicates the pressure switch is not activated.

5
A "Closed" reading:
Indicates the pressure switch has enough pressure behind it to be activated.

6
If the pressure switch is activated, there is a problem with the electrical and the problem is more complicated.

7
If the pressure switch is not activated, check for pressure in the system

if loss of refrigerant due to a leak-- so you need to troubleshoot that

[txab]

Yes -- I forgot to mention that both the fuel pump and the fuel filter were replaced in April '12.

 

I'll try that trick with the newspaper. How does the engine fan impact the a/c system, though?

 

 

Just like your home AC unit uses a fan to pull air across it's condenser to remove heat, you truck's engine fan does the same job. At idle is where it can show that it is weak. You can have bad fan clutch without engine overheating. Lack of airflow across the condenser will cause an increase in vent temps because not enough heat is being removed from the refrigerant. Once the engine RPMs go up better cooling occurs again. Your fan clutch should be fully locked up (turning) when the engine is hot. When cold it should freewheel somewhat since not as much cooling is required

 

If you test the clutch, make sure the engine is at operating temps. Be very careful of course. If it's weak, it will stop or slow down without a lot of pressure against the fan blades

 

 

A couple of vids: