How to Diagnose A/C System On 2000 VW Jetta GLSPosted 11/16/2005
By Jim Newkirk
This summer has been a hot one, and I'm sure you have seen your share of air conditioning problems. Unfortunately, A/C systems have grown steadily more complex and difficult to diagnose.
The problem involved a 2000 Jetta GLS 2.8L manual A/C system with no compressor clutch operation. On this vehicle, the cooling fan and A/C compressor operation are controlled by the J293 fan control module, which is usually located in the driver's front corner of the engine compartment on the lower frame rail. The J293 module has both a 14-pin connector identified as the T14 connector in Volkswagen wiring diagrams and a four-pin connector identified as the T4a connector.
To diagnose this system, starting on the T14 connector:
1) Start and idle the vehicle. Select "A/C on" at maximum cooling and "blower on" at high speed.
2) Check for 12 volts at the T14 connector pin No. 8 (T14/8). This 12-volt signal comes from the A/C switch and requires both cooling fans on at low speed and compressor activation.
3) Check for 12 volts at pin T14/9. This voltage is a switched ignition source and will have 12 volts when the ignition is in the "on" position.
4) Check for 12 volts at pin T14/4. This is a constant battery source and should read 12 volts at all times from fuse S16.
5) Check for a good ground at pin T14/6.
6) Check for 12 volts at all times at the four-pin connector, T4a pins T4a/1 and T4a/3 from fuses S164 and S180, respectively.
7) Turn off the ignition. Remove the T14 connector and check for continuity between pins T14/14 and T14/5 on harness side to ensure proper operation of the F38 ambient temperature switch. Continuity must be present if the ambient air temperature is above 32 degrees Fahrenheit. Reconnect the T14 connector and restart the vehicle.
8) Check pin T14/2 using a duty cycle meter. If the refrigerant charge in the system is normal, about 30 percent to 35 percent duty should be indicated at pin T14/2 without the compressor engaged. The duty cycle signal is supplied by the G65 pressure sensor in response to system pressure changes. A duty cycle above 90 percent or below 20 percent will command the compressor off.
9) Check for an 11-volt reference voltage at pin T14/3. The reference voltage originates in the J293 fan control module and can be grounded by |the power control module (PCM) under certain circumstances (typically wide open throttle or vehicle overheat conditions) to turn the A/C compressor off.
If zero volts are present, the PCM is commanding "compressor off" or the wiring harness is shorted to ground. Raise vehicle idle speed above 2500 rpm and observe compressor operation and voltage at pin T14/3. If the voltage at pin T14/3 returns to 11 volts with the idle speed above 2500 rpm and compressor operation resumes, then a throttle basic setting procedure is needed and must be performed with a factory-compatible, by-directional scan tool. Note: A loss of throttle basic settings will keep the compressor from activating.
If all previous tests have passed, check the T14 connector pin T14/10 for 12 volts. This pin is the output signal to the compressor clutch coil. If all the other tests have passed and there is no voltage at pin T14/10, this indicates a faulty fan control module.
As you can see, turning on an A/C clutch is not a simple function on late model vehicles, and systems that you might not associate with an A/C problem can stop you and your customer from keeping cool. Be sure to check Direct-Hit's Hotline Archive section for more diagnostic procedures and tips.
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