FWD VANS
To improve idle quality, Wide Open Throttle (WOT) performance and for system protection, the A/C compressor clutch is controlled by the Engine Control Module (ECM).
Refrigerant switches used are a low pressure switch (front A/C option), high pressure cut-out switch (front and rear A/C options), and an A/C pressure cycling switch (rear A/C option). Different pressure switches are used because vehicles are equipped with different A/C compressors depending on A/C option installed.
The A/C compressor clutch relay is controlled by the ECM. This allows the ECM to raise idle speed before engaging compressor clutch and to disable compressor clutch during WOT and if coolant temperature becomes excessive. ECM inhibits A/C compressor clutch operation until engine speed is less than 4000 RPM for 3 seconds.
ECM engages Torque Converter Clutch (TCC) at 2 different speeds depending on A/C system operation. When A/C is off, ECM will engage TCC at about 30 MPH. When A/C is on, ECM will engage TCC at about 35 MPH.
Note. Some FWD vans use an Engine Control Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM) to maintain engine idle speed during A/C compressor clutch operation. These trucks do not use ECM, PCM or VCM to control A/C compressor clutch circuit.
RWD VANS & TRUCKS
On vehicles with an Electronic Control Module (ECM) controlled A/C compressor clutch, the A/C compressor clutch relay is controlled by the ECM. The ECM improves idle quality by delaying A/C compressor clutch engagement until idle speed is increased and disengaging A/C compressor clutch when idle speed is too low. A/C compressor clutch is cycled by ECM. ECM smoothes cycling of A/C compressor clutch by adding fuel the instant A/C compressor clutch is applied.
On vehicles without an ECM controlled A/C compressor clutch, turning on A/C system supplies battery voltage to the A/C compressor clutch and A/C request signal terminal of Engine Control Module (ECM,) Powertrain Control Module (PCM) or Vehicle Control Module (VCM). When the ECM, PCM or VCM sees battery voltage on A/C request signal terminal, it maintains idle speed. The ECM, PCM or VCM does not control A/C compressor clutch operation. An A/C pressure cycling switch is used to control A/C compressor clutch operation.
SCAN TESTER
A variety of information is transmitted through Data Link Connector (DLC) connector terminal "E" or "M", depending on engine. This data is transmitted at a high frequency which requires a Tech 1 scan tester for interpretation. Several scan testers are available for diagnostic work. Scan testers other than Tech 1 scan tester will function and provide information for diagnostic work; however, some charts will specify a Tech 1 scan tester. Failure to use a Tech 1 scan tester when specified may result in incorrect diagnosis of a system.