1999 Chevrolet Astro Van AC System Schematic and Wiring Diagram Explained
Locate the compressor clutch relay under the driver’s side dashboard near fuse panel F22. Pin 85 connects to a 15 A fuse (position 17) via a gray/black wire; verify continuity before replacing the relay. The evaporator temperature sensor wires–yellow and yellow/white–join at connector C201 behind the glovebox. Disconnect the battery negative terminal before probing these circuits to prevent shorting the ECM.
Test the blend door actuator moter resistance between terminals A and B. Specifications call for 75–125 Ω at 20°C. If readings fall outside this range, replace the unit. The actuator plugs into wiring harness junction J102, where a brown wire delivers 12 V switched power from fuse 12 (10 A). Use a multimeter in AC voltage mode to confirm the signal from the ambient air temp sensor–expected range is 0.5–4.5 V with the ignition on.
Pressure switch PS1’s wiring–light blue and light blue/white–ties into the low-pressure cutout circuit. At 28 psi, the switch opens, interrupting power to the clutch coil via a red/white wire. Jumper the switch temporarily to bypass the safety during diagnostics, but never operate the system below 20 psi. The condenser fan draws 18 A maximum; ensure the fan motor resistor pack is seated firmly in its mount beneath the passenger seat to avoid overheating.
Trace the high-pressure cutout to the compressor discharge port. The switch closes at 320 psi, grounding the green wire to the body control module. If pressures exceed 400 psi, inspect for restrictions in the orifice tube or receiver-drier. The accumulator inlet and outlet hoses should differ by no more than 5°F–use a laser thermometer to check this delta. Replace the accumulator if inlet frost is visible, indicating moisture contamination.
Electrical Layout of Chevrolet G-Series Climate Control
Locate the compressor clutch relay under the dashboard near the blower motor resistor–circuit identifier CR-5 on the wiring blueprint. Verify continuity between terminal 87 and the clutch coil using a multimeter set to 200 ohms. Resistance should read 3.2–4.8 ohms; deviations indicate a faulty clutch or corroded connector at C202. Replace the relay if voltage drops below 11.8V during engagement.
Trace the high-pressure cutoff switch wiring from the accumulator to the pressure cycling switch (PCS) at port A14. Disconnect the 4-pin connector at the PCS and probe pins 2 and 3 with engine running: 12V should toggle at pressures above 45 psi and below 320 psi. Bypass the switch temporarily with a jumper if cycling persists–persistent failure confirms a defective PCS. Inspect the Schrader valve for debris if readings erratically fluctuate.
Evaporator Core and Expansion Valve Diagnostics
Access the expansion valve by removing the glovebox liner; note the thermal bulb clamped to the evaporator outlet tube. Heat the bulb gently with a heat gun–valve should audibly click at 40°F, regulating refrigerant flow. Absence of clicking despite 12V at the thermostat suggests a seized valve piston. Replace entire TXV assembly if ice forms at the inlet orifice despite proper superheat settings (target 12–15°F at 32°F evaporator temp).
Check the blend door actuator at firewall harness connector WH603–power window motor drives it via two-wire bi-directional pulse. Confirm 5V reference voltage at pin A with key on; missing signal indicates failed HVAC control module or broken door linkage. Remove actuator and manually test door range–the door should travel 50° without binding. Recalibrate the module by cycling ignition three times if door stalls mid-travel.
Identifying Key Climate Control Parts in a Chevy G20 Cargo Model
Begin under the hood near the driver’s side. The compressor sits adjacent to the serpentine belt, identifiable by its cylindrical shape with aluminum fins and two refrigerant lines connected via threaded fittings. Trace the high-pressure line–typically a smaller diameter metal tube–that routes toward the condenser, mounted directly ahead of the radiator. Verify airflow paths; debris clogging the condenser fins will degrade cooling performance.
Move to the passenger cabin. Remove the lower dashboard panel beneath the glovebox to expose the evaporator housing. Expect two separate harnesses: one for the blower motor and another for the temperature blend door actuator. Disconnect the negative battery terminal before servicing–electrical shorts occur frequently with aged wiring harnesses near the firewall grommet entry.
Critical Linkages and Sensors
- Accumulator: Attached to the evaporator outlet, a black cylindrical canister holding approximately 1 pint of desiccant.
- Thermal expansion valve: Brass valve with capillary tube wrapped around the evaporator inlet; corrosion often attacks solder joints.
- Pressure cycling switch: Located on the accumulator, triggers at 32 psi to prevent evaporator freeze-up.
- Low-pressure cutoff: Mounted near firewall on the suction line; interrupts compressor clutch at approximately 25 psi.
Follow the low-voltage wiring from the compressor clutch. A green 18-gauge wire runs to the underhood relay center, labeled “A/C Relay” on the fuse block diagram. Test continuity at relay terminals 85 and 86–expected resistance: 50-120 ohms. Failure here mimics refrigerant undercharge symptoms.
Inspect refrigerant lines for oil residue at couplings. The high-pressure discharge line–hot to the touch–should not exceed 220°F under 2,000 rpm load. Use a non-contact infrared thermometer at three points: compressor outlet, condenser mid-point, evaporator inlet. Record values: 180-210°F, 110-135°F, 32-40°F respectively. Deviations indicate restriction or improper charge.
Access Points and Hidden Components
- Drain tube: White plastic nipple beneath the evaporator housing; verify unobstructed flow to prevent condensate pooling.
- Vacuum reservoir: Black plastic canister behind right kick panel stores vacuum for door actuators; cracked diaphragms cause erratic temperature control.
- Ambient temperature sensor: Threaded brass probe mounted forward of radiator support; corrosion alters readings, prompting premature compressor cycling.
- Cabin air filter: Behind evaporator housing; replace annually to maintain airflow and prevent mold growth.
Use a refrigerant identifier gun before recovery–contaminated R-134a damages recovery equipment. Access the Schrader valves by removing protective caps; high-side near condenser, low-side on accumulator. Verify valve cores hold 50 psi nitrogen without leakage before evacuation.
Step-by-Step Guide to Interpreting the Climate Control Circuit Layout
Locate the compressor symbol first–marked as C on most blueprints. Trace its power feed from the fuse block (F1 or F2), typically a 20A or 30A fuse, then follow the red or yellow line to the high-pressure cutout switch (S1). This switch disconnects at 325 psi to prevent damage; verify its normally closed contact before proceeding.
Identify the blower motor regulator (R1)–usually a variable resistor or solid-state module. Check its wiring: key inputs include a 12V ignition-switched feed, ground via a black/pink stripe, and output to the blower motor (M1) in 3-5 speed configurations. Test resistance across the regulator’s terminals; values should decrease progressively from high to low settings (see table below).
| Blower Speed | Expected Resistance (Ω) | Voltage at M1 (V) |
|---|---|---|
| High | 0.5–1.2 | 11.5–13.8 |
| Medium-High | 3.0–4.5 | 9.0–10.5 |
| Medium-Low | 6.0–8.0 | 6.5–8.0 |
| Low | 10.0–12.0 | 5.0–6.2 |
Examine the thermostatic control unit (T2), often paired with a capillary tube clipped to the evaporator. Its wiring loops through the compressor clutch relay (K1) and vents at 35–40°F. Use a multimeter in continuity mode to confirm the switch opens/closing at these thresholds–deviation exceeding ±3°F indicates a faulty sensor.
Trace the condenser fan circuit (F2) next. It shares a fused feed (15A) with the compressor but splits at a dual-pressure switch (S2). This switch engages the fan at 250 psi or 220°F refrigerant temps; verify its wiring runs to a dedicated relay (K2) before grounding at GND-3. If the fan fails to activate, probe the relay coil voltage–expect 12V at terminal 85 when the system reaches 200 psi.
Critical Junction Tests
Check the A/C request signal loop. The climate control head (H1) outputs a 0.5–3.0V signal to the PCM (U1) via a purple/white stripe wire. Interrupting this line should deactivate both the compressor clutch and condenser fan simultaneously. Use a scan tool to monitor PID A/C Request: toggle between ON/OFF while observing clutch engagement–latency exceeding 200ms suggests a corroded splice at connector C304.
Common Faults in the Chevrolet Express Cargo AC Electrical Path
Start by inspecting the compressor clutch relay (K6)–this component fails most frequently due to thermal cycling and corrosion at the socket terminals. Use a multimeter to verify coil resistance (should read 50–120 ohms); if outside this range, replace the relay. Next, check the low-pressure switch (S15) mounted near the accumulator–its contacts oxidize over time, causing intermittent cutouts. Bypass it temporarily with a fused jumper wire to confirm; if the system engages, clean or replace the switch. The blower motor resistor pack (R7) also degrades under sustained heat–look for melted solder joints or discolored traces on the PCB. Reflowing the connections may restore function, but a full replacement is often necessary.
Power distribution faults often trace back to the underhood fuse block, where the AC clutch fuse (F3, 20A) blows due to shorted wiring between the compressor and firewall grommet. Probe the harness at connector C201 (white/black wire) for continuity; resistance above 0.5 ohms indicates frayed insulation or pinched wires in the engine bay harness. The cycling clutch switch (S6) on the dryer assembly sometimes sticks, preventing pressure equalization–test with compressed air to ensure it clicks at 25–30 psi. Finally, the thermostatic expansion valve (TXV) seizes due to moisture ingress, leading to erratic cooling; if superheat readings at the evaporator inlet (3–8°F) exceed targets, flush the system and replace the valve.