Complete Wiring Diagram Guide for 1990 Chevy 1500 AC System
Start by locating the under-dash fuse block–this year’s C/K series keeps the climate control relays and pressure switch on the passenger side. Trace the two-wire pink connector from the A/C compressor clutch; one leg carries the 12 V trigger from the high-pressure safety switch, the other grounds through the evaporator thermostat. Bypass the switch only for testing–long-term jumper use risks compressor damage from insufficient refrigerant flow.
Stock schematics group the condenser fan with the coolant temp sender circuit. Pull the 30 A fuse marked “A/C Fan” before probing; shorting this leg can blow the 60 A fusible link feeding the entire under-hood harness. Replacement fusible links should match the original 8-gauge cross-section–undersized wire creates a fire hazard between the battery and the junction box.
Remove the glovebox liner to access the HVAC actuator motors. Each motor receives a dedicated five-wire harness: power (red), ground (black), and three vacuum signal leads. Swap polarity only after disconnecting the battery–capacitors inside the actuators retain charge for up to 30 seconds, risking short circuits if terminals touch improperly.
Test the blower motor resistor pack with an ohmmeter. Readings between adjacent terminals should fall between 1.2 Ω and 2.4 Ω. Values outside this range indicate corrosion; replace the pack as a unit–individual resistors are not serviceable. Keep a spare 20 A fuse handy; resistor failure often blows the fuse before tripping the circuit breaker.
The under-hood relay center houses three critical relays: high-pressure cutout, condenser fan, and A/C clutch. Label each relay position before removal–incorrect reinstallation can disable the cooling system entirely. Use dielectric grease on relay sockets to prevent future corrosion; any moisture ingress here mimics intermittent compressor failure.
Full-Size GM Truck Climate Control Electrical Guide
Locate the blower motor resistor under the passenger-side dashboard–three wires (orange, dark green, and tan) connect to resistor terminals C, B, and A respectively. Use a multimeter set to 200 Ω; resistance between C and B should read 0.9–1.1 Ω, between B and A 1.4–1.6 Ω, and between C and A 2.3–2.7 Ω. Replace the resistor if readings exceed ±0.2 Ω.
| Wire Color | Terminal | Function | Voltage (Engine Running, High Fan) |
|---|---|---|---|
| Orange | C | Power feed from fuse 14 (25 A) | 13.5–14.2 V |
| Dark Green | B | Low-speed relay coil trigger | 0 V (relay open), 12 V (closed) |
| Tan | A | Ground return | 0–0.2 V |
| Light Blue/Black | C101 (A/C switch) | Compressor clutch coil | 13.0–13.8 V (clutch engaged) |
Trace the light blue/black wire from the pressure switch: it routes through the inline fuse holder (10 A) behind the glovebox, then splits to the A/C control switch (orange connector, pin 5). Probe the switch side of the connector; ignition on, A/C switch depressed–pin 5 should show battery voltage. If absent, backtrack to the 10 A fuse or inspect the splice at S105-Y (located near the radiator support).
Finding the Climate Control Harness in Your Classic Pickup
Begin under the dashboard on the passenger side. The harness exits the firewall near the heater box, bundled with a thick black conduit. Trace the conduit downward–it splits just above the blower motor resistor, where four smaller wires diverge: two light blue, one dark blue, and one brown with a white stripe. These feed the compressor clutch, pressure switch, and cabin temp sensor respectively.
The main harness continues along the frame rail before looping back toward the front grille. Look for a cylindrical aluminum canister secured by a single bolt–this is the accumulator. The harness attaches to its side via a two-prong connector, typically sealed with dielectric grease. If corrosion is visible, probe the pins with a multimeter; readings below 10 ohms indicate a healthy circuit.
Common Trouble Areas
Behind the glove box, a junction block connects the in-dash controls to the harness. Disconnect the negative battery terminal before removing the glove box liner–two 7mm screws hold it in place. The junction often accumulates dust; use contact cleaner on the male/female spade terminals, not sandpaper, to avoid damaging the thin silver plating.
The radiator support hides another critical link. Follow the harness past the condenser fan–it should be clipped to the upper support bracket. A chafe point frequently occurs where the harness rubs against the hood latch mechanism. Re-route it with split loom tubing if the insulation shows bare copper. Reconnect the latch with a zip tie to prevent future wear.
At the compressor, the harness terminates in a three-wire plug. The center pin carries 12V from the clutch relay, while the outer pins communicate with the high/low pressure switches. Probe the clutch coil resistance; 3.5-4.0 ohms confirms operational condition. Replace the relay if voltage is present at the connector but the clutch fails to engage–located in the underhood fuse block, marked “A/C” on the cover.
For ground verification, locate the black wire with a yellow stripe branching to the driver-side fender. Use a jumper cable to test–clamp one end to the wire and the other to the battery negative; if the compressor clicks, the vehicle’s ground strap requires cleaning or replacement.
Step-by-Step Guide to Tracing the Climate Control Compressor Circuit
Locate the clutch relay first–it’s typically mounted in the underhood fuse block near the battery. Remove the relay and inspect its socket for corrosion or bent terminals. Use a multimeter set to continuity mode to test pins 85 and 86 (coil) for a complete circuit; if open, the relay or its power feed requires replacement.
Trace the heavy-gauge red cable from the relay’s pin 87 back to the alternator’s output terminal. This wire carries high-current supply to the compressor clutch. Verify voltage at pin 87 with the ignition on and the relay reinstalled; readings below 12.4V indicate excessive resistance or a failing alternator.
Identifying the Pressure Switch and Thermostat Lines
Follow the thin white-with-black-stripe wire from the relay’s pin 85 downward along the firewall. It terminates at a bi-metal thermostatic switch screwed into the accumulator. Probe both switch terminals while cycling the climate controls–normal operation shows voltage only when the system calls for cooling.
Disconnect the pressure switch connector near the condenser’s dual-port valve. Test for 5V reference at the green wire pin with the A/C switch engaged. Absence of signal confirms a broken wire between the switch and the climate control module or a blown 15A fuse in the instrument panel fuse box.
Testing the Ground Leg of the Compressor Clutch
Strip the black wire at the compressor clutch coil terminal and attach a jumper to the engine block. Momentarily apply 12V from a battery to the jumper–an audible click confirms the clutch coil’s integrity. If silent, replace the clutch assembly; resistance readings outside 3.5–4.5 ohms indicate internal short or open.
Reinstall all connectors and clear diagnostic trouble codes with a scan tool. Operate the system at 2000 rpm and monitor clutch engagement cycles. Sustained cycling every 10–15 seconds under ambient temperatures above 75°F suggests a low refrigerant charge or a stuck expansion valve.
Identifying Common Color Codes for AC Circuit Wires
Locate the light blue wire–typically the primary power feed for the climate control compressor clutch–running from the pressure switch to the relay or fuse block. Dark green, often paired with a thin white stripe, carries the switched 12V output from the relay to the compressor clutch coil; probe this circuit first if the clutch fails to engage, as corrosion at the coil plug is a frequent issue. Black wires with a solid orange stripe ground the compressor clutch and blower motor resistor; verify these connections at the chassis ground near the firewall, as loose or painted-over studs disrupt cooling functions.
Yellow wires supply voltage from the fuse panel to the A/C control switch and condenser fan, while tan with black stripes handle signals from the ambient temperature sensor to the climate control module–expect voltage drops here if the fan cycles erratically. Pink wires, sometimes with a black tracer, link the high-pressure cutout switch to the compressor relay; a defective switch will kill compressor operation entirely. Use a multimeter to check for 5V reference on violet wires feeding the evaporator temperature sensor, as shorts here cause the system to default to defrost mode.
Troubleshooting Blown Fuses and Relays in the AC System
Locate the fuse box under the dashboard on the driver’s side–it’s typically behind a removable panel near the kick panel. Use a test light or multimeter to check each fuse related to the climate control circuit: fuses #3 (10A), #10 (20A), and #15 (30A) are primary candidates. If the test light doesn’t illuminate on both sides of the fuse, replace it with an identical amperage rating. Avoid substituting higher amperage fuses, as this risks damaging the compressor clutch coil or wiring harness.
Relays fail more often than fuses but leave no visible signs of damage. Swap the suspect relay with a known working relay of the same type–usually the horn or cooling fan relay–which shares the same pin configuration. If the AC engages after the swap, the original relay is defective. Check for continuity between terminals 85 and 86 (coil side) and 30 and 87 (switch side) with a multimeter; infinite resistance indicates an open circuit.
- Terminal 85 should show continuity to ground when the relay is activated.
- Terminal 86 must receive 12V from the pressure switch or control module.
- Terminal 30 should receive constant battery voltage from the fuse.
- Terminal 87 outputs voltage to the compressor clutch when engaged.
Inspect the AC pressure switch next–it cuts power to the relay if refrigerant pressure is too low or high. Disconnect the switch and jumper the two wires with a paperclip. If the clutch engages, the switch is faulty. If it doesn’t, trace the wiring from the switch to the relay and compressor clutch for breaks, corrosion, or poor connections. Pay special attention to the clutch’s single-wire connector, which often corrodes due to road salt and moisture.
Low refrigerant levels mimic electrical faults by preventing clutch engagement. Attach a manifold gauge set: low-side pressure below 25 psi at operating temperature disables the pressure switch. Recharge the system with the correct oil-refrigerant mixture (R134a with PAG oil for this vintage system) if pressures are low. Overcharging causes the high-pressure switch to trip–typically above 350 psi–so monitor gauges while adding refrigerant incrementally.
Corrosion at the ground connection near the battery tray can cause intermittent relay failures. Clean the ground bolt and mating surface with a wire brush, then coat with dielectric grease before reattaching. Verify ground integrity by probing the ground wire with a multimeter while the AC is commanded on; voltage drop should not exceed 0.1V. If it does, run a new 10-gauge ground wire from the chassis to the battery’s negative terminal.
If the compressor clutch still doesn’t engage after ruling out fuses, relays, and pressure issues, measure voltage at the clutch connector with the AC on. No voltage suggests a faulty thermostat or control module–test the thermostat by bypassing it with a jumper wire. If voltage is present but the clutch doesn’t pull in, the clutch coil may be shorted or open. Measure its resistance: 3-5 ohms indicates normal operation, while infinite resistance or zero ohms confirms a failed coil requiring replacement.