Understanding the 1991 CRX PGM-FI Architecture
The 1991 Honda CRX represents the pinnacle of the EF chassis generation. Whether you are maintaining a D15B1-powered HF or a D16A6-equipped Si, the Programmed Fuel Injection (PGM-FI) system relies on a centralized Main Relay to distribute power to the ECU, fuel injectors, and oxygen sensor heaters. When a 1991 CRX experiences a 'hot no-start' condition or an intermittent failure of the fuel pump to prime, the factory service manual immediately points to the PGM-FI Main Relay and its associated wiring harness. Using the Haynes Honda Civic and CRX Repair Manual as a baseline, this guide provides a severe-duty, step-by-step troubleshooting flow to isolate voltage drops, open circuits, and ground faults within the primary EFI harness.
Prerequisites and Tooling
Before probing the 1991 CRX wiring diagram circuits, ensure you have the correct diagnostic equipment. A high-impedance Digital Multimeter (DMM) is mandatory to prevent back-feeding voltage into the sensitive PM6 or PX6 ECU. You will also need T-pin back-probes for testing sealed connectors, a piercing probe (used sparingly), and an oscilloscope if you suspect injector driver signal degradation. All replacement wiring must meet SAE J1128 specifications for low-tension primary cable, specifically utilizing TXL or GXL insulation rated for 125°C to withstand the under-hood thermal environment.
Phase 1: Constant Battery Feed Verification
The PGM-FI Main Relay in the 1991 CRX is a dual-coil, 7-pin unit located under the dashboard, mounted to the left of the steering column bracket. The first step in our troubleshooting flow is verifying the constant 12V+ memory and power feed.
Step 1.1: Test the BLK/YEL Circuit
Locate the 7-pin PGM-FI Main Relay connector. Identify Pin 4, which utilizes a 12 AWG Black with Yellow stripe (BLK/YEL) wire. This wire routes directly from the main under-hood fuse box (Battery Fuse 1) to the relay. Set your DMM to DC Voltage. Back-probe Pin 4 with the ignition OFF. You must read a minimum of 12.4V. If voltage is absent or below 11.5V, perform a voltage drop test from the battery positive terminal to Pin 4. A voltage drop exceeding 0.2V indicates high resistance, likely due to corrosion in the C401 under-dash harness connector or a failing main fusible link.
Phase 2: Ignition Switch Signal Verification
The ECU and fuel pump cannot activate without the ignition switch signaling the PGM-FI relay to close its internal contacts.
Step 2.1: Test the YEL/BLK Circuit
Identify Pin 5 on the 7-pin relay connector. This is a 14 AWG Yellow with Black stripe (YEL/BLK) wire originating from the ignition switch. Turn the ignition key to the ON (II) position. Your DMM should read 12.0V to 12.6V. Crucially, hold the key in the START (III) position. The voltage must remain above 10.5V during cranking. If the voltage drops to near zero during cranking, the ignition switch internal contacts are burned, or the YEL/BLK wire is sharing a degraded crimp splice in the dash harness. According to Honda-Tech PGM-FI Archives, failing ignition switches are the second most common cause of relay non-activation after cracked relay solder joints.
Phase 3: ECU Power Delivery and Ground Integrity
Once the relay closes, it routes battery voltage to the ECU and the fuel pump. For the 1991 CRX Si (PM6 ECU), this power distribution is highly specific.
Step 3.1: Verify ECU Main Power (WHT/RED)
The relay outputs power via a 14 AWG White with Red stripe (WHT/RED) wire on Pin 7 of the relay connector. This wire travels through the firewall grommet and terminates at Pin A1 on the PM6 ECU's 32-pin 'A' connector. Back-probe ECU Pin A1 with the key ON. You must read within 0.1V of the battery voltage. If the relay clicks but Pin A1 reads 0V, the WHT/RED wire has an open circuit, often caused by a broken copper strand inside the insulation near the tight bend radius behind the glovebox.
Step 3.2: Validate ECU Sensor and Main Grounds
A common misdiagnosis in 1991 CRX wiring diagrams is assuming a bad ECU when the issue is actually a high-resistance ground. The PM6 ECU relies on three critical ground pins: A15, A16, and D21 (all 18 AWG Black wires). Do not simply test for continuity to the chassis. Perform a dynamic voltage drop test. Place your DMM positive lead on the battery negative terminal and the negative lead on the ECU connector back-probed ground pins while the fuel pump is priming. A reading above 0.15V confirms a corroded ground point (G101 or G401), which will starve the internal ECU drivers and cause injector pulse width anomalies.
Step-by-Step Decision Tree: Fuel Pump Does Not Prime
Follow this diagnostic logic tree when turning the key to ON yields no audible fuel pump hum from the rear hatch area.
- Does the PGM-FI Main Relay click?
- YES: The ignition switch signal (YEL/BLK) and internal coil are functional. Proceed to test the Fuel Pump power wire (YEL/GRN, 14 AWG) at the pump connector in the rear hatch. If 12V is present but the pump doesn't run, replace the pump or check the rear ground point G601.
- NO: The relay is not receiving the trigger signal or the internal coil is open. Proceed to Step 2.
- Is there 12V at Relay Pin 5 (YEL/BLK) with Key ON?
- YES: The ignition switch circuit is intact. The internal PGM-FI relay coil is faulty, or the relay ground path (Pin 6, BLK wire to G401) is open. Test continuity from Pin 6 to chassis ground.
- NO: Trace the YEL/BLK wire back to the ignition switch. Check the dash fuse box for blown Fuse 15 (15A) which protects the ignition switch accessory feeds.
OEM Wire Color Codes and Pinout Reference Table (PM6 ECU)
When tracing the 1991 CRX wiring diagram for the D16A6 Si model, reference this critical pinout table for the PM6 ECU power, ground, and primary injector circuits. Always verify wire colors against your specific chassis, as previous owners may have repinned connectors during OBD0 to OBD1 conversions.
| ECU Pin | Connector | Wire Color | AWG | Circuit Function |
|---|---|---|---|---|
| A1 | 32-Pin A | WHT/RED | 14 | ECU Main Power (from PGM-FI Relay) |
| A15 | 32-Pin A | BLK | 18 | Main Logic Ground (G101) |
| A16 | 32-Pin A | BLK | 18 | Main Logic Ground (G101) |
| D21 | 32-Pin D | BLK | 18 | Sensor Reference Ground |
| B1 | 32-Pin B | YEL | 18 | Injector #1 Driver Signal |
| B3 | 32-Pin B | BLU | 18 | Injector #3 Driver Signal |
| C15 | 32-Pin C | YEL/GRN | 14 | Fuel Pump Relay Output to Rear |
Common Failure Modes and Expert Repair Techniques
The most notorious failure point in the 1991 CRX EFI system is the PGM-FI Main Relay itself. The factory relay is potted in a gray plastic casing. Over decades of thermal cycling (heating up from the cabin heater core and cooling down), the lead-free or high-lead solder joints on the internal PCB crack, creating a microscopic open circuit that only manifests when the relay gets warm (hence the 'hot no-start' symptom). To repair this, carefully pry apart the plastic casing using a flathead screwdriver. Inspect the PCB for hairline rings around the solder joints of the main power traces. Use a soldering iron set to 350°C and apply fresh 60/40 rosin-core solder to reflow all heavy-gauge connection points. Do not use acid-core plumbing solder, as the flux will corrode the copper traces and destroy the relay within months.
Additionally, inspect the C401 and C402 connectors located near the left strut tower and under-dash kick panel. Honda's OEM weatherpack seals from 1991 are prone to drying out, allowing capillary action to draw moisture into the female terminals. If you find green copper oxide (verdigris) on the pins, do not simply spray electrical contact cleaner. The female tension is already compromised. You must depin the connector using a proper Japanese Automotive (JAE) depinning tool, cut back the wire to clean copper, and crimp new nickel-plated terminals using a ratcheting open-barrel crimper. Seal the new connections with dual-wall, adhesive-lined heat shrink tubing to restore the factory IP67 moisture resistance rating.