Cars · Diagnostics
Honda Main Relay Diagnostic & Repair Guide
Comprehensive diagnostic and repair guide for Honda main relay failure, solder reflow procedure, and fuel pump prime issues.
A faulty PGM-FI Main Relay is one of the most common causes of intermittent crank/no-start conditions in Honda and Acura vehicles produced between 1988 and 2000. This issue typically surfaces on hot days or after the engine has been sitting for a short period after driving.
Symptoms of a Failing Main Relay
- Engine cranks but will not start: The starter turns the engine over normally, but the fuel pump does not prime.
- No check engine light bulb check: When turning the ignition key to Position II (ON), the Check Engine Light (CEL) does not illuminate for 2 seconds and then click off.
- No fuel pump prime sound: You do not hear the 2-second fuel pump "hum" from the fuel tank when turning the key to Position II.
- Heat-sensitive failure: The car starts fine in the morning, but fails to start after parking under direct sunlight.
Main Relay Location by Vehicle Model
| Model | Model Years | Relay Location |
|---|---|---|
| Civic / CRX | 1988–1991 (EF) | Under driver-side dashboard, near hood release bracket |
| Civic / Del Sol | 1992–1995 (EG/EH) | Behind driver-side dashboard kick panel |
| Civic | 1996–2000 (EK/EJ) | Behind glovebox near heater core casing |
| Integra | 1990–1993 (DA) | Under driver-side dashboard left wall |
| Integra | 1994–2001 (DC2/DB8) | Under driver-side dashboard left wall |
Step-by-Step Diagnostic Test
- Turn key to Position II (ON).
- Listen closely for three distinct clicks:
- Click 1: Ignition switch energizes the main relay coil.
- Click 2: Fuel pump turns ON (humming sound for 2 seconds).
- Click 3: Fuel pump turns OFF (CEL turns off).
- If Click 2 or 3 is missing and the fuel pump does not hum, the main relay board has cracked solder joints.
Repair Procedure: Solder Joint Reflow
Instead of replacing the main relay with an aftermarket unit (which often fails prematurely), reflowing the factory Mitsuba relay PCB solder joints provides a permanent repair.
Tip
The original factory relay uses unleaded solder that oxidizes and cracks under heat cycles. Using 60/40 rosin-core solder resolves this permanently.
Required Tools
- Soldering Iron (30W–60W)
- 60/40 Rosin-Core Solder
- Desoldering braid or suction pump
- Flathead screwdriver (to remove relay plastic casing)
Procedure
- Unplug the main relay connector and remove the mounting bolt (10mm).
- Gently pry open the plastic clips securing the circuit board inside the housing.
- Inspect the copper solder pads on the back of the circuit board under magnification. You will notice tiny ring cracks around the large relay terminal pins.
- Apply flux and touch the hot soldering iron tip to each cracked joint.
- Add fresh solder to form a smooth, shiny solder cone over all 7-8 primary pins.
- Reassemble the relay, reinstall it in the vehicle, and test fuel pump priming.