How to Replace a Honda Main Relay
If you are having trouble starting your Honda the first step to take is diagnostic and, possibly, replacement of the main relay. Design improvements after around 1995 has made this relay more reliable and, after around 2005, the main relay was divided into two parts. However, if your Honda is much older than about ten years, this is a common problem area.
The main relay in Honda automobiles is essential to your car's operation. Actually two relays in one, the main relay controls power to the car's computer and to the electric fuel pump. Diagnostics and replacement of the relay is quite simple, especially if you have a little bit of experience with electrical troubleshooting. The only tools that should be necessary for this would be an electrical meter capable of measuring voltage and resistance and a pair of jumper wires, preferably with alligator clips on the end.
In addition, since the specific terminals vary for different years and makes you should have a shop manual with a wiring diagram for the car. It might be possible to search the Internet for the specifics on your vehicle but that's not guaranteed. You could skip the resistance troubleshooting portion of this diagnostic but if you do you might end up spending between $50 and $100 replacing a part that wasn't the problem.
When you turn the ignition key to on, but before you try starting your car, you should hear two solid clicks from the relay. One is turning on the ECU and the second powers the fuel pump. Before you turn the key to start you might hear a softer click after two seconds. This is the fuel pump turning back off after it pressurizes the system. When you turn the key to start and crank the engine you should hear a third click. Since the starter will be cranking at this point you may actually need to put your hand on the relay and feel this one.
If any of these actions are missing there is a good chance that the relay has failed. If all are missing you may wish to check for a failed fuse.
The better test for the function of the relay is to provide power to the proper terminals to activate each portion of the relay and check for a closed circuit. This diagnostic isn't particularly difficult but if you don't have the diagrams to tell what terminals are the correct ones, or if you've never done this type of testing, you can likely skip this test. If the relays are working properly in this case you will need to check elsewhere to diagnose your problems.
Removal of the relay is quite simple. It is located under the dash near the fuse panel. You will likely have to remove the cover from below the steering wheel, under the dash. To remove this cover, carefully pull out on the top of the panel, tilt the cover slightly back and lift off of the hinges at the bottom. Under that you will see a metal plate held in place with four bolts. Remove the bolts and the plate. The relay is generally located to the left of the steering wheel and you simply pull it out. When you're ready to replace it, simply push it back into place, replace the covers and you're on your way.
If you're handy with a soldering iron you may actually be able to fix your relay. The most common problem with these devices is defective solder joints. After you remove the relay you can take off the plastic cover. Using a small screwdriver you gently pry open one side and then the other. Small plastic retaining tabs hold the cover on. Once the cover is off you can see a small circuit board inside with numerous solder joints. To do the repair properly you should remove all of the old solder on one connection at a time and resolder the connection. You might be able to just reflow the old solder with a little bit of flux, however, and solve the problem. Replace the cover and there's a good chance you're ready to go.
