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Honda Main Relays

Now that summer is upon us once again, there are a lot of Hondas that won't start after they get hot. This explains what happens and how to locate the problem.
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» Part 1: What's Up?
» Part 2: Basic Tests
» Part 3: Testing The Relay
 

The most common time to have a no-start condition in a Honda is after a hot soak, like when you pull in for gas or a quick run into a store. The best way to duplicate this condition is to use a piece of wire to hold the throttle linkage and set the engine speed at 2,500 rpm. Let the engine run for about 20 minutes with the hood shut. Then remove the wire and turn the engine off. Let it sit for five to 10 minutes. Then try to restart the engine several times. If the engine doesn't start, turn the key on. The check engine light will come on for two seconds and go out. You should see the fuel pressure go up and hear the fuel pump run during the two seconds. When the light goes out you should hear the main relay click. If it doesn't click, check terminal seven on the main relay (fuel pump) for power and terminal eight (computer) for ground. If you have no power and you have ground, the main relay is bad.

If the main relay is bad on an Accord, you will lose fuel pressure. If it's bad on a Civic, you will lose power to the injectors and the fuel pump but you may not lose fuel pressure because the injectors can't open without power. When the main relay goes bad and there isn't any voltage at the injectors, it will set a code 16 for an injector because the computer doesn't read voltage on the groundside of the injector.

It's also possible that the car has more than one thing that is causing a hard start. You could also have a bad ignition switch, a bad igniter and a bad ignition coil.

If there isn't any spark, move your spark tester to the coil tower to rule out plug wires, cap or rotor. If you still don't have spark, you have a bad ignition coil.

To check the igniter itself, you will need a scope. The igniter terminal of the igniter will have a 10-volt reference voltage. The computer will ground this terminal when it receives a pulse from the permanent magnet sensor in the distributor. The ignition coil produces spark when the igniter releases ground from the negative terminal of the coil. By probing the igniter signal and coil negative you check the input and output of the igniter. If there is a good igniter signal and nothing at the negative side of the coil, the igniter is bad.

The main relay will give you the same symptoms as a bad coil or a bad igniter. A main relay usually only fails when the weather is really hot. You might have a hard start every now and then, but not enough to really cause you any concern, but when an igniter or a coil fails, the car won't start at all until it cools down and that will be a major concern.

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» Part 1       » Part 2       » Part 3
Additional Information provided courtesy of
ALLDATAdiy.com and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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