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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 main relay on Honda vehicles may often cause intermittent no-start conditions, especially when the weather gets hot. But the trouble really starts when another component fails, like an ignition coil or an igniter.

Before we diagnose a no-start, let's look at how Honda's main relay works. The relay is a dual relay, that is to say that it is two relays in one. The first relay powers up the computer and supplies power for the second relay. Once the computer is powered up, it will ground the second relay, which supplies power to the fuel pump. There are three conditions that will cause the computer to activate the fuel-pump half of the main relay:

  • When the key is turned on, the computer will run the fuel pump for two seconds.
  • When the starter is engaged.
  • When there is a tach signal to the computer.

Now let's diagnose our no-start. Before I do anything else, I check for a computer reference voltage. This can be done on any input sensor. An easy one to get to is the Throttle Position Sensor. If I have 5 volts there, then I know the computer has power and ground. If the computer isn't powered up, don't even think about trying to trace out fuel or spark problems. There won't be any spark or fuel until the computer has power and ground. Most no-starts are caused by little or no fuel, little or no spark, low compression or spark at the wrong time. These are all easy things to find if a component has failed, but it can be real hard to find a problem that fails intermittently.

Intermittent starting problems are usually spark or fuel. Low compression doesn't come and go. If you had low compression yesterday, you'll have it today. To check for spark and fuel you will need a fuel pressure gauge, a voltmeter and a spark tester. When the engine doesn't start, you want to be able to check the igniter, the coil, the injectors and the fuel pump.

Hook up your fuel pressure gauge. There are two places to hook up a fuel pressure gauge on a Honda. One is on the fuel rail and the other is on one of the bolts on the fuel filter. After you have connected the fuel pressure gauge, start the engine and make sure that there are no leaks. You don't want any leaks while you are testing for spark. Connect the voltmeter positive lead to the ground lead of one of the injectors. Connect the spark tester to one of the spark plug wires.

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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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