Engine Maintenance

Troubleshooting Main Relay Problems


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The main relay is a device that opens and shuts the fuel system, and in most cars it also opens and shuts the ignition system. A common main relay problem is that the car doesn’t start if the interior of the vehicle gets hot. This is generally because of poor solder application at the time of manufacture. This can result in a dry, open joint. This joint causes main relay problems in cars. The problem may also be due to a buildup of a layer of hard oxide that can be removed by desoldering.

How to Troubleshoot Main Relay Problems:

  • If the car doesn’t start when the ignition switch is turned on, and if the check engine light doesn’t switch on and off, it indicates that the ignition switch, the main relay, the fuse or the ECU are faulty.
  • If the car doesn’t start when the ignition switch is turned on, if the check engine light switches on and off and if there is no relay click when the check engine light is on, then the main relay is dead.
  • If there is no relay click when the check engine light is off, the main relay is most likely dead.
  • Similarly your main relay is dead if you bang on the dashboard or the main relay and the car starts. Also if the check engine light comes on and stays on and there are no accompanying clicks, you have the same problem.
  • If after doing all of the above, you switch on the ignition and then switch it off without hearing any clicks, you need to remove and fix the main relay.
  • If the main relay is dead, you can rectify the problem by replacing, or removing and repairing it.
  • To replace the main relay, you should first disconnect the negative cable on the car’s battery and also remove the cruise control if your car has one. You should then remove the 10mm mounting nut and the post that holds the main relay in place. Once these are removed, it’s relatively easy to slide the main relay bracket off and remove. After the wire harness is disconnected from the main relay with a screw driver, you can attach the new one to the wire harness and mount the main relay bracket in pace. Fix the nut and post back. Lastly, reattach the cruise control and reconnect the car battery.
  • If you choose to repair the main relay, remove it by following the procedure listed above. The weak joints should then be de-soldered and a fresh rosin core solder should be applied. After doing this, the main relay should be reinstalled in the car.
  • Once the main relay has been replaced or repaired, the car should restart and idle easily on both hot and cool days.

You can take the assistance of a professional mechanic to repair your vehicle, but if you have some mechanical knowledge, you can save money by solving your car’s main relay problems on your own.

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