Repairing Honda/Acura Main Relay

The Honda or Acura turns over but won’t start in hot weather is a common problem caused by the main relay going bad. This is the result of the poor solder application from the factory. Which intern causing the vehicle to not get any fuel because there is an open in the power circuit to the fuel pump

As a consequence, raise areas on the joint indicate a “dry joint” which can lead to an open circuit. The dry, open circuit is the main cause of no start.

This is partly caused by the buildup of heat on the terminals which expand, contract and subject to vibrations. It is likely that a layer of hard oxide has built up inside the solder joint. You will have to remove the oxide on the solder joint by desoldering or removing the old solder before adding the new This is a very simple process. This is the only known problem with the main relays.

In short if you would like to save $100.00 or so dollars by spending less than an hour you can repair your main relay and have many more years of service out of it. Just follow these simple steps below.

The easiest way to confirm that your main relay is bad is by checking the power signal to the fuel pump. If when trying to start the vehicle there is no power signal to the fuel pump you either have a main relay solder problem (99% of the time this is the problem) or a bad wire not carring the signal to the fuel pump. This is virtually never the case.

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1) Fuel relay. Start then let go of key, rpm shoots up, car dies. Next attempt it cranks and won’t start but rpm don’t shoot up. Terminals fault is same as the ECU relay terminals. Identical image used.

2) This powers the ECU, injectors: Not enough solder.

3) The visible crack from the resistor. Symptom: start fine when held in START position. Let go key, car dies which is the same symtptom of a bad ingition switch.

4) Start fine if held in START position, let go key, car dies and can be repeated is the symptom of an open resistor terminal. This can be confuse with the symtptom of a bad ingition switch.

5) ECU Relay: The near invisible and visible crack. Symptoms: Cranks but no fire, no start, check light stays on. The usual cause of no start.

6) Notice the not so shiny solder. This creates a high reisistance, flaking the solder. Also, not enough quality solder.  It’s recommended that all joints be redone but that is up to you.

Now then These are the steps to repair

You will find the main relay behind the kick pannel on the driver side of vehicle please see Location for instructions on finding the main relay.

After Main relay is removed from the vehicle you use a small flat head screw driver gently prying on both sides of the cover you will see something just like the pictures above. When you find your problem here is the first step.

Step1

Desoldering using a desoldering iron. ↓

1.) Press and hold the air vacuum bulb. Apply the desoldering iron tip so that the terminal penetrates within its orifice.
2.) When the solder liquefies, start gently to rotate the desoldering tip so that the component’s terminal can be eased away from the sides.
3.) Release the air vacuum bulb just long enough to suck the solder.

Note: If any solder remains are left on any terminal after attempting to desolder it, resolder it with fresh solder and repeat the desoldering process.

A clean removal should look like this:

Step 2.

If you do have a joint which looks in need of rework don’t be tempted to just reapply the iron. This is unlikely to succeed as the solder in place will have no flux in it, so the flow across the joint will be worse than when it was first made. Better to remove the solder, using a desoldering pump or copper braid, and make the joint again.  The flux’s job is to strip away all of the grease from the surfaces to be soldered, thus ensuring that the solder will flow properly. A concave should be formed with an angle of 40 and 70° from the horizontal. 

A good solder job should look like this:

step 3

Clean with steel wool, inspect for imperfections then use a conformal coating material. Conformal coating enhances performance, improves electrical stability as well as accidental shorts and thermal shock. Their ingredients include varnish, epoxy, parylene, polyurethane, silicone, acrylics, or lacquer. Coatings are applied in a liquid form; when dry, they exhibit characteristics that improve reliability. These characteristics are:

• Heat conductivity to carry heat away from components

• Hardness and strength to support and protect components

• Electrical insulation to preventing accidental shorts

• Low moisture absorption

• Prevents oxidation * tip: If your Honda digital clock currently works, now is a good time to protect it with conformal coating.

• To remove the conformal coatings simply use acetone. BE CAREFUL

Now you are all done. Simply install the main relay back into the vehicle and zoom zoom you go.