The Garage – Electrical Faults by Mike Owen of Owen Automotive

My most frequent telephone calls are often about electrical problems.  “My car won’t start” or “the lights don’t work” or…

For starters, understanding the wiring diagrams and being able to identify wires is essential for this work.  British wiring harnesses follow a standard colour coding system.  The colours on older cars with original cloth covering are often faded which makes identification difficult.  Newer harnesses with PVC covering are easy to trace on a wiring diagram when fault finding. 

These tests involve using a basic 12-volt test light to test electrical circuits – not the self-powered variety.   A test lead is also needed.  Ensure the test light is working by connecting it to the battery.   To avoid corrosion or cutting a wire, it is better to test the end of a wire rather than penetrating a wire with the sharp probe.  Note: The brightness of the test light is a good indicator of the power or voltage of the battery.

Fuse box – The fuse box is very often the main culprit for electrical problems.  Attach the alligator clip of a test light to a suitable ground such as the engine.  With the ignition on, probe the power-in set screw and power-out screw to see if the test light lights up.   If the power-in screw is okay but the power-out isn’t, the problem is either a blown fuse or corrosion on the fuse holder clip.  Clean up the contact with a roll of sandpaper and gently squeeze the clip if needed to ensure a snug fit for the fuse.  This is important if the car is used infrequently or stored in a damp area. 

As a precaution, test the fuse by placing one end in the fuse holder clip and probe the power-out end of the fuse.  Always use the correct amperage fuses.  Apply a small amount of dialectic grease to the fuse or clips to ensure a good contact.  Also, check the end of the screw for corrosion.  Trim the wire end if needed and ensure the screws are snug.   

On MGBs, if the plastic dust cover for the fuse box is missing, replace it. 

Tail lights – The bullet connectors for tail lights originally had the wire folded over the bullet and were not soldered.  Check the tightness of the connection and apply dialectic grease.  Check the light socket for corrosion and clean with a pointed wire brush as needed.  With the light switch on and using a test light that is grounded to the light socket itself, probe the terminal in the socket for power.  Apply dialectic grease to the socket.  Also, test to ensure the socket is properly grounded.  These procedures can also be used for checking parking and headlights.

Check the light bulb to ensure it is not burned out.  First, clean the contacts of any corrosion.  A simple test is to hold one end of a test lead against the base of the bulb.  Then connect the bulb terminal and the other end of the lead against the battery terminals.   

Brake light switch – A common problem with brake lights is a brake light switch failure.  After-market switches are often poorly made or allow silicon brake fluid to leak into the switch.   Using a test lead with the ignition on, bridge the set screws on the switch.  If the brake lights work, the switch is faulty.  Another approach is to disconnect the wires and use an “open” cotter pin inserted into push-in connectors to bypass the switch.  For later MGBs with a mechanical switch, test the wires on the brake pedal box on the firewall.  These frequently corrode.

Ignition switch – The most common problem with the ignition switch is a loose connection caused by years of vibration.  When driving, the engine may intermittently cut out.  Use a flashlight under the dash to check for a loose wire to the switch.  Tighten the screws as a precaution.

Tail and licence plate wiring – A break – from a rear end collision – in the red wire to the tail lights and licence plate light can result in a burn out of the wiring harness.   It is a good idea to install a 10 amp in-line fuse in the wire.  Austin Healeys are particular susceptible with the licence plate light set low on the bumper.

Bad ground check – A test light is most often used to test for power.  It can be used “in reverse” to test for a bad ground connection.  Power-up the light by connecting the alligator clip to a power source.  On the MGA, the most convenient source will be the “live” power-in terminal at the fuse box.  Test any ground with the probe – using a long test lead if necessary – and if it lights up, the ground is good. 

Batteries – Battery posts and clamps should be cleaned of corrosion regularly with a mixture of baking soda and hot water.   Apply dialectic grease or vaseline to the posts.  As well, remove, clean and re-tighten the bolts used to clamp the cables.   Put red paint on the positive battery post, clamp and cable to clearly mark the positive terminal.

Today’s batteries are sealed and the cells cannot be topped up.  Because batteries will sulphate over time, they have a lifespan of about five years when used regularly – likely less if used infrequently.   Take an older battery to an auto electrical shop for a load test to ensure it has the needed cranking power. 

When storing the car, it is a good idea to use a battery maintainer to maintain the charge.  Disconnect the battery by removing the ground cable and connect the maintainer to both battery posts.  Better yet, remove the battery from the car.  Another idea is to install a battery cut-off or isolator switch in an inconspicuous location for charging and security purposes. 

One last thing, consider doing these checks as part of an annual maintenance plan.