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Chasing Electrical & Wiring Faults

Started by JohnTwist, August 09, 2020, 02:08:01 PM

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Sorting our electrical problems requires a step by step, methodical approach.  It is necessary to identify the problem, examine the wiring diagram, then trace the circuit, connection by connection (usually from the HOT side of the battery) until the problem is found.

Half of all electrical problems arise from faulty battery connections; half of the remainder arise from a dirty fusebox.  Service the battery!  Clean the posts, the clamps, and the earth strap!  Service the fusebox!  Remove it and sandblast or etch away the corrosion.

As a rule:  Wires do not fail.  The bullet connectors corrode; the female connectors oxidize; but the wires themselves NEVER fail.  There is an exception to every rule.  Clean the bullet connectors with sandpaper.  Replace the female connectors.

As a rule:  Wires begin and end OUTSIDE the loom or harness.  There is NEVER a reason to cut through the PVC wrapping to expose the wires.  This rule has no exceptions.

When something works when it shouldn't, when odd things occur at the same time, the problem is often a faulty earth.
Body shops cannot wire.  Expect the most bizarre connections (and results!) after a trip to nearly any bodyshop.
It may be easy to visualize the wiring as plumbing; wires as pipes; switches as valves; the battery providing "pressure;" and all "juice" MUST return to the battery.

Light bulbs work or not.  A dimly glowing bulb indicates a faulty earth.

A glowing High Beam indicator (on both high and low beams) indicates a faulty earth at the headlights.

The dash rheostat on the later cars fails easily.  Simply wire through it to give full brilliance to the dash lights.

Dash light bulbs film up on the inside of the glass.  New bulbs will make a world of difference.

Carburetter cleaner removes paint and undercoating from wires to expose the true colour code.

An ignition relay failure (77-80) will result in a dead GREEN circuit.   A faulty fusebox evidences itself the same way.  Test the WHITE/BROWN on the third fuse – if that is HOT, the relay is good and the fusebox/fuse dirty. 

The MGB → 74/2 takes a group 26 battery; the MGB 74/2 → takes a group 24. The wiring at the starter solenoid sometimes loosens.  This will evidence itself in a winking ignition light; occasional failure to start; occasional failure to charge.
If the horn(s) just chirp, there is not enough power getting through them; clean the earth contacts in the horn push.

There are two ignition coils:  12volt, no ballast resistor, 68 →73, DLB 101; 8 volt, use with 1.3-1.8 ohm ballast resistor, 74→79, DLB 102.  The 1980 uses the DLB 101.

Neither the temp gauge nor the fuel gauge fails.  It's always the sending unit.  There is an exception to every rule.
If the earth connection in the boot is unattached or loose, the fuel pump, side markers, courtesy light, reverse lights, licence lights, or taillights may malfunction.

If the turn signals do not work, SNAP the hazard switch on and off 20-30 times.

TOOLS:  A 12 volt test light and a wiring diagram for the SPECIFIC year and model.

WARNING:  Approaching an electrical malfunction without a test light, without a wiring diagram; or helter skelter, is a certain route to madness!

QUESTIONS?  John Twist, University Motors Online,  Technical Hour 1-2 pm EST M-F  616 307 6737
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


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