MGB & GT Forum

Your MGB's electrical system has a short to ground . . . . somewhere in the system.

You have a wire that is carrying 12 volts DC:

Somewhere on that wire is a break in the insulation on part of the wire that is "touching" the metal part of the MGB body.

You can either send the car out and pay a good mechanic to find your problem, or you can go through these steps to find the problem:

MGB Dead Battery

READ THIS ALL THE WAY TO THE BOTTOM, BEFORE YOU ATTACK YOUR MGB ELECTRICAL PROBLEM:

MOST OF THESE PROCESSES DON'T NEED ANY KNOWLEDGE; BUT, YOU DO NEED TO KNOW HOW TO MAKE ELECTRICAL CONNECTIONS ON YOUR VERY OLD CAR, CLEAN AND SHINY.

Very simply stated, your Ignition Light is connected between the battery and the alternator.

When one or the other is not working correctly, and you don't have the same voltage on both sides of the light bulb, the light bulb "glows".

First:

Remove both battery cables from the battery, and clean the connections, inside the battery cable clamps, and the outer part of the battery terminals.

Look at the connection between the end of the ground cable and the connection to the MGB body.

If it is not clean, it will need the same service directed above for the battery cable clamps at the battery.

The end of the second cable, at the starter, also may need a "shoe shine" as well.

Don't have a battery terminal brush/cleaner:

http://www.autozone.com/autozone/accessories/Tools-Garage-and-Equipment/Battery-Terminal-Brush-Cleaner/_/N-25ni

Reinstall the battery cables.

Second:

Put a battery charger on the battery over night, to see if it will come up to a full charge.

The next morning, before you start the car, remove the battery charger leads, and measure the Battery Voltage across the terminals, inside the clamps.

Start with a multimeter set on DC volts....don't have one, this is a reasonably priced choice:

http://www.radioshack.com/product/index.jsp?productId=421466

If the battery voltage, after being charged overnight, is not well above 12 volts DC, it will need to be replaced.

Bring up this file, and locate the schematic for your car.

www.advanceautowire.com/mgb.pdf

Go to the 1977 schematic:

THIRD:

With you at the battery, with the multimeter leads on each terminal, red on (+) and black on (-), have someone else start the car

Measure the DC voltage across the battery terminals, while somebody holds the accelerator down at 1500 RPM.

A healthy alternator should have an output voltage between 13.8 and 14.3 Volts DC.

If the voltage at the battery is less than 13.8 Volts DC, with the engine running at 1500 rpm, then open the hood, and check the Voltage on the large terminal on the alternator.

Again measure the Voltage while someone is pushing on the accelerator and controlling the engine speed., and see what the alternator voltage is doing when the Ignition Light comes ON.

Before you go crazy:

IT MAY NOT, with all the clean battery cable clamps and battery terminals, and a fresh battery charge, overnight.

If the battery won't hold a charge, it will need to be changed.

If the alternator does not have an output of at least 13.8 volts DC, then it will need to be replaced....don't make this decision until all the other steps are completed.

My twin brother and I, with our father's encouragement, and some tutorials from a 70 year old mechanic in 1955, learned how do all this when we were 15.

By the time we left high school in June 1958, the Model A Ford had a later model Ford flathead V8, added hydraulic brakes, and rewired the car from scratch, nothing but a roll of wire and a soldering iron.

We both became Mechanical Engineers, I at NASA's Johnson Space Center, in 1963, with a transfer to CA in 1983. My twin brother went to work for Admiral Rickover at the Navy's Bureau of Submarine Nuclear Reactor Development in Washington, DC. He is still goes to work at age 74. I retired in 2010 at 70.

Roger N. Tanner, Professional Engineer (retired)
Oxnard, CA USA
roger.n.tanner@gmail.com

---

Roger N. Tanner
Professional Engineer, Retired



Edited 1 time(s). Last edit at 2014-10-24 04:46 PM by rntanner.