Going to the negative side with the MGB GT

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I’ve always thought I’m positive, my wife says I’m negative. The 1967 MGB GT has always been positive but now I’m making it negative. I guess my wife wins. What gives?

A few things. At some point in the MG’s life a few changes were made that would have been done on a negative earth car. While these changes don’t really make much difference and would be easy to switch back it did get me thinking about the situation a little.

The only real reasons to keep the MGB as a positive ground car are for originality and the fact that the tachometer is wired for a positive ground system. After doing some research I found that converting the tachometer to negative ground is not that big a deal, just a small amount of soldering and changing a few wires. Why bother though?

The main reason I’ve decided to change is that I’m thinking I’ll take out the antiquated generator and put in a alternator. This has many advantages, primarily more amperage, especially at low RPM. More amperage means more power for brighter headlights and the system will work with modern electronics like GPS units and stereos etc.

Presently the generator isn’t producing amperage at all and at this point that’s one of the main reasons the car isn’t on the road yet. No charging, eventually the car will die. I expect it’s worn brushes and I should be able to get it running again easily but long term an alternator should be less troublesome and I’m planning ahead for the switch.

The hardest part of the tachometer modification job was getting the tachometer out of the dash. It’s one of those jobs that if you’ve done it before only takes a few minutes but the first time can be time consuming as you try to figure out the system. Certainly the internet helps as a number of folks have discussed the methods.

The actual positive ground to negative ground conversion is well documented on mgaguru.com. Will it work? I’ll let you know a little later.

A few photos:

Rats nest of wiring under the 1967 MGB GT dash
Rats nest of wiring under the 1967 MGB GT dash
Removing Smiths tachometer form an MGB GT
Removing Smiths tachometer form an MGB GT
Smiths positive ground tachometer
Smiths positive ground tachometer
Smiths tachometer wired for Positive Ground
Smiths tachometer wired for Positive Ground
Smiths tachometer wired for Negative Ground. Wires switched for the diode and ground.
Smiths tachometer wired for Negative Ground. Wires switched for the diode and ground.
Work notes from previous service on a Smiths Tachometer
Work notes from previous service on a Smiths Tachometer

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