Moss Motors, Ltd

I am re-building a 1951 TD with MGA engine and drive train.  I started over and am completely rebuilding the car from the ground up.  The engine (1500) will be ready in a couple weeks so I am now deciding what to do with the electrical system.  I will also be replacing the wiring harness.  Since this car is far from original I am not terribly worried about changing things and want a car that will be reliable and fun to drive.  Consequently, I am considering going with a negative ground system with an alternator and electronic ignition.  I know this car came with two 6 volt batteries.  I would like to go with a single 12 volt battery.  Any suggestions or recommendations?  All my 40 something years of car building has been with American muscle cars so this project is a real departure for me. 

All of the TD series (1950-1953) came with a 12 volt battery in a positive ground configuration.  The MGA used two 6-volt batteries wired in series for 12 volts, so the only change you'll need to make to the car is reverse the wires to the ammeter for a negative ground setup.

The electronic ignitor kits that replace the distributor points are polarity-specific.  It's amazing how much better these Brit motors run with the mod - money well spent.  And, if you've got some bucks to spare, replace the transmission with the 5-speed Moss conversion or similiar.  MAJOR improvement in drivability! 

That TD with the MGA motor will really move (for a TD, anyway).  Make sure the brakes are up to the extra workload - not one of the "T" series strong points ...

There is a lot more to reversing the voltage than you might think: 

The two sockets on the dashboard will have the wrong polarity, and the clock may stop working (it says positive earth on the back).

Whilst the wiper motor will still work, the motor inside will spin the wrong way, so it will quickly run its bearings due to the reversal of thrust.  Reverse the two wires. 

Obviously, the voltage regulator will require re-wiring, which could be a major job if it is the earlier RF95 type with integrated fusebox.  Also, an alternator can produce double the current (or more), so the main feed wire will not be thick enough and could melt.  It needs to be uprated.

Accessories - If you have a heater fitted, it will suck instead of blow, and the same goes for any screenwash pump or air horns.  Once again, just reverse the wires.

There is a remote possibility that you have a replacement SU fuel pump fitted with a diode instead of a capacitor.  If so, this will quickly overheat and burn out.  I have seen 2 of these.  Best to remove the diode and replace it with a 23 volt bi-directional Transil.

Have you worked out how you are going to power the rev counter?  An alternator has no take-off for a reduction box, and anyway it needs to spin at a much higher rpm. 

 

Having never put an MGA 1500 block in a T-type, but I have both an MGA and a TF.  Don’t agree with all that Ormolu writes. The screenwash pump on the A is pneumatic; the horn will still blow; you can most likely run the rev counter off the tach. drive on the block. (May only give an approximate RPM).

"The two sockets on the dashboard will have the wrong polarity, and the clock may stop working"

The wiring to the sockets  will need to be reversed.  The clock in a TDis not polarity sensitive.

 

"Whilst the wiper motor will still work, the motor inside will spin the wrong way"

The wiper motor in a TD is not polarity sensitive

 

"the voltage regulator will require re-wiring"

The regulator is not polarity sensitive, contrary to some information that says they have to be flashed if the polarity is reversed.

 

"If you have a heater fitted, it will suck instead of blow, and the same goes for any screen wash pump"

There is no OEM heater in a TD.  The most used after market heater is  one for the Arnolt TD and the blower motor in it is not polarity sensitive - it turns one way regardless of polarity.  The windshield washer in a TD consists of the person sitting behind the steering wheel getting out armed with a rag.

 

"Best to remove the diode and replace it with a 23 volt bi-directional Transil."

Or just reverse the diode leads.  The fuel pump will continue to work even if the diode (actually, diode/resistor assembly) burns out, but the points will burn prematurely.

Cheers,

Dave 

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Cheers,
Dave
http://homepages.donobi.net/sufuelpumps/

JJ

My twopennorth, why not go for a complete rewire, it surely needs it?? Use an MGB harness,later alternator car. I suspect the harness would be cheaper than a TD one and will take higher charging current. Get an enlarged MGB wiring diagram, eBay?

 You will also be able to incorporate traffic signals and reversing lights easily. You will possibly need to extend the cables in places, stick to the colour code. Seriously think about incorporating a realistic fuse panel, i.e. >4 fuses, under the dash somewhere. Dont try and incorporate the existing ammeter in the alternator charging circuit, currents will be too high after start up particularly.

What about MGB Discs +remote servo on the front, I think motorbill is going to give us the gen for this? Overdrive gearbox?

Great winter job!! cheers, steve

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To a man equipped with only a hammer, most problems look like nails

"Get an enlarged MGB wiring diagram, eBay?"

 MGB, color coded wiring diagrams are available free from  Advance Auto Wire at: http://www.advanceautowire.com/  Click on stock Schematic, then on MGB, MGC, MGBV8 and finally scroll down to the year you want.  The diagrams can be printed on a home printer or you can copy the whole file, take it toKinko's and have them print it full size (11 X14 I believe) and have it lamnated if you desire.

Cheers,

Dave

 

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Cheers,
Dave
http://homepages.donobi.net/sufuelpumps/

Guys,

We've done a number of these conversions. It's not a difficult job, and you can use almost all of the existing wiring. I wouldn't advise trying to stuff an MGB wiring harness into a TD. You'll end up with a hodge podge of unused wires and things in the wrong place. Get a new TD harness and when the time comes, put it into place. The alterations including extra fused circuits and an alternator, if you want one, are pretty simple. Let us know what you decide. We can help.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon