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positive to negative ground conversion

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jvandyke

jvandyke

Luke Skywalker
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One of my winter projects is going to be conversion to negative ground (for stereo -been running it "isolated" all summer, and Crane Cams XR700 ignition -salvaged from my wrecked Opel).
I know I have to rewire the tach, I've found instructions on that, just probing for any advice beyond that.
I plan on renewing the cables on the battery, cleaning up the tray area a little. Spinning the battery 180 degrees, rewiring the tach, have to switch the wires on my fuel pump I believe, other than that, all other devices should be happy, no?
 
Don't forget to polarize the generator.
 
Depends upon the Crane unit you have. If its an older one (I don't know the EXACT time that CRANE switched) it is ground specific. That is made for use as positive ground OR negative ground . Newer ones can go either way. .. My guess is that if it came from you Opel it IS an older model. I have one of the older units and it is one of the things that has kept me from making the change. I just don't want to have to buy another one, and this one just won't quit!
 
Right. Thanks
Opel was a '70 GT and negative ground. Midget has an Allison (early version of Crane I believe) bolted to the engine bay but not hooked up,maybe because it was the wrong ground. My Crane is for negative ground I'm sure, or could go either way. Hopefully the shutter fits in the dizzy. Doesn't look like there's a Lucas specific model that is different then the one I had on the Bosch dizzy from the Opel. Ah, the Opel, I had custom tuned that dizzy for limiting total mechanical advance, oh the things that car taught me.
 
It really is quite a simple conversion to make provided as Jack says, you remember to polarize the generator, take care of the tach conversion, and in my case at least, switch the wires for the fule guage which read empty when full and vice versa! Most of the electrical system in the car does not care which path runs to ground!
 
You do not need to switch the fuel gauge wires.

On the fuel pump, if you have an SU, you do not switch the external wires. Check under the black plastic cap on the pump and you MAY find a diode or capacitor wired parallel to the pump's points. Not all pumps have either of those components. Where you do find them, you remove the diode or capacitor, flip it so it's connected "the other direction" and refit the pump's plastic cover.
 
Yes, the fuel gauge is not a conventional analog meter, like the ones used on test instruments. The latter have a coil of wire moving in a field created by permanent magnets. If you reverse the current in such a meter, it reads backward; if you could reverse the magnets, it would again read correctly.

There are no permanent magnets or moving coil in the fuel gauge. It depends only on the strength of the magnetic field generated by two electromagnets, not its direction. So, polarity doesn't matter.

Good description of this at https://mgaguru.com/mgtech/electric/fg_01.htm and the following links.
 
just to clarify, this instruction:
https://www.mgexperience.net/article/neg-convert.html

check the picture, is this correct?
tachguts.JPG
 
Thanx for the pic.. i've been fluxomed by the descriptions for years and this is the BEST information i've ever seen... I may get around to doing this now that I KNOW what needs to be moved.


Of course things are never as simple and cheap as they first appear. Changing would mean that I have to replace my Crane unit, as mine is an older model that is for Positive Ground ONLY. I understand the newer runs go both ways? !
 
I think this is what the instructions are saying, hopefully someone will verify, is that how you read it too?. Then switch the white wires on back. My chrome ring came off pretty easy too, a little oil, pryed up the tabs a bit, turned to line up with notches and it came right off, the instructions don't say it but to get the guts out you have to take the white wire loop connection thing off the back too.
I've got an Allison ignition bolted in the engine bay, not hooked up (running points) and a Crane that is negative ground only. Not sure what's up with the Allison. Do you have a picture of the shutter inside the dizzy? I'm hoping I can use my current shutter, haven't tried to fit it yet.
 
Here's how I did it, hope this is right.
tachguts2.JPG
 
When we installed the shutter and eye in the dizzy, we had to CUT the post off and file it smooth there. (the post onto which the points slide and is their pivot point) We just could NOT position the pick up anywhere close to where it was supposed to be in either direction without cutting off the post. After the initial pain in the A#$ getting it set up, its been completely trouble and maintenance free. Worth the effort. Sorry no pic.. but be SURE to use the tie wrap to hold the wires away from the rotor!
 
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