What's The Best Way To Change 78 T140e Bonnie To Negative Ground

[scottholl]

hello group, (1st post)

I searched the forum looking for a definitive explanation of a classic question but couldn't find what I was needing so forgive bringing back old ghost threads.

I have a 78 T140E 750. The vin # suggests a May 1978 build date.

1.) What are the steps to convert the current positive ground and convert to negative ground?

2.) If I bought a 1979 harness (negative ground and built to support the electronic ignition) and assembled it to my 78 would it be a seamless adaptation or are their critical incompatibilities?

thanks in advance

scottholl

 

[Rudie]

78 T140E 750. The vin # suggests a May 1978 build date.

1.) What are the steps to convert the current positive ground and convert to negative ground?

Why?

There are quick-'n'-dirty ways of doing it and better ways of doing it but, apart from you regularly work on negative ground electrics and have trouble swapping between negative and positive, I don't know of a good reason to do it.

2.) If I bought a 1979 harness (negative ground and built to support the electronic ignition) and assembled it to my 78 would it be a seamless adaptation or are their critical incompatibilities?

No, '79-on harnesses (plural intentional ) have significant differences:-

. Completely different handlebar switches - left-hand is likely ok but ignition and lighting switches are completely different, '79-on idiot lamps and ignition switch are in a console bolted between the clocks on the top yoke.

. The seamlessly supported electronic ignition is the Lucas Rita - 'amplifiers' available off eBay, updated trigger unit available new from a guy in England. Connecting any other e.i. needs more thinking about than connecting the same other e.i. to a positive ground harness.

. '79-on alternator is 3-phase, any harness is set up for a negative ground rectifier; an early harness needs a negative ground Zener diode (reliable positive ground Zeners (infinitely more common) are hard to find, a reliable negative ground one is likely to be a nightmare). The 'later' harness didn't have a DC connection for a Zener because it used a pack of three, one connected to each alternator-Zener lead. The obvious fitment is a electronic combined regulator/rectifier, but you have to adapt a '79-on negative ground harness for that, same as you do a pre-'79 positive ground one.