MG MGB and MGB GT Tech Talk
I am curious if my MGB is a negative or postive ground. It is a 74 1/2 B, US. If it is positive grond, what exactly does that mean? I have been having a hell of a time getting the electrical system to work. If I run a line from the positive terminal of the battery while installed (both pos/neg terminals connected) to say a light, it works. However, turning the ignition to accessory nothing happens.
Thank you.
A 74 1/2 is negative ground. It sounds like you need to go through the entire harness and clean every connection.
Thank you. Yes, I have thoroughly cleaned it.. Over and over... I have the engine out currently, so I am wondering if the circuit is not complete given the starter/ingition coil, etc., is disconnected.
I have previously had a shorting problem. I found that problem. It appears the wires going to the starter/solenoid, etc., were fraid, melted and exposed. Since disconnected the starter, I no longer have the clicking and sparking I had before.
thru the 67 model year, Bs were positive ground. After that, negative.
grab yourself a copy of the definitive MGB electrical explanation and manual
http://www.amazon.com/MGB-Electrical-Systems-Essential-Manual/dp/1845840577/ref=pd_bbs_sr_5/002-5321106-2991218?ie=UTF8&s=books&qid=1177465881&sr=8-5
about $35 at Amazon (above link)
MGB Electrical Systems (The Essential Manual) by Rick Astley,
All your power comes from the main cable that goes from the battery to the starter, if the engine is out the circuit is broken and nothing will work.
Ok, kinda what a I figured... Good thing I asked here.. http://www.mgexperience.net/phorum/read.php?1,549363
Thank you.
We went through this very recently. In my post then I said something to the effect that I had understood that the reason for positive earth was because some engineer somewhere decided that he wanted the arc at the spark plugs to be initiated from the other side of the gap. I then added something about it possibly being related to the six volt systems being used way back when. Frankly I don't know of a compelling reason to retain positive earth except perhaps a gauge or two that was designed for that arrangement.
Your car is of course a negative earth system.
Jack
Jack - Back in the early ays of MG when the electrical system was 6 volts, they were negative ground. They ddn't go to positive ground until the T series came out. Cheers - Dave
Didn't know that Dave, now I do. :-) I was thinking of the early Bs and the early fifties Chrysler products.
jack
It's all to do with electrochemical potentials across cable junctions at the battery - it was thought that pos grd gave less corrosion. Ford used it until sometime after 48. The coil/spark/points deal doesn't matter, since it can be made to give the right spark direction (from hot center electrode to cold grd electrode) either way. That's why old Lucas coils are marked CB & SW for when they were all pos grd, but later coils are marked + & -, to accommodate either. Everything is now neg grd since semiconductors want it that way.
FRM