I am starting the assembly of my 59 XK150 FHC tomorrow morning. I am not going to be into wiring for a while but I was wondering about changing to negative ground. I do believe that my car is positive ground. What is involved with the change ?
I purchased XK150 explored and it is going to be a great help but it doesn’t have anything on changing grounding.
On a second item: are there parts available to go to a dual master cylinder for the brakes ?
Stephen
Flashing the fields in the generator, and swapping sides of the ammeter.
As paul has stated, but why change? you might try a google search and or search this site for reasons and methods. A simple way of looking at it is to determine IF you are going to be adding any modern day accessories )fuel pump, ignition or radio) if so negative earth is the way to go
Stephen
I switched my 58 150 to negative ground. I installed an alternator rather than the generator, high touque starter motor, lot smaller to deal with. I even used an modern fuze box. None of the changes are so permanent that a future owner could not restore the car to original. I wanted a reliable driver rather than a hanger queen show car.
Len Wheeler
Tucson
Can’t remember the details but I was told by a very reliable scource that very early cars were negative earth. Somewhere along the line it was decided that positive earth was better. (For reasons I can’t recall)
Subsequently, this was discounted and a reversion to negative earth come to pass.
I think it’s something to do with electrolysis
And the fact that steel corrosion is reduced by grounding negatively.
I am not married to the idea of changing to negative grounding. I do want to do a concourse restoration. 95% + of the car is original and it is a 48,000 mile car. I got it in a basket and spent 9 hours going through small parts yesterday. I am in good shape other than some fasteners. I am not planning on using the car as a driver. I just would like to do one car that is pure show. I pulled the original starter out of a box yesterday and it is enormous. As long as there are no huge detriments to positive grounding I think I’ll stick with it.
Cars were originally positive earth for better corrosion resistance. the move to negative earth came from modern electronics (radios etc) where negative earth was standard due to transistor configuration/requirements. try a google search for a better definition/explanation. Provided you don’t want any modern stuff you will not have an issue using a positive ground.
Positive earth was better, but I think there was some sort of dispute among manufacturers, leading to the convention of negative earth for no good reason. Non-British cars in the US were negative earth well before transistors were introduced in radios (which was ca.1960 IIRC). The valve radios didn’t care about polarity, as they used vibrators to generate AC. Transistor circuits can also be designed to operate from either polarity, but one or the other permits more straight forward circuit design, depending on whether the main transistors are PNP or NPN. Early transistors were PNP Germanium, for which positive earth permits a more straightforward circuit design. IMHO.
Certainly late-to-change cars are another story.
I thought positive earth was the system that was more, not less, corrosion-prone?
Certainly every lead-acid battery I’ve ever owned had/has more corrosion around the positive post than negative.
Positive ground is considered better for the ignition circuit, better spark at the plugs and less wear on the rotor.
The Joseph Lucas company published a training course manual in 1956, for the general Motor Trade personnel.
Here is what the Lucas engineers had to say about coil polarity.
In the battery section of this manual, ground polarity is not mentioned, although corrosion at the battery posts is. They argued in favor of their new helmet type connection over the old clamp type.
Corrosion around the positive post may be lead peroxide, a condition occurring in all batteries regardless of ground polarity.
Without doubt there is an advantage to making the centre sparking plug electrode negative with respect to the outer electrode, which is connected to chassis earth. That’s because electrons (negative charge carriers) that collect on the small centre electrode become concentrated, and actually repel themselves off of it. This effect is in addition to the force of attraction to the positive outer electrode. Reversing the polarity would make the electrons jump off the larger outer electrode towards the smaller centre one–not as good. As mentioned, the rotor is better off with this polarity as well.
But two points:
First, the spark voltage (several KV) is generated by the secondary winding of a coil–it can supply negative to the plug wire and positive to the chassis completely independent of the low voltage wiring–which can be either positive or negative earth. You can choose the polarity by switching the wires (points and ignition) between the two (+ and -) primary terminals on the coil. But, as also mentioned, you can get slightly higher voltage by making use of the primary winding configured as an autotransformer. To do this, the coil needs to be designed as either positive or negative earth to begin with.
Second, the whole argument regarding spark polarity is somewhat moot, as it’s easy to get a good enough spark with either. Many modern cars fire two plugs at once, one of which receives the “wrong” polarity.
Perhaps that is what the Lucas engineers meant in their paragraph entitled Negative Earth Coil when they wrote:
“…such coils being connected internally to give a similar spark polarity to those used with the positive earth system.”
I dunno. I think that’s why there was an argument. As in electroplating (or anodizing) the anode (+) is sacrificial and the cathode (-) gets metal and goop plated onto it. So with + earth, the body/chassis corrode. With - earth, the wiring/connectors corrode, or so the argument goes. I was pretty young at the time, but IIRC some companies had some models one way and others the opposite. It was all six volts, and you had to check to be sure. So the advantage of one over the other must have been subtle. When the switch from 6 to 12V occurred, I suspect many switched from + to - earth at the same time. All of my 12V recollection is negative earth.
You may be thinking of US cars. Jaguars were always 12V positive earth from the SS era in 1936 to about 1965 when alternators replaced generators.
Lots of vintage British cars were positive earth as well.
As were lots of old American cars.
Good point! I wonder what percentage of cars in that era had 12V? Jaguar was ahead of its time, at least compared to many US cars.
British bikes also used dynamos and 6v +ve earth dynamo systems, when the Lucas alternators and negative earth equipment came in during the mid/late Fifties, using selenium plate rectifiers. Charge control was via chunky light switches that brought in extra pairs of alternator coils to match the three conditions: lights off (2 coils to support ignition and stop lamp use), sidelamps on (4 coils ) and headlamps on (all six coils in circuit).
By 63/64 they switched to 12V and Zener diode charging control with all coils permanently in use and the Zener clipping any excess voltage and dissipating it as heat through a fancy heat sink that became a styling feature under the headlamp.
The last few years in the early eighties they went to three-phase/three diode systems for reasons that escape me, as I never found anything wrong with the Zener system.
One thing that’s wrong with it (theoretically at least) is that it’s a “shunt regulator.” The alternator supplies full power all of the time, which is subtracted from that which would otherwise go to the rear wheel. Power not needed by accessories is dissipated as heat.
Shunt (and series–another variant) regulators are analogue and dissipate heat. Switching regulators superseded these because they regulate without wasting energy. The early bikes you described had “manual” switching regulators.
Those plate rectifiers (copper oxide, then selenium) were also inefficient and generated heat whilst rectifying–the plates were heatsinks.
Thanks for that information. I believe I am going to keep the car as
originally made. I do want to do a true concourse restoration and I may as
well hold true to the original build.
Stephen
