P100s to earth or not to earth

Re-assembling my P100s after re-chroming.
I see there is a socket inside the bucket with a set screw for connecting a ground/earth wire.

But my wiring is original and there was no ground wire on either side.
The grounding was just done through the mounting bolts to the fender/wing.

The wiring diagram I have, D378, shows a grounding wire on each, but doesn’t give any indication of where it went to, just shows the standard ground symbol.

I’m wondering if a grounding wire would be better than relying on the mounting through the rusty chassis bolts.
Has anyone else done anything similar?

I would have thought connector should have two connections, one for the bulb and the other for the dim solenoid. I’ve never had any earthing problems just using the heavy mounting bolts.


The earth wire would go from the lamp socket clamp to the junction block, then to the earth post. This eliminates any variable contact through the rim hinge. Hanging earth symbols on the diagrams usually infer that there is point nearby. Sometimes they are only referring to a return which would be through the component’s mounting screws, and not an independent wire.

It would be acceptable to assume that the three stud-nut-spring washer assemblies on the mudguard bracket would have more than enough contact for the return current. Clean the area before tightening.
I usually coat with a bit of greae to keep water away. Some people run another earth wire out of the lamp and onto another convenient point on the inside wing, as a backup.

You can easily check the quality of the internal earth post by checking for resistance between the post and any part of the mudguard or body, with a multimeter. The resistance should be an absolute zero. I’ve never found a problem with the original system.

Sorry for if my photo was confusing. It’s the right hand light, which does not have the reflector dipping solenoid. The left one does.

The grounding socket is the little thing between the big nut and the Bakelite terminal block. Lucas must have put those sockets in there for a reason.

I get about 12 Ohms between the left headlight mount and the battery ground cable, and 8 Ohms on the right. Maybe that’s why the headlights never seemed all that bright. I think I will put in the ground wires. I have cloth covered black wire, so it will look period correct.

While some inexpensive modern ohm meters give poor accuracy near zero, I suspect you know that already.

At 8 or 12 ohms drop on ground side, I would add the ground wires. If a headlamp runs at 48 watts on a 12 volt drop that is 4 amps at a 3 ohm load (when hot). Adding 8 ohms in series would give a dull headlamp output since 11 ohms total would draw about 1 amp giving a only about 3 watt output.

Thanks Roger.
My meter is a Simpson.
It’s been a long time since Ohm’s Law.
I remember as mechanical engineering students we joked that as far as we were concerned it should be Ohm’s Suggestion or Ohm’s Minority Opinion.
Between us and the civil engineering students, we thought we ought to be allowed to think of current and resistance like in a river with flood gates or a pipe with a valve, but the professor wouldn’t go for it.

1 Like

WHen I am explaining electricity to people who don’t know/can’t quite grasp it, I use water as the current flow, a hose as the conductor, and a hose bibb as the power source. Connect the hose to the hose bibb, thrun the valve on, and the sprayer nozzle (the light, etc) works, but cut the hose and you have a mess.

Calling Ohm’s Law by the names Ohm’s Suggestion or Ohm’s Minority Opinion is a very good step towards recognizing reality often is complex. Physicists look for simple relations first and then ask what the range of validity is for the simple relation (if one is found). Ohm’s Law is one such simple relation which works well in many circumstances. Yet, resistance is not always approximated as a constant value for a material or object and knowing when the value varies is important.

The resistance of a tungsten filament, such as in old headlights, is close to zero at room temperature and becomes several ohms when heated to light up. This shows that tungsten’s ability to resist electric current flow depends on temperature when the temperature range is from room temperature to glowing hot. Ohm’s Law in its simple freshman-physics form does not discuss the underlying dependencies of materials on conditions, rather it just uses the description of V = IR for simple description (which works well in many cases).

Another example of simple laws is friction coefficients. A car tire may have a grip given as a single constant number, often expressed in “g” relative to gravity’s acceleration, Every race car driver knows that the tire “g” depends on temperature and number of heat cycles the tire has experienced, the tire grip is not a constant in racing applications.

As a physics professor with an experimental laboratory at the University of California since 1980, I’ve preferred to show simple simple physics laws followed immediately with the questions of what is the range of validity for the simple law and how well do you need to know the actual relation.

Ohm’s Suggestion or Ohm’s Minority Opinion is a wonderful way to urge checking when a simple law is useful and when reality needs more careful thought than a simple law gives.

1 Like

As I recall, we made out an arrest warrant for breaking Ohm’s Law.
Our warrant said something like “Unless you have reluctance there is the potential to be inducted into the current wet cell and resistance is futile.”
Then there was a choice between the Ideal Gas Law and the YouDeal Gas Law.
And yet somehow they gave us degrees. :man_student:

Anyway I got my headlights all put together, but the dipper was stuck, turned out to be the wiring was bunched up behind and catching the reflector swivel. Then the fuse blew. It’s a Buss GBC 8 amp glass fuse with pointy ends, overall length 15/16" tip to tip. I found some on ebait.
The fuse listed in the SPC is FA.6 so does that mean 6 amps?

1 Like

Ohm’s Law actually can be applied directly to each step of resistance change with a constant DC voltage to determine the current at that resistance, then it could be plotted out.

It would be easier to use current to determine resistance as the current can more easily be measured incrementally at that constant voltage, then apply Ohm’s Law to determine and plot the resistance.


F indicates Fast or Fast acting (blowing).

T is slow-blow, TT is very slow-blow FF is very fast acting.

Why T for slow-blow? Slow-blow fuses are ‘timed’ or time delayed.

I looked in the SPC and the control box voltage regulator has FA.25 fuses, which it says are 25 amps, and the cigar lighter fuse is FA.35 which it says is 35 amps. Good things to know.

I’m not sure what the “A” on FA indicates. Maybe it is part of FastActing, or maybe not. I’ll see what I can find, if anything, on it.

So far I just learned three things I’ve not seen on fuses before.

M for medium acting (who knew?)

H and L for short circuit blow rating over the fuse rating.

H is High current over fuse rating
L is Low current over fuse rating

Fuse designations

This is the one that I use… :slight_smile:

1 Like

A few years ago, a friend sent me a photo of two of the 350 amp (Audiovisual Auto-Alert) fuses in a fuse box.

We figured he must have found Dumb and Dumber’s place.

:grimacing: :flushed:

1 Like

My GBC-8 fuses arrived today, so I won’t have to use a 22 caliber bullet shell. :wink:

But it only comes into play when I dip the headlight, which I never do.

I noticed something interesting about my P100s; there is a number stamped in the solder joint on both the bucket and the front cover at the hinge.

The dipper is matching numbers.

But it is missing on the non-dipper cover, maybe due to an old repair.

1 Like