TR2/3/3A Unexpected continuity in 3 places

I am wiring the dashboard of a frame-off restoration of '57 TR3. After finishing the wiring, I tested for electrical continuity – both expected, or unexpected - to see if I'd made any mistakes.

I found unexpected continuity in three places, but this might be the consequences of my own lack of familiarity with how the components are supposed to work. I would appreciate any feedback that would shed light.

The first surprise was to discover continuity on the generator control box between terminals D and F. The D terminal receives two yellow wires, one from the ignition warning lamp, and the other from one of the connector bolts on the generator. The F terminal receives a yellow–green wire from the other connector bolt on the generator.

When I discovered continuity between terminals D and F, I of course disconnected the wires to test the terminals in isolation. The continuity remained.

The second instance of unexpected continuity was on the generator itself, when I tested for continuity between the two connector bolts (disconnected from any wires). However, I suspect that this is normal because I got the same results when I tested a newly overhauled spare generator.

The third unexpected continuity occurs on the panel light switch when the switch is wired according to an otherwise reliable wiring diagram. There is continuity between the two poles of the switch whether the switch if on or off. I tested the switch unwired, to see if it was faulty. It was not; when the wires were removed, there was no continuity in the off position.

I think I might understand why the switch has continuity when it is wired: the red wire from the switch goes to the parking/tail/license plate lamps, which are grounded, and the white-red wire on the other pole of the switch goes to the dash illumination lamps, which also are grounded. So, I assume that the circuit is completing through the grounding connection (essentially, the car sheet metal). Still I remain perplexed: Doesn't this mean that the dash lamps will automatically and always be illuminated, as soon as I turn on the main lighting switch? Which means, essentially, that I could never turn the dash lamps off -- or, said another way, it would seem that the panel light switch is superfluous.

Attached is a snippet of the wiring diagram that illustrates the loom section in question.

Again, I would appreciate any feedback to illuminate the situation.

Much thanks in advance.

Steve G.
Welllesley, Massachusetts
USA

Quite normal.

The regulator contacts inside the control box should connect F to D any time the voltage on D is less than roughly 14 volts, and the current from D to A/A1 is less than 20 amps or so. In normal operation, D is at battery voltage, and connecting F to it supplies more field current to the generator, telling it that more output is wanted.

The larger terminal on the generator is the armature connection, the other side is grounded and the armature has a very low resistance. The smaller terminal is the field, also low resistance and grounded. Your continuity tester sees the two low resistances in series and reports continuity.

Same thing with the dash light switch.

The reason it works is because normally, the return path to the battery is through the ground connection at each lamp. So any time the main switch is on the tail and marker lights get 12v which flows through the lamps to ground. If the panel switch is off, the panel lights get no power and don't light.

Randall,

Thank you for your thorough reply (which I've ready 3x and will have to read 3x more to fully understand). But, the main point that I take is one of reassurance -- which is the essential thing for the moment.

Again, thank you.

Steve

Feel free to ask again if I wasn't clear about something (as often happens, I'm afraid).

Another way to look at it is the circuits you are testing only have 1-6ohms of resistance. If the battery in your ammeter tester is not brand new, the tester will have trouble supplying enough current to read that low. If you are using the “beep” function, then that few ohms triggers the beep as a continuous circuit.

I can’t do any better than Randall did on the D and F connectors.

It varies of course, there is no standard definition of "continuity". But usually anything under 100 ohms or so will be reported as "continuity".

Also, incandescent lights have a curious characteristic. Their resistance is much lower when they are not lit. So even though the normal operating resistance of a dash bulb is roughly 80 ohms or so, they will be more like 10 when not lit. Then with 6 of them in parallel, the overall resistance will only be an ohm or two ohms!

Randall and John,

There are many great moments in this project (let's not talk about the low points...), and one of the greatest is the sense of community among owners who share the passion. As a complete newbie, another great joy for me is learning the plethora of skills and knowledge needed to do the project well. I really appreciate your eagerness to share your knowledge and experience.

Steven