1960s no power in the door jamb switch wire

[Georges63cad]

Hello to all, 63 Fleetwood.....with test light I get no power at the single wire going to the door jamb switch on both sides. Could it be a fuse, broken wire or both? Shouldn't there be power in both wires all the time? I made sure I had a good ground when I tested it.

[V63]

Those switches control the ground and therefore should NOT have power

[TJ Hopland]

Well there should be power there because it would be coming through the bulb waiting to be grounded at the switch.   

[V63]

Well there should be power there because it would be coming through the bulb waiting to be grounded at the switch. 

The "door jamb switches" or the wires to them  will NOT illuminate a "grounded"  test light because they control the ground circuit. (Notice the wire diagram I will try to attach)  The door jamb switches open and close the grounding  circuit.

It is possible to  test the grounding of the circuit if you clamped the lead of the test light to a live positive battery circuit  and strategically probed the door jamb switch. Then the test light should illuminate while testing that controlled ground circuit .

[TJ Hopland]

I'm not trying to insult anyone just trying to help people understand how this stuff works. It is a little different to think about but its still pretty simple compared to trying to figure out a CAN bus on a modern car that does the same thing.  Lets just make up a simple circuit similar to how it would be in the car. I say similar because in the car there will be multiple switches involved and we don't need to complicate things yet. 

Battery + feeds a fuse. Fuse feeds one side of a light bulb. In theory we have almost 0 resistance between the battery and end of the light bulb.  Other end of light bulb goes to our door switch and the door is currently closed so there is no connection to anything. Light will be off. If we open the door the switch grounds that wire which grounds one end of the bulb. Other side still has + so the bulb lights. Close door, ground disconnects so no ground so no light.

If we were to measure the resistance between the battery + terminal and the door jam switch we would read the resistance of the light bulb.  Lets say that's 10 ohms.  Same reading you would get if you just measured the light bulb itself.  10 ohms would be about 15 watts. 

Now lets take an old school automotive test light.  Lets say its resistance if we measure it is 20 ohms which is around 7 watts.  If we stick one end of the test light on a good ground and we stick the other side to the door switch terminal on the closed door we have now completed the circuit and both lights will light but at half power.  Its not actually half power but for this discussion we can just say its half power.  If you leave the connections the same and open the door the test light will go out and the dome light will light at full brightness.

If you have 2 light bulbs or 2 test lights you can do this experiment on the bench.  Hook battery + to one side of one light.  Light to light.  Light to -/ground.  Lights will light half brightness.   Add a ground in the middle between the light bulbs.  The light on the ground side will go out and the remaining light will go full bright.   

If we were to measure resistance of the whole circuit we would read both lights which would be 30 ohms.  If you were to pull out laboratory grade equipment you could also read the resistance of all the wires and contacts and such but for these purposes we don't need to be that exact.  Also all this only works this way with old school bulbs and test lamps.  You throw LED's into the mix things can change because LED's can be polarity sensitive and sort of act like directional electrical check valves. 

The reason they switch the ground on these is its an easier way to make multiple places to turn the lights on.  Lets say its a 4 door car there are 4 door switches plus the switch on the dash so 5 places that can turn on the light.  Having the ground at the switches means they can do it with just 6 wires.   One wire is the + from the fuse to the light socket then from the other side of the socket you run one wire to each switch location that gets grounded. 

If they went the other way its only 5 wires to the light socket because the other side could be ground but it takes twice as many wires to each switch because using + there has to be an in and and out to each switch.  In most cases they would not be literally be running 5 wires to the light socket.  There is likely one one wire to the socket that goes to the dash and that is where the 5 individual lines from each switch splice to that one.                

[TJ Hopland]

Just a follow up to clarify the testing process more.....

You can still test this type of circuit with a volt meter.  If you hook one side of your meter to ground and probe the terminal on the door switch with the other and everything is good you should read what ever the system voltage is, 12.6v would be typical.

You are reading though the bulb(s) but a modern(ish) digital voltmeter doesn't put enough load on the circuit that the bulbs will effect the voltage reading enough to worry about.  Because you are reading through the bulb(s) if the bulb or socket is bad you won't a reading.

If you get system voltage at the door switch try a jumper wire from ground to the switch terminal.  If that lights the light you have an issue with the switch.  A possible switch issue I have seen is if the car has been painted the switch body may not be making contact with bare metal.

If you got no voltage at the switch then move to the light socket.  One terminal of the socket should have + voltage on it.   The other is the one that runs down to the door switch(s).  If you don't have voltage at the socket you need to find what is supposed to be supplying that and check there, likely  a fuse.