Gauge lighting weirdness

[DerPuffinator]

As mentioned in my intro thread, my car came with a North Hollywood boost gauge in place of the clock. Being something of a traditionalist, I sourced a clock (thanks again Todd!) and set about removing the boost gauge.

(As an aside, the PO had spliced the vacuum source from the brake booster hose, using not a simple barbed t-fitting, but an AN flare tee and hose clamps. )

Anyway the NHS gauge had a different kind of incandescent bulb socket from OEM, meaning the nice little button connector to the VDO base had been removed

The bulb happily turned on when the headlights were turned on, so that was pretty normal, but I wasn’t sure of the wire polarity so I checked continuity to ground for both sides.

Imagine my surprise when *both* wires beeped to ground. I’m unclear how that’s possible if the car isn’t on fire.

No magic smoke escapes when the lights turn on, and one wire (the same one each time!) goes hot, so I am at a loss to explain the ground behavior.

I’ve added “pop out all the gauges and document all the wiring” to my task list to make sure there isn’t some awful short lurking somewhere, but I figured I’d ask here if anyone had any similar experience.

[FrankM_]

Did you check gnd continuity with the bulb in place ?
Then a ‘continuity’ beep from the multimeter is normal as these bulbs have a low resistance. Typically a multimeter will signal continuity if it sees less than 200 ohms or so.

Prob best to remove the bulb first and check if any connector has a gnd connection.


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[DerPuffinator]

Negative, bulb was removed and terminals detached from the socket.

[targa80]

The gauge bulbs are all daisy chained off of the same line which means they are all in a parallel circuit. The overall resistance of all the bulbs would be lower than the resistance of one bulb. Use a multimeter with either an analog or digital display. The ground line will show a lower resistance to chassis ground vs the hot line of the circuit to chassis ground.