Packard Forum Index » Pre-War (1899-1942)

Hi all,

Recently was installing a tail lamp which had a dead short owing to one of the copper strip bulb contacts being loose, twirling and touching the lamp base. I didn't notice this problem as it was under the bulb and, as the tail lamp would not light up, I was measuring voltage at the light switch block on the steering column. Soon noticed that "hot electrical" smell and looked at the dash to see the wire inside the circuit protector was lit up red hot like in a toaster (no fuse was installed). I quickly disconnected the battery and found the short.

Clearly the device protected my car's wiring. Yet I'm a bit puzzled at how this device works. The manual says the resistance wire in the protector provides enough current for some lighting when the fuse blows, but is it supposed to get red hot in protecting the circuit from the short? Maybe it gets red hot instead of the wiring leading to the short getting red hot and burning up. Even if that is so, I don't see how enough current will pass the resistor to power the lights, when all circuits including the one the short is in are drawing power through that same resistor which probably gets more resistant the hotter it gets.

Can any electrical guru out there provide the answer?

I believe that bypass or protector is essentially a length of nichrome resistance heater wire. If the fuse is good and in place the fuse shorts across the wire effectively bypassing it because there is no resistance in the fuse as compared to the wire. Current always flows thru the path of least resistance so none goes thru the wire as long as the fuse is good. If the fuse blows, that is a big open and lots of resistance so current then starts flowing thru the wire. Just like in a heater, the wire gets red hot as it passes a limited amount of current depending on the specs of the wire. Because of the resistance, the voltage will be lower but will be enough for a few lights albeit at a dim level. If the load gets too high such as a solid short the heater wire will eventually be unable to carry the current. Best case will make bulbs too dim for any use or worst case, wire will burn open. I think I recall one of the regular posters a few years ago needing to replace one which had burned open and asking about a source and specs for the wire.

Thank you, Howard. That makes sense. If there was a dead short, like on mine, a fellow probably didn't have much time to make it home before he was left in the dark!

Might be nice to get the wire gauge and if you have a good quality meter the approximate resistance of the existing wire just in case a short happens again.

I don't know if the other poster found an exact replacement or what he did to fix his. Classic and Exotic doesn't show it in their electrical section but maybe they have wire by the foot or it is part of their rebuild service. Maybe someone else carries a drop in replacement.

Nichrome wire is readily available but you need to know some of the parameters of the original to get a reasonably correct resistance for the length of wire that can fit in the Packard enclosure.