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

Replacing light bulbs in sockets in back of the dash. When instrument panel lights are turned on, They show dimly for about 5 seconds and then begin flashing in 1-second intervals as if a relay is operating in the circuit. What is going on?

It sounds as if the circuit breaker or as Packard called it, a thermostatic relay, built into the headlight switch is operating. Typically the CB or relay will operate and self reset at a frequency depending on how bad an overload might be. Question would be what other lights are on when this is happening.

If the wiring is stock the instrument light rheostat is fed from the headlight switch along with other lights. Wiring diagrams show there should be an inline fuse in the wire from the headlight switch that feeds the instrument light switch. If there was a problem with the instrument lights that fuse should blow first and the CB would never get a chance to operate. When the headlight or other wiring has a problem or old age or a bad adjustment has changed the capacity of the CB to a lower value that is typically when it operates.

If there is no fuse protecting the instrument switch on your car, one common issue with the old wiring and the single wire sockets the instrument bulbs use is the insulation shrinks back or cracks and allows a bare portion of the wire to contact the grounded metal shell of the bulb socket where the wire exits the hole at the back of the socket.

As mentioned it sounds like you have a grounding issue.

You can check for a ground by popping the lamp holder free of the back of the bezel and turning on the lights... If one of them is the offender than the system will not pulse

I had several under dash light socket wires that had lost the insulation during a bulb swap. I purchased LIQUID Electrical tape at home depot and coated the wires after moving the sockets down the wire.

Note: this was done laying with my feet laid over the seat back and head on the floor boards.
A word of caution, the liquid tape is runny and will drip, cover your carpets and its good to have a friend over to make sure the can does not tip...

If the lights worked with out issues before the bulb swap than you don't have to worry about the under hood wiring to the other lights right now.

Good hunting and as recommended else where look at adding a in line fuse to the wiring harness,.

Once you have identified the offending wire(s), I would suggest adding a bit of additional protecting by putting some friction tape over the frayed wire. This could be done before or after the liquid tape. If you do it before the liquid tape, make sure you go beyond the tape edge with the liquid tape to ensure a good bond with the wire.

Best of luck.
Gar

Thanks a lot. You were dead on. Frayed wire and problem bulb. 20 AMP blown fuse in line. What should proper fuse rating be?

Crushed old insulation off, removed spring on bulb socket and insulated at the socket and 6 inches back with heat shrink insulation.

Wiring behind dash is a rat's nest. Like to do the rest of the wiring restoration. Why this thing did not burn up decades ago?.

How much trouble is it to remove the left dash to gain access to the rat's nest?.

Can anyone post a photo of the rear of the speedometer assembly? Hate to remove this thing again to locate the bulb sockets...

20 amp is the rating for the inline fuse and both of the aux fuses that are mounted directly on the headlight switch. The tail light wire fuse is in the wire feeding the instrument switch. Per the specs saying the thermostatic overload relay or CB should handle 25 amps yet it apparently cycled before a 20 amp fuse was blown its value may have drifted downward over the years. That is a common issue on the separate style breakers used in later models and may be happening to yours too.

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More good stuff.
20 AMP was in line but blown.
Replaced with 20 AMP fuse.
Will go deeper.

Check the size of your existing wiring. Prior owner on my 1928 use #14 wire for my feed to the ammeter (which can draw up to 25 amps). The wiring got so hot that the internal insulation melted and leached out between the cloth wiring.

At a high level: #14 is good for 15 amps, #12 20 amps, and #10 30 amps.

Thanks, Good advice. Bigger the better and no one will see behind the dash.

I was surprised to find the wiring behind the dash is aluminum.

I don't know if I asked this on another thread but are the backlit #s on the gauges, clock, and speedometer radioactive? My little bulbs do not seem to do much in the way of illumination.

What is the size of these little bulbs on the prewar Packards?