Dino Saga 060402___Instrument Lights, Dimmer

Dino Saga 060402___Instrument Lights, Dimmer

Instrument lights have never been too bright. Replacing a bulb every 3-5 years was my only maintenance for years. Noticed that lights were dimmer than normal one night. Decided to check all bulbs and see how dimming worked. Bulbs were fine. Pulled rheostat from dash, let it dangle, set it to high and measured the voltage from each end to ground. Engine off, 12 volts at one end, 9 volts at the other. No wonder instruments looked dim. Shorted the two wires together and Oh Boy! 12 volts everywhere and now there was light. Pulled the two wires and took the rheostat into the shop.

Measured the resistance at various positions of the wiper. Went from about 1.5 ohms to about 6 ohms. Why such a high minimum? Seems it should be 0 ohms at full ON. Took a good look at how the resistor wire was wound on the ceramic core. Then looked at how the wiper was shaped and bent. There were two problems. Wiper lifted off wire before wiper tip got to the last turn of wire. Hot end of wire was looped through a hole in the ceramic core and then went leisurely down to the terminal screw. The loop and the leisurely run added about 2 inches of unwanted resistance no matter what the wiper did.

First thing was to bend the wiper so that the contact point went all the way to the last turn of wire before lifting off. Used rounded tip long nose pliers to re-shape wiper. Had to go easy; the wiper is hardened and will crack if bent too sharply. The bump on the wiper now hit the end wire. The flat of the wiper hit the wire midrange. Resistance minimum got down to 0.5 ohm. Put the rheostat back in car and tried again. Still losing about a volt. Measured the current. 2.4 amps! Lots of bulbs! Counted 10. More to do.

Pulled resistance wire off of terminal and unwound it from the hole in the ceramic. Put a piece of #18 solid copper wire in the ceramic hole and bent the ends up. Wrapped the resistance wire around the copper wire for about ¼”. Wrapped the stripped end of a piece of #22 insulated wire over the resistance wire and soldered the mess together. The resistance wire is sandwiched between copper and all is enclosed in solder. Good mechanical and electrical connection. Not likely that joint will get hot enough to melt the solder. Mechanical means will still hold things if solder softens. Tinned other end of wire, formed a hook and ran it to the terminal. Oriented the ceramic core so that wiper hit resistance wire ¼” from joint.

Checked everything again and all was fine. 0 to 5.5 ohms and instruments were easy to see at full on. Dimed just fine when wanted. Bet most other Ferraris of the 60s & 70s have the same problem. Takes about a half hour to fix, once you know what to do.

Pics show the final product. Been good for a few years now. You learn.

John
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