testing rheostat

How many wires populate the plug from the wiring harness? If there are only two you have your answer.

IIRC, the typical wiring configuration for a potentiometer is the center pin and one of the outside pins on the plug, the center pin being the input voltage. Of course there's no guaranty that the Italians follow this practice but it's something to consider. You could check that by testing for +12VDC at each of the terminals on the plug (lights on, obviously) while the negative input of your VOM is connected to a known ground.

It's doubtful you'd hurt anything by jumping across the terminals of the plug but you should confirm that in advance by checking for +12VDC across each permutation of terminal pairs on the plug.

Or you could chase the wire colors on the schematic diagram if you have one for your car.

 

It isn't too hard to figure out. First it is a potentiometer, the difference is a rheostat is for high power regulation and a potentiometer is a variable resistor. What you have is a variable resistor which varies the voltage to the instrument lights bulbs. There a usually three taps or post terminals connected to a circular body. The current enters on the supply end of the wire wound resistor and the other end of the resistor is connected to the light bulb. The center terminal is connected to a device called a wiper which slides across the wire wound resistor and is also connected to the lighting side of the wire wound resistor. When the wiper is close to the end of the wire windings on the light bulb side the resistance is high or lower voltage giving you a dim bulb. When the wiper is close to the supply side of the resistor the resistance low and the voltage is higher giving you a bright bulb. There is also point where the wiper is off the windings and is directly conducting the supply end to the light bulbs.

The failures in most cases is the wire wound resistor gets dirty from contamination and the wiper fails to make contact with the windings. Some potentiometer and cemented with a ceramic compound which breaks down and the windings come loose and as the wiper slides it has little electrical contact with the windings. Inspection and cleaning the potentiometer is your first step. Electrical contact cleaner is best because it has a little lubricant to help things out. If the windings are in place and the wiper slides well across the wire windings it should work. Measure terminals for full battery voltage. That would be the supply side. Then measure the resistance to ground on the remaining two terminals. It may be easier to remove the light switch and bench test it. The blinking of the light bulbs is a sign of a dirty wiper or it's has lost its tension to contact the resistor windings.

 

Couldn't find any useful wiring info in the 456 manual I have but if the plug looks like the one depicted in the 456M manual the configuration is of no help because it's actually a four pin plug populated by three wires (black, white, and yellow). There's no actual schematic though, so it's not possible to determine where those wires go. If you want to bypass the potentiometer to confirm that it's bad it looks like your best bet is to check each of the three wires against both ground and +12VDC.

 

The Ferrari wiring section sucks. None of the components have internal schematics. I do understand that the systems are complex, but all you can do with this data is trouble shoot harness problems. These are really wire listings and not even close to a schematic.