Help!! I have a couple of extra '56 Headlight switches that I managed to acquire through the years. Everything tests fine except I'm not getting anything at the rheostats for the dashboard lights. And I'm not smart enough to remember how I tested them years ago. Can someone refresh my memory?

Thanks,

 

Frank - There should be three posts off the rheostat (potentiometer). Place one (COM) lead on the middle post place the other (RED) lead on either of the outboard posts (either side will work). I think you should get about 1K Ohms (somewhere thereabouts). Then, turn the dial on the rheostat - you should see either an increasing amount of Ohms or a decreasing amount of Ohms (since one outboard post will be the opposite of the other outboard post) - depending on which outboard post. If you test resistance from one end to the other without the rheostat (potentiometer) functionality, you'd hookup both leads on each of the outboard posts - this would be total resistance of the circuit.

 

Thank you, Norm! Tests completed- one good, one bad.

 

 

FYI, a rheostat and a potentiometer are not the same.

Potentiometers and rheostats have similar construction and function in similar ways: by turning a **** you can control the current and voltage distribution in a circuit. Seems simple enough, but the way these components are built makes them better options for different applications.

The most obvious difference between a potentiometer vs. rheostat is the number of terminals; potentiometers have three (input and 2 outputs) while rheostats have two (1 input, 1 output). Thanks to the potentiometer’s three terminals, it can be used as a rheostat when one of the terminals is left floating. However, the reverse is not entirely true; a rheostat cannot be used as a potentiometer unless you add at least one other resistor and ground the rheostat’s output.

A potentiometer is basically a variable voltage divider; as the **** on the device is turned, a sliding contact creates a voltage divider between the input and the two outputs. In other words, a potentiometer is simply meant for voltage division. With a rheostat simply being a potentiometer with infinite resistance at the second output, it modulates total power received by the load connected to the output. If the rheostat and load resistance are equal, then maximum power is transferred to the load component.

 

Dynoking - Well stated & the reason I identified the rheostat as a potentiometer (above) w/ regards to the headlamp switch. The below video is a great demonstration of the three: Rheostat, Potential Divider & Potentiometer.

 

I like my instrument illumination full intensity, as do many people. Long ago I got sick of jiggling the **** right at the sweet spot of full intensity to make them remain lit, so now I just connect it bypassing the variable resistor. Voila' full brightness at all times and no wiggling the ****. Easy to do on our cars with screw terminal connections.

 

Frank, I bought a new headlight switch from Fusick and it kept tripping the breaker. It was from China of course. I found a NOS on ebay and have not had any more issues. I would hold on to that good spare.
Rick

 

Rick, that's EXACTLY what happened to me. I've bought some very good stuff from Fusick through the years, but their headlight switches have been awful. Like you, I found a good switch on ebay. I was testing out a couple of old DELCO's that I had laying around as a backup, just in case.

 

I typically buy such things from NAPA or another local auto parts store.