Instrument lights C 172S

Sstomlinsoniii

My instrument lights stop working recently On a Cessna 172 S. It has a double rheostat and change the knob produce is nothing on both knobs. I had an AP mechanic Change the rheostat with no luck so we also changed the circuit breaker and we still have a problem. Any ideas.

Scott T

Scott Sherer

Hi, you need to meter voltage coming off of the rheostat. But really, start at the circuit breaker and check voltage there, then test at the rheostat, then follow the wiring to the lamps. There's a break in the circuit somewhere and while very simple to diagnose, expensive because the wiring is buried and difficult to get to.

Centurian1973

Does your 172 use an electronic power supply
Maybe located above glove box?
If so ck the power transistors thier is two of them
If something shorts out downstream it I’ll blow them instantly
Pete

Larry

There should be a dimming module. The rheostats only control the bias voltage to the transistors on the dimming module. If one of the transistors is blown, that pretty much means there is probably a short in the lighting strings. That said, there is typically short-circuit protection to prevent damage to the transistors.

You really need to check for shorts in the wiring.

Sstomlinsoniii

Thanks

Sstomlinsoniii

@Scott Sherer said:
Hi, you need to meter voltage coming off of the rheostat. But really, start at the circuit breaker and check voltage there, then test at the rheostat, then follow the wiring to the lamps. There's a break in the circuit somewhere and while very simple to diagnose, expensive because the wiring is buried and difficult to get to.

@Scott Sherer said:
Hi, you need to meter voltage coming off of the rheostat. But really, start at the circuit breaker and check voltage there, then test at the rheostat, then follow the wiring to the lamps. There's a break in the circuit somewhere and while very simple to diagnose, expensive because the wiring is buried and difficult to get to.

Sstomlinsoniii

Thanks