Low Fuel Warning Light

by **Gordon** » 16 Jul 2008 13:23

What was the triggering mechanism for the low-fuel warning light in the overhead console on the Challenger SE models? I'd love to install some type of similiar warning light on my non-SE car. Anyone have any ideas on how to accomplish this?

by **fbernard** » 16 Jul 2008 16:00

Gordon wrote:What was the triggering mechanism for the low-fuel warning light in the overhead console on the Challenger SE models? I'd love to install some type of similiar warning light on my non-SE car. Anyone have any ideas on how to accomplish this?

Judging from the available technology back then, I'd say the relay was triggered by the sender's resistance going over a certain threshold (73 Ohms Empty, 10 Ohms Full). Or to put it even simpler, the voltage at some point (dashboard gauge, or fuel sender's terminal) becoming high enough to trip the relay's coil. No additional component required.
I doubt the fuel sender was any different between SE and non-SE cars.

On my car, I'd wire the "Low fuel" sign straight to the accelerator pedal :s007:

, but if you want to do it more seriously, start by measuring the sender wire voltage (from the dash harness) at near-empty (or where you want the light to turn on), and full (also half-empty if possible). Then you can start working on a voltage comparator.

What I don't know, though, is if the sender works using 12V or if it uses the 5V supply of the gauges.

by **Gordon** » 17 Jul 2008 12:30

Thanks fbernard ! I didn't think that it would be that difficult. I appreciate you pointing me in the right direction. I do like your idea of wiring the damn thing right to the gas pedal, though!!

by **dave-r** » 17 Jul 2008 14:19

The sender (and gauge) works at an average of 5v for spark safety in the tank.

I say average because it is a very crude and not very smooth voltage supply. More like a square wave.

To off topic slightly...

You have just got me thinking. Does my Painless kit deliver the right voltage or does it have to connect to the existing low voltage system?
I will have to see if there is any mention in the instructions. I can't remember seeing any.
Currently i am making a note of the shortcomings with the Painless wiring kit and will produce a full assessment when I finally finish the car!

by **71_DEPUTY** » 17 Jul 2008 18:27

the low fuel relay has it's own built-in voltage regulator. same gas guage and tank sender was used.

as the resistance got to around 20 ohm's or so- the relay contacts turned on and provided power to the light.

in the cars that had this option- it was a separate harness that was used.

most times the light would stay on all the time- either the wire from the tank was shorted out to ground or the low fuel relay was toast.

John Mac

by **fbernard** » 17 Jul 2008 22:40

71_DEPUTY wrote:most times the light would stay on all the time- either the wire from the tank was shorted out to ground or the low fuel relay was toast.

John Mac

It still works on my car (and takes a while to turn off, as the gauge needle goes up)

Re: Low Fuel Warning Light