2nd Stewart Warner Problem

[Aggressor]

I have a long term unresolved problem with 3 of my Stewart Warner Deluxe gauges. These are electric gauges not mechanical. My Tach, Fuel Level, and Temperature gauges all jump up a few points when the light switch is moved from off position to the parking light or headlight position.
When the lights are switched on:
The tach will raise 200 Rpm.
The fuel level will raise to show approx. 1 1/2 to 2 gallons of additional fuel.
The temperature gauge will raise another 15 to 20 degrees.

The ammeter is uneffected as are all other gauges which are mechanical.
All gauges are grounded with seperate and individual wires.

The only thing common to all 3 gauges effected and the light switch is the gauge lights that they all carry.

Any ideas are most welcome.
-Geary

[Slither]

It seems like the electric gauges would work from the source voltage available in the system. I suspect that when you turn on the lights, there is a slight voltage sag as the alternator/battery supply current to drive the lights. In other words, the lights pull the source voltage down slightly due to ESR and other parasitic losses, causing the gauge readings to move with it. Just a guess...

[eliminator]

Geary,

How about the Altimeter Reading, is it effected?

Couldnt help it....

[Aggressor]

Rick it's all good. You're a clever and resourceful man with a quick wit that I've come to appreciate over the years.

Turn on the lights and the gauges will jump the stated amounts throughout the entire rpm range.
It's as if the twelve volt electrical "leg" to the light is running through the chassis of the gauge and giving it a cumulative voltage boost on its way to a common ground with the gauge.

Paul - Gut reaction makes me think the gauges are getting an electric "push" as opposed to sag.

It's these small things that do so much to make a person nutz.
As the say "I don't suffer from mental illness-I'm enjoying every minute".

-Geary

[RJacobsen]

Geary, have you tried removing the gauge lights temporarily to see what happens?

[mkassab]

I think Rod is on the right path. Also... have you put a volt meter on your gauge for readings pre and post lights on?

Is your alternator and wire heavy enough for the load?

Is it possible to try a different gauge? Or brand? To see if the issue follows it.

Keep us posted.... Mark

[Slither]

Geary, have you tried removing the gauge lights temporarily to see what happens?

Doubt it will change anything, but I'll be interested in hearing the result if he tries this... the lights are separate CKTs.

Paul - Gut reaction makes me think the gauges are getting an electric "push" as opposed to sag.
-Geary

That's because you are assuming that the rise in the gauge reading must be caused by an increase in voltage... which at first glance would seem logical. But the gauges likely move in the opposite direction of the source voltage because of their design, i.e. when the source voltage goes up the readings go down and vice-versa.

The senders are typically resitive elements, and I suspect they work in a voltage divider or a bridge CKT. If the source voltage sags, the reading changes. Easy to sort out with a good schematic (I'll see what I can scare up online...). As was suggested, measure the voltage at the gauges under both conditions: lights on, lights off. I suspect you'll get different readings, and have your answer quickly.

If it stays rock solid, then I have another idea... but let's see this first.

Do you know what the resistance range is on the fuel sender empty/full? If I recall, SW uses something like 240/30 ohms empty/full, so a lower voltage reading would appear to the gauge as less resistance showing more petrol in the tank.