Fuel gauge drives to zero with ignition switch on

Electrical and electronic topics for small boats
pcrussell50
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Fuel gauge drives to zero with ignition switch on

Postby pcrussell50 » Tue Aug 07, 2018 1:09 am

1987 Mercury 150hp, two-stroke-cycle motor.

Previously, I had the fuel gauge and sender system wired to the Perko battery master switch. So when I turned on the battery, the fuel gauge went live. At that point in time, I had nothing powered by the ignition switch except the ignition. That meant no tach, either. Fast forward to this past winter (such as they are in the south west), and I had a professional mechanic (who races these 2.0L Mercury 150's) fiddle with the wires in the quadrant in order to get the tach working. Which he did, easily enough. But he said he set up all the gauges to power up through the ignition switch, too. Which I suppose is how it ought to be. Only now, when this happens, the fuel quantity needle drives to zero. So, it still reads right when I turn on the battery master, until I turn on the ignition when it goes to zero.

-Peter

jimh
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Re: Fuel gauge drives to zero with ignition switch on

Postby jimh » Tue Aug 07, 2018 8:07 am

I will reply in a few days when I get to a better terminal to compose a detailed comment.

jimh
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Re: Fuel gauge drives to zero with ignition switch on

Postby jimh » Sat Aug 11, 2018 7:42 am

pcrussell50 wrote:...[a mechanic] set up all the gauges to power up through the ignition switch...now...[when the ignition switch is moved to RUN] the fuel [tank level gauge] needle drives to zero..[but] it still reads right when I turn on the battery master...


Hmm...this is an odd one. Typically a fuel tank level gauge will have several terminals on the rear of the gauge housing. A common notation for the terminals will be

L = Lighting circuit, connect 12-Volts here from a switch to energize the internal gauge illumination lamp; this is usually wired with BLUE wire

I = Ignition circuit, connect 12-Volts here to energize the tank level circuit; this is usually wired with VIOLET wire

S = Sender circuit, connect the remote tank level sender, a variable resistor; this is usually wired with WHITE with TAN wire (or PINK in Mercury)

G = Ground circuit, connect to the boat 12-Volt negative battery or "ground" circuit, also in some cases an isolated ground return from the sender is connected here; this is usually a BLACK wire of if an isolated ground from the sender, then BLACK with WHITE wire.

Here is a typical schematic diagram, although I drew this one for the TRIM circuit, but it is very similar to the FUEL TANK gauge wiring:

Image
Typical analogue gauge with remote sender circuit for trim or fuel gauge

About all I can imagine has occurred in your wiring is that the installer made an error in connecting the gauge terminals. A couple of possibilities come to mind.

If you look at the circuit diagram, you can see that the lamp circuit can work a bit backwards. If the installer mistakenly connected the switched 12-Volts from the ignition switch to the L terminal instead of the I terminal, the circuit would be powered up through the lamp filament, and the gauge would tend to read off-scale towards the E side. Then, if the I terminal were later connected to 12-Volts as well, the gauge circuit would tend to return to normal operation, except be affected somewhat by the voltage at L.

I suggest you check the wiring at the back of the gauge and see if the VIOLET wire from the ignition switch is connected to the LAMP or L circuit instead of being connected to the I or ignition circuit.

Another possible problem that could cause the behavior described: loss of the main ground on the gauge. Check that there is a BLACK conductor at the G terminal.

Also, I don't understand how you could get power from the ignition key circuit when the battery master switch was in the OFF position. Something is odd about that wiring, too.

Nota Bene: this thread has been moved to SMALL BOAT ELECTRICAL for further discussion.

pcrussell50
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Re: Fuel gauge drives to zero with ignition switch on

Postby pcrussell50 » Sat Aug 11, 2018 2:03 pm

jimh wrote:Also, I don't understand how you could get power from the ignition key circuit when the battery master switch was in the OFF position. Something is odd about that wiring, too.


Sorry if my description was confusing. There is no power anywhere on the boat if the battery master is in the OFF position. As soon as you turn the battery master ON, the fuel gauge comes alive and reads accurately. That was how I wired it a few years ago, not using any of the wiring in the remote control, and not knowing if any of it worked anyway. That meant that I had no readout on the tachometer either, of course. Fast forward to a few months ago and I decided to let a pro sort out the wiring in the 30 year old remote control. Which he did, and now the tach works swimmingly. Only, as soon as you turn on the ignition switch on (after first turning on the master) the fuel gauge goes to zero. Something about how I wired it, is interacting destructively with how he wired it.

jimh wrote:Nota Bene: this thread has been moved to SMALL BOAT ELECTRICAL for further discussion.


Good call. I have so little interaction with the SMALL BOAT ELECTRICAL section I forgot it existed.

-Peter

jimh
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Re: Fuel gauge drives to zero with ignition switch on

Postby jimh » Sat Aug 11, 2018 2:30 pm

The problem sounds like what I described above: you are supplying power to the gauge in some way that it ends up on the wrong terminal on the back of the gauge, or there may be a missing ground which is getting completed by the LAMP circuit on other lamps. To better see how these other sneak circuits work, look at the illustrations in another article about miswiring on gauges. See

E-TEC Rigging: Trim Gauge
http://continuouswave.com/ubb/Forum6/HTML/002089.html

If there is no ground on the G terminal on the back of the gauge, it will work by the sneak circuit through the un-illuminated cold filaments of all the miniature light bulbs in all the gauges. But as soon as the LAMP circuit gets power--which is likely what happens when you turn on the ignition key switch--then the dial pointer dives toward the left-side meter peg--usually rather hard.

Look at the back of the gauge. Verify there is a ground lead (BLACK) on the G terminal.

It is possible there could be an isolated ground conductor attached, and that conductor is not really connected to the tank sender at the other end, so it really is not a ground.

steelhead55
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Re: Fuel gauge drives to zero with ignition switch on

Postby steelhead55 » Mon Aug 13, 2018 2:42 pm

It sounds to me like the ground side of the gauge now gets 12 volts on it when the ignition is turned on. A simple check would be to clip a volt meter to the back of each leg on the gauge. I bet you will read 12 volts to ground from each, when the ignition is turned on. If you were to put the voltmeter across the gauge, it would read 0 volts when the ignition is on.