Fuel Gauge needle moving with revs issue

[mainline]

Hi! I've been having issues with the fuel gauge on an old vespa scooter, and I'd like to ask those who understand these things for advice on what I did wrong, and/or how to fix it.

The scooter is 12V AC. The stator produces (on paper) 120w, which is passed through a 3-pole regulator (12V; 20A), and goes to the gauge according to the schematic below. The float unit is brand new, and has been tested.

The diode is a 1N4007, and the resistor after it is a 5.2 ohm unit.
View attachment 116326



View attachment 116328

The issue is that, starting the engine, the needle goes to the correct position on the gauge (the tank was full for the video), and generally gives the correct reading while at idle. However, whenever the revs go above idle, the needle moves to the left. I have this same issue with a spare gauge, so I'm inclined to think this particular gauge isn't at fault, but the way the circuit is set up.

Here's a vid showing how the gauge reacts when revving the engine:

Any idea what may be causing this? BTW, the regulator is NOS, and although I haven't checked its output, it seems to be working fine (no blown light bulbs).

Is there no voltage stabiliser fitted ?
Usually the stabiliser gives a regulated 10 volts supplying the gauges.

 

[DPG]

Is there no voltage stabiliser fitted ? Which is different to the charging regulator.

Good point. Do they have these on scooters?

 

[Mike Johnson]

Did not answer the question if there is an earth strap to the engine and while we are at it, is there a suppressor on the spark plug?

 

[plugsandsparks]

Turn the meter selector switch 2 clicks to the right where it shows 200 V, this will measure AC , depending on the meter it should give an accurate reading then connect to the lamp as you were doing it before and note the reading. As you have a DC gauge you need a better rectifier and a voltage regulator but you do need a good idea of the voltage range in AC before a suitable regulator will work across idle and full revs.
just one diode will not cut it as you have a frequency change with revs which changes the voltage in DC as presented to the gauge.

 

[Tarlow]

Is there no voltage stabiliser fitted ?
Usually the stabiliser gives a regulated 10 volts supplying the gauges.

Apart from the regulator, no such device on this vehicle. Here's a wiring diagram for this type of scooter:

 

[Tarlow]

Did not answer the question if there is an earth strap to the engine and while we are at it, is there a suppressor on the spark plug?

Isn't an earth strap for battery-equipped vehicles? In any case, no, there's isn't one on these scooters. Just a ground from the stator plate to the body, and another ground from the cdi to the engine.

No suppressor on the spark plug either.

 

[Tarlow]

Turn the meter selector switch 2 clicks to the right where it shows 200 V, this will measure AC , depending on the meter it should give an accurate reading then connect to the lamp as you were doing it before and note the reading. As you have a DC gauge you need a better rectifier and a voltage regulator but you do need a good idea of the voltage range in AC before a suitable regulator will work across idle and full revs.
just one diode will not cut it as you have a frequency change with revs which changes the voltage in DC as presented to the gauge.

OK, now I get it! I'll check that tomorrow and report back.

 

[Tarlow]

So apparently using the right setting on the voltmeter helps a lot

Also, looks like the regulator is in fact fine.

I let the scooter idle a little longer before the video, and the output was consistently around 10V at idle. There are a couple more revs available above what was attained here, but in regular traffic this would be the rev range (giving in the realm of the 12-12.9V mark). Btw, the needle was nearing empty at those "high" revs already.

 

[mainline]

So apparently using the right setting on the voltmeter helps a lot

Also, looks like the regulator is in fact fine.

I let the scooter idle a little longer before the video, and the output was consistently around 10V at idle. There are a couple more revs available above what was attained here, but in regular traffic this would be the rev range (giving in the realm of the 12-12.9V mark). Btw, the needle was nearing empty at those "high" revs already.

Check the dc voltage at the rectifier.

Has there been any alterations to the wiring etc.

Is there room to have a battery ?

 

[Tarlow]

There is no rectifier, and no flasher relay either (no turn signals). I posted the diagram to show that the current wasn't transformed on its way to the gauge. The rectifier was fitted to some models for the horn for some markets, but not on mine.

The only alterations to the wiring were to add a brake switch to the handlebar in line with the foot pedal brake switch; also, the brake pedal is fed through the main circuit instead of having its own wiring, and the rear light has its own wire coming from the regulator. Nothing that interferes with the gauge circuit as far as I can tell.

A battery could always be fitted, but the point of this bike being all AC is to keep it simple and easy to repair and maintain. I'd rather avoid that route.

 

[plugsandsparks]

Phew, thats better. Let me look for a proper rectifier and suitable regulator to give a consistent DC voltage, you may have to play around with the resister to get a full scale reading when talk is full, or you may want to get an empty indication when the tank is very low, its often hard to get both without alot of playing around.

 

[mainline]

There is no rectifier, and no flasher relay either (no turn signals). I posted the diagram to show that the current wasn't transformed on its way to the gauge. The rectifier was fitted to some models for the horn for some markets, but not on mine.

The only alterations to the wiring were to add a brake switch to the handlebar in line with the foot pedal brake switch; also, the brake pedal is fed through the main circuit instead of having its own wiring, and the rear light has its own wire coming from the regulator. Nothing that interferes with the gauge circuit as far as I can tell.

A battery could always be fitted, but the point of this bike being all AC is to keep it simple and easy to repair and maintain. I'd rather avoid that route.

Try shorting the diode out and see how it reacts.

Connected correctly, the coils will balance, leaving only the variation in the sender resistance to unbalance it and move the needle.

 

[plugsandsparks]

Just had a look and the proper Vespa rectifier is cheap enough and at least it is designed for the frequency range

Regulator / Rectifier 4-Pin for Vespa PK 50, PX 80, PK 125, P 200, PX 200, Cosa, XL2

Regulator / Rectifier 4-Pin for Vespa PK 50, PX 80, PK 125, P 200, PX 200, Cosa, XL2

euroscooterparts.co.uk

 

[Tarlow]

Try shorting the diode out and see how it reacts.

This particular gauge didn't have a diode at first. When connected to AC, no reaction at all. When connected to a battery, it would move with the float.

 

[Tarlow]

Just had a look and the proper Vespa rectifier is cheap enough and at least it is designed for the frequency range

Regulator / Rectifier 4-Pin for Vespa PK 50, PX 80, PK 125, P 200, PX 200, Cosa, XL2

Regulator / Rectifier 4-Pin for Vespa PK 50, PX 80, PK 125, P 200, PX 200, Cosa, XL2

euroscooterparts.co.uk

Isn't that for a battery model? There are 4 pins, and I only have 3 wires for the regulator.

 

[mainline]

This particular gauge didn't have a diode at first. When connected to AC, no reaction at all. When connected to a battery, it would move with the float.

Did it move the gauge correctly. ?
Is the AC fuel gauge not available ?

You could use a bridge rectifier and variable voltage regulator circuit, but better to get the one suggested.

 

[DPG]

I think I'd just lift the filler cap and have a look

 

[Tarlow]

Did it move the gauge correctly. ?
Is the AC fuel gauge not available ?

You could use a bridge rectifier and variable voltage regulator circuit, but better to get the one suggested.

It seemed to more or less, but this was off the bike, with no revs to affect it.

The AC fuel gauge acts exactly the same on this bike. It also has a diode and a resistor, which is why I fitted a diode to this one (which as a DC unit originally, didn't have one) to see if it would work.

The regulator suggested is for battery models apparently, and has 4 pins, while my circuit is just ground, hot wire from the stator, and wire out.

 

[Tarlow]

I think I'd just lift the filler cap and have a look

Haha it certainly would be more simple, but I'm not acrobatic enough to do all that whilst driving

 

[plugsandsparks]

Isn't that for a battery model? There are 4 pins, and I only have 3 wires for the regulator.

The diagram you posted for the non battery model has 4 pins, there are only 2 wires to the regulator and what looks like 2 wires to the outputs of which one is the horn. For what it costs, its got to be worth getting it , connecting the 2 wires to the regulator as per diagram and then measuring the DC output using your meter as per diagram. If you get around 10VDC solid through the rev range , then jobs a good un.. you just then need to adjust the resistor to get the correct readings on the gauge- OR take a road trip to Derbyshire England and i do it myself....