Fuel Gauge needle moving with revs issue

[Tarlow]

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.






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).

 

[DPG]

Is there a voltage regulator anywhere on the scooter? I suspect it will need one to avoid variations due to supply (battery) voltage.

 

[Tarlow]

Hi,

Yes, There is a 3-pole regulator (12V; 20A), right after the stator, and there's no battery. I never questioned the regulator because it was new and I've never had the issue of bulbs blowing, but could a faulty regulator cause this to happen?

 

[DPG]

Hi,

Yes, There is a 3-pole regulator (12V; 20A), right after the stator, and there's no battery. I never questioned the regulator because it was new and I've never had the issue of bulbs blowing, but could a faulty regulator cause this to happen?

I would say it could cause the issue yes. Check the output of the regulator with a voltmeter while you vary the engine revs.

 

[Tarlow]

I guess that would make sense, and explain why several gauges had the same issue on the same bike...

So should the regulator (if good) deliver no more than 14.6V like a battery, or is there a different tolerance?

Access to the regulator isn't easy, so I'll check the output at the rear light first (2 screws to undo) before dismantling the bike to get to the regulator. If the voltage is too high there, that will confirm that the regulator isn't doing its job.

 

[DPG]

I guess that would make sense, and explain why several gauges had the same issue on the same bike...

So should the regulator (if good) deliver no more than 14.6V like a battery, or is there a different tolerance? I'll try to check that this evening.

The key thing is that it should be fairly constant regardless of engine revs. Let us know what readings you get.

 

[Mike Johnson]

The faults on most of the older vehicles I worked on in my youth could be traced to a bad earth connection to the unit showing a fault, just a thought.

My old Lotus would wind down the electric windows when you turned on the radio all down to an earth return fault.

 

[Tarlow]

My old Lotus would wind down the electric windows when you turned on the radio all down to an earth return fault.

I'm pretty confident in that part of the wiring. Continuity was checked on all relevant leads when troubleshooting the gauge, and there was continuity between the frame and negative plots at the float sender and the gauge itself.

I checked the voltage at the rear light and at the regulator (Ducati), and checked again with an older (apparently working) regulator (no name):

 

[DPG]

I'm pretty confident in that part of the wiring. Continuity was checked on all relevant leads when troubleshooting the gauge, and there was continuity between the frame and negative plots at the float sender and the gauge itself.

I checked the voltage at the rear light and at the regulator (Ducati), and checked again with an older (apparently working) regulator (no name):

Lot of variation there.

 

[Tarlow]

So both regulators are out of whack?