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?

 

[DPG]

So both regulators are out of whack?

Is the ground connection to the regulator definitely good? And earth strap to engine, if there is one.

 

[Tarlow]

I'll check continuity between the black wire at the regulator and the frame again.

The stator has a hot wire going to the regulator, and a ground going to the body's ground wiring (shared with the regulator). Is that what you mean by earth strap to engine? I'll check continuity between stator ground wire and regulator as well.

 

[Tarlow]

Checked both, there's continuity between the regulator ground and the frame, and the ground wire from the stator.

 

[DPG]

Checked both, there's continuity between the regulator ground and the frame, and the ground wire from the stator.

How much is a new regulator?

 

[Tarlow]

Just checked a third regulator I had laying around, and same figures obtained as the last 2.

A new regulator shouldn't be too expensive, but these 3 pole ones usually gave to be ordered.

 

[plugsandsparks]

Was the scooter ever fitted with a battery ?

Also what are the readings when you put the meter on the AC range ?

Also what voltage lamps do you buy for this scooter, i.e. is it a 12V system or 6V system.

 

[Tarlow]

The scooter wasn't fitted with a battery and is 12V AC, but the one the gauge came from was 12V DC. I added a diode to this gauge to make it work with AC.

Another gauge was tested on this scooter with the same results.

Not sure what you mean by putting the meter on the AC range. You mean plugging the red wire on the left? I've been conducting all these tests plugged to the right.



I just tried on the left, and the readings are usually 0

 

[DPG]

The scooter wasn't fitted with a battery and is 12V AC, but the one the gauge came from was 12V DC. I added a diode to this gauge to make it work with AC.

Another gauge was tested on this scooter with the same results.

Not sure what you mean by putting the meter on the AC range. You mean plugging the red wire on the left? I've been conducting all these tests plugged to the right.

View attachment 116340

I just tried on the left, and the readings are usually 0

If the gauge isn't intended for DC operation then that is probably your problem.

Don't use the left hand socket, that is for current only, and will effectively short circuit anything you connect between it and the centre socket.

 

[Tarlow]

The gauge IS intended for DC operation, which is why I added the diode. The setup is similar to another gauge tried, for use on an AC scooter. Same needle wavering.

 

[DPG]

The gauge IS intended for DC operation, which is why I added the diode. The setup is similar to another gauge tried, for use on an AC scooter. Same needle wavering.

Sorry, I'm out of ideas.