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Changing bulbs for the warning lights, and testing the low fuel light circuit

583 Views 8 Replies 4 Participants Last post by belter, 4 d ago

OddBird

Discussion Starter · 8 d ago

I had a low fuel light that was on all the time, and the bike came with a new sending unit in a box, so when I was in the tank cleaning it and replacing all the hoses, filters and pump (the inside of pump pickup screen was filled with fine dirt) I installed the new low fuel sending unit.

Now the the low fuel light Never comes on.

I haven’t pulled up the wiring diagram yet, or done any troubleshooting- I thought I’d check to bulb first then go from there. Any tips when looking at this system? Any idea how low the fuel level must be before the sending unit closes the circuit for the light?

Thanks for any insights… can you get to the bulbs without removing the whole dash?

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Lelebebbel

· #2 · 8 d ago

The bulbs are all in an insert that slides in from the bottom of the dash. If I remember correctly, you can get it out if you remove the headlight.

If you have one of the old style metal fuel sender units, I think they weren't ever very reliable.
They work by supplying some power to a thermistor element inside the tank. If the fuel level drops below that element, the temperature of it increases, and the resistance decreases which allows enough current to flow to illuminate the light (this also creates a delay effect so the light doesn't flicker when the fuel sloshes).
Because of this, the system will not work if the bulb is replaced with an LED - I think it would be on all the time if you did that.

The light should come on as soon as the top 1-2cm of the fuel sender unit are exposed to air for maybe 30s? You can see it through the tank lid.

I've had solder connections break on the little circuit board with the light bulbs, so have a close look at that. They must've had the same people solder that who also welded the frames...

Side note, I believe Ducati later replaced these sender units with a plastic float switch that does the same thing, but I'm not sure if it's a straight swap.

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OddBird

Discussion Starter · #3 · 7 d ago

Lelebebbel said:
The bulbs are all in an insert that slides in from the bottom of the dash. If I remember correctly, you can get it out if you remove the headlight.

If you have one of the old style metal fuel sender units, I think they weren't ever very reliable.
They work by supplying some power to a thermistor element inside the tank. If the fuel level drops below that element, the temperature of it increases, and the resistance decreases which allows enough current to flow to illuminate the light (this also creates a delay effect so the light doesn't flicker when the fuel sloshes).
Because of this, the system will not work if the bulb is replaced with an LED - I think it would be on all the time if you did that.

The light should come on as soon as the top 1-2cm of the fuel sender unit are exposed to air for maybe 30s? You can see it through the tank lid.

I've had solder connections break on the little circuit board with the light bulbs, so have a close look at that. They must've had the same people solder that who also welded the frames...

Side note, I believe Ducati later replaced these sender units with a plastic float switch that does the same thing, but I'm not sure if it's a straight swap.

Thanks. Both the original and the replacement were plastic. So no thermistor in the plastic units? Good to know. I have drained fuel down to 10mm above the bottom of the column, and still no light.

thanks for the reply.

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belter

· #4 · 7 d ago

check the circuit outside the tank by jumping the two wires in the 4 pin connector that run the light. #3 and 4 in the connector. not the red and black wires that are #1 and 2, the pump power. with 3 and 4 connected, the light should come on. very rare for any issue there.

otherwise, check the sender. specifically, the hole in the side of it. about 2/3 down in this report:

Playing with a Ducati fuel level sender

. I put a post on my Facebook page a while ago about Dcuati parts not being covered by warranty if not fitted by an authorised workshop. T...

bradthebikeboy.blogspot.com

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OddBird

Discussion Starter · #5 · 7 d ago

belter said:
check the circuit outside the tank by jumping the two wires in the 4 pin connector that run the light. #3 and 4 in the connector. not the red and black wires that are #1 and 2, the pump power. with 3 and 4 connected, the light should come on. very rare for any issue there.

otherwise, check the sender. specifically, the hole in the side of it. about 2/3 down in this report:

Playing with a Ducati fuel level sender

. I put a post on my Facebook page a while ago about Dcuati parts not being covered by warranty if not fitted by an authorised workshop. T...

bradthebikeboy.blogspot.com

Great write-up….. if I jump it and get a light, I’ll be pulling mine out to have a look at the bottom hole. Thanks!

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OddBird

Discussion Starter · #6 · 5 d ago

Well, I jumpered the 3&4 sockets and the light came on, so I removed the sender and sure enough- a little divot was there, but no hole. Drilled it out, and fuel pissed out of it. Re-installed it and the light came on after about 30 seconds. Started filling it with fuel and the light went out. Success!

I’m guessing the manufacturer tests them on the bench by just inverting them while they are dry, so somebody assembling these things decided there’s no need to drill the hole….?

Thanks guys!

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belter

· #7 · 5 d ago (Edited)

i got one for a customer recently from gowies, and just before i put the top back on the tank i thought "better check that". no hole. emailed gowies my link and they checked the other one they had in stock - no hole.

i don't understand how ducati are selling them and not getting the constant feedback. or maybe they got a batch made a few years ago and they're slowly filtering out to customers.

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