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XRV750 RD04
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Discussion Starter #1 (Edited)
I had the tank off recently (RD04) and thought while I did it I'd try to get to the bottom of the fuel level senders not working.

I seem to have tracked it down to the senders themselves being at fault by touching the two spade connectors together (this results in the fuel light coming on permanently when I keep them touched together as they should) so I think the senders are at fault.

I wondered if the spade connectors might have been connected the wrong way at some point, so thinking "they're not working anyway - nothing to lose" I swapped the positive and negative around where the spades connect to the senders. Still no Joy.

Came on here to do a bit more research about them and discover a couple of posts that suggest that with the design of the sender there's a live current running through exposed wires that sits inside the fuel (looking at photos of them this appears to he the case :shock:).

I've had the bike running since putting the tank back on (before discovering this) and it was fine and no explosion. But it's got me a bit concerned that if I do have them connected up the wrong way around, whether I might be sitting on a time bomb waiting to go off or something (maybe after years in the tank the senders have become coated with gum or something and this is the only thing sitting between me and a giant petrol bomb sitting between my legs if there actually is a live current running through that sender inside the tank and I've wired the spades up the wrong way around).

Common sense says that honda would have thought of that and the sensors would have been designed in such a way that if the spades are connected back to front then it wouldn't be dangerous, as it's such a simple mistake to make (particularly as there doesn't seem to be any indication of which wire connects to which spade - or at least anywhere I can find in any of the fiches, haines manual or my service manual) - plus there'd probably be reports here of petrol tanks going up all over the place - but then I can't help thinking that running an exposed electrical wire through fuel like they appear to, doesn't strike me as common sense either.

Anyone got any ideas about whether if the wires had been connected the wrong way around at any stage whether this would have damaged the sender unit?

Or if it poses a danger if they're connected back to front?

Or if polarity is important, which coloured wire connects to which terminal on the senders?

I'd really like the fuel senders to work so I'm thinking about removing them from the tank to have a closer look at them and see if cleaning them up or re-soldering the wires inside helps to get them working again, or if not trying to find some replacements.

Would be handy to know about this polarity issue before I do that though. Any pointers anyone can offer would be a great help (can't seem to find this discussed in any of the other threads I've searched for).
 

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Premium Member
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7,170 Posts
Zen

Polarity is not an issue with the senders (I'm pretty sure). Most work with resistance, however the AT ones are "make or break" connections I believe, and if they work like those on one of my other bikes, they actually measure temperature, i.e, when submerged in fuel they are colder, and as soon as the fuel level has dropped below them, they warm up in the air.

One thing worth noting is that there are 2 fuel level circuits (anber and red) so chances are, one of the senders is OK. Is it the rd or amber light thats not working??

Obviously, the amber one is set higher in the tank, so if it is that one you suspect, unbolt them and swap them over, and see if the amber circuit starts working again.


Bob :thumbup:
 

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XRV750 RD04
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1,549 Posts
Discussion Starter #3
Thanks for the info Bob.

I think you're right and they're make or break circuits (though I still wasn't sure if that meant they could be wired up backwards without causing a problem).

I'm guessing the polarity wouldn't cause a fire hazard? (maybe I'm a bit paranoid but an exposed wire looking like its running through fuel like on the pictures I've seen of them makes me cautious).

I think the fuel lights and circuits are OK as when I disconnect the spades from the sensors on the tank and touch the positive and negative spades together the fuel lights come on and stay on as expected for both lights when making the circuit with the corresponding spades.

I drained the fuel from the tank into cans and connected up the spades to it (being careful that I got every last drop out I could so it should show up as empty with both lights on) then when I turn the key in the ignition, the fuel lights come on for a quick self-check as usual, then go out again. Even started the engine for a moment using the fuel that was left in the carbs to see if that helped but didn't have any effect (though couldn't leave it running for long as I didn't want to risk damaging the fuel pump).

Unfortunately it's both that aren't working properly so I don't think swapping them over will help much in this case (though getting them out for a good clean and check might be helpful so will get around to that eventually and keep an eye out for spare ones on ebay).

If they're meant to make the circuit when the tank is empty, then everything else appears to be working how it should from the fuel sensors upwards so it seems to point to the fuel sensors playing up unfortunately :(
 
J

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Yes Bob you are correct. Had the same problem on my bike and discovered that the sensors just complete the ground circuit to bring the light on. I had a problem with the top sensor working and not the bottom which resulted in an unscheduled stop on the countryside in China because of no fuel. Swapped the bottom for the top and now have at least the final warning. Another habbit I have gotten into is to turn off the right hand petcock when I ride long distance just incase the sensor fails again. The sensors are only in the left side of the tank so this gives you a way for some reserve fuel.
 

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Wing Commander
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14,437 Posts
I'm watching this - how do you get the bloody things to works would be a better answer.

On the zebra I have to remember how long ago I put fuel in and how many miles I have done.

When I left her idling the other day and then went for a spin I ran out of fuel (right next to a garage) and put in 24.18litres, which suggests I filled the tank and put 0.18 litres into the fuel hoses and pump.:confused:
 

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XRV750 RD04
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1,549 Posts
Discussion Starter #6
Yeah, that's what I'm relying on at the moment too - setting the odometer to zero when I fill up, then I just try to fill up more regularly than I probably need to. This isn't so bad most of the time but on longer journeys it'd be nice to have a better idea when you're reaching the actual limits in the tank when the fuel lights come on so you know exactly how much you've got left. Particularly for when I next visit scotland. Last summer there was a sunday I rode and every petrol station I came to for miles was either closed for the day or closed down completely and I was chugging along everwhere at low revs hoping the tank wasn't going to run dry and leave me stranded, without knowing exactly how much I had left and if that was really necessary.

From what I can gather the original honda units are very expensive, so I'm going to do what I can to sort them out myself or find a unit from another bike that fits first if possible (I've noticed some for sale used but not sure if they'll fit - the connection looks the same but the stork with the capacitor thingy on looks a bit longer).

I've read that some of these things work using dialectric capacitance rather than temperature, so perhaps it's not as simple as a make or break circuit and there's some sort of other range that's being measured outside the sensor. I know there's a sensor tester block hidden away near the cockpit, so perhaps that is performing some function and when I'm touching the wires together assuming it's a make or break circuit, the lights are coming on just because that's giving a measurement over whatever range it's looking for.

I wish I knew more about electronics or had more time to learn about it to be able to work this out. First I think I'm going to try taking them out and giving them a clean and re-soldering any soldered bits and see if that helps (though might have to wait a while now as I'm keen to get out on the bike again after it being laid up for a while with bits taken off it).
 

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Stone Crazy
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5,310 Posts
I'm watching this - how do you get the bloody things to works would be a better answer.

On the zebra I have to remember how long ago I put fuel in and how many miles I have done.

When I left her idling the other day and then went for a spin I ran out of fuel (right next to a garage) and put in 24.18litres, which suggests I filled the tank and put 0.18 litres into the fuel hoses and pump.:confused:
Dont worry about the bit extra fuel Whealie most tank sizes are nominal
i am sure you were not stitched up. unless the previous user of the pump managed too empty the pipe, which i have seen old gits do, watch a bloke stand with the pipe held in the air shaking it for 1/2 a minute one day to try and get the last drips out
Merv
 

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Wing Commander
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14,437 Posts
on longer journeys it'd be nice to have a better idea when you're reaching the actual limits in the tank when the fuel lights come on so you know exactly how much you've got left.
I find it easier on long journeys. It is the weeks when you do 20 or 30 miles and then a different trip and then another and suddenly you don't really have to time to stop for fuel but your not sure if you have enough to get where you have to be urgently.
 
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