How to: Repair low fuel warning sender

[Sanity Clause]

Not to be confused with fuel gauge sender, which is also located on the same unit in the fuel tank.

The low fuel sender unit can fail, sometimes leaving the low fuel warning light on, even when you've refilled with fuel [it shorts out somehow - which happened in my case], but more often, the warning light remains off all the time.

OK, first you need to check one simple thing, and that's if the bulb on the dash has blown.

In the boot, unplug the white 3-pin connector [seen below, on the left].
Turn on Ignition.
Short out the white/black and blue/yellow wires. If the bulb on the dash is OK, it should light up.
Turn off ignition.




So if your warning light isn't blown, then you'll need to pull out the sender unit from the fuel tank.

Disconnect the other electrical plug, and then undo the 3 cover screws.

It'll be pretty dirty under there...


Give the area a good clean [a smallish paintbrush is handy] before proceeding, otherwise you'll get dirt and dust in your
fuel tank.



Remove the grommet from the cover plate [still inserted, in the pic below], and pull the black cable and socket through.

Next, undo the 5 screws locating the sender unit in the tank, and carefully remove the unit.



It's best to stuff a rag in the tank opening to help prevent fumes coming out, and anything going in.

The low level sender unit is the small canister at the end of the arm with the cable wrapped along its length.

This canister was originally made by Panasonic, and had a part number ERTLG12N5, but is no longer available. [Don't bother trying to contact them - they don't reply]

Inside the canister is a small thermal resistor, called a thermistor. While a current is running through it, it generates a small amount of heat. So when it is immersed in fuel, the heat is dissipated, and doesn't allow any current to flow. But as the fuel level drops below the thermistor, it warms up a little, and allows a small current to flow to the warning bulb.

The original specs are:
3.4w lamp "on" current should be 135mA minimum, and lamp "off" current 60mA maximum.

This thermistor can be replaced. It's a 1K NTC thermistor [1K = 1000 ohms].

The one that I used came in a pack of 5, and were cheap as chips. It may not be identical to the original, but still works.





So, with the sender unit on the bench, unsolder the cable at the far end, and then the canister from its bracket.



The tricky bit now, is to open the canister without doing too much damage to it. I secured it in a bench vice, and gently tapped/levered around the cap end with a hammer and small screwdriver. I did manage to split the canister slightly, but it can be soldered up again, so no big deal. Try not to damage the cap, if possible.

Once the cap has been freed up it should come off, but the other end of the canister will still need unsoldering to remove the remains of the broken thermistor.

You should now have something like this [not my photo]...



Unsolder the remains of the old thermistor and attached cable from the canister cap. It looks like someone cut the one in the photo. Possibly not a good idea, if you want to re-use the cable.

Unfortunately, my camera didn't like taking the close-up pictures, so apologies for the blurred ones.

Cap removed, showing broken thermistor.




Take the new thermistor, and [if required] bend out the wires so that they make a straight line. Then shorten one of the wires to the appropriate length, and solder the thermistor and its connecting cable into the canister cap.



Make sure that the hole in the bottom end of the canister is clear of solder [or remelt it], and poke the long end of the thermistor wire through.



Fold the metal back around the canister cap as best you can [pliers are probably best], and once secure, solder and cut off the excess wire from the other end. If you did split the canister at all, it can be resoldered now.

There should be a few small holes in the cap, and also at the bottom of the canister, to allow fuel in and out. They should be clear of solder and/or debris.



Now you can resolder the repaired canister back onto the sender unit. Mole grips hold it in place, while soldering.



Wind the connecting cable around the arm and resolder the connection



Plug the sender unit back into the socket in the boot and test it. After a short while, it should light the warning indicator on the dash. Give it about 15-30 seconds to warm up, though. Some thermistors could take longer, depending on the Beta value of the particular thermistor.

IMPORTANT: DO NOT PUT THE SENDER BACK INTO FUEL WHILE THE IGNITION IS SWITCHED ON. It's not dangerous, but you could fry the thermistor with the thermal shock.

While you've got the whole sender unit out of the tank, and you have problems with your fuel gauge readings, now is the time to investigate.

That'll be another thread, when I get round to it - although it has been documented before, I think?

So far, the repair seems to be working well, but has only been through a couple of low fuel cycles so far. Time will tell...

......................................................................................................................................................................
Parts required...

1K NTC Thermistor

This is the one I used, from RS Online in the UK...
Manufacturer: AVX
Manufacturer Part No: ND03J00102K
Description: THERMISTOR, NTC, 1K, 3.5MM Body diameter, 3480K Beta Value
Also stocked by Farnell. Edit: [credit card orders have to have a minimum order value of £20, so uneconomical.]


In the USA...
RL2004-582-97D1 or later part RL2004-582-97K, from Mouser Electonics

If you google these part numbers, you may find somewhere more local.

Inspiration for this repair came from motorbike forums, where I found the same part being used in Honda bikes XRV650J, VT750C SHADOW, V65 Magna, and probably others... p/n 37810-MB4-008...
st-riders.net
goldwingfacts.com

For reference, some other vehicles that the Panasonic part was used in... Mazda RX7, 626, and Nissan Maxima. Although if you wanted to source used parts, you might find that these are tank-out jobs...

By the way, if anyone has trouble sourcing the thermistor, I have a few spares.

[Bean]

nice write up, well done

[ZaX]

Excellent. I will have to do this!