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Faulty Fuel Gauge


tomperkins

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Hi there,

A few weeks ago my Caterham fuel guage started reading full all the time. I had already purchased a Raceline gauge so I swapped to that in the optimistic hope that this may cure the problem. It didn't.

After reading numerous posts on here I then took the boot floor out and started checking earths. The earth going into the sender appears to be good, as does the earth going into the gauge.

I took out the sender and the resistance appears to be correct (based on some numbers I've seen on here) and changes as the float moves.

That leaves me a bit stuck on the next thing to check.

Any ideas would be appreciated,

Tom

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That sounds like a short to earth of the wire that runs from the gauge to the sender... can someone please confirm it works that way round? It could be anywhere within that but I'd guess the most common are near the gauge and where the wire is clamped to the chassis.

Assuming that's right:

  • What model and dash?
  • Have you got a wiring diagram?
  • Start with a good look around the back of the gauge and identify the wires (again).

Jonathan

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Thanks Jonathan, I'll see if I can find that wire earthing somewhere.

It's a 2004 car with a carbon dash and k-series engine if that helps.

Do you know if there's a way to use a multimeter to connect to either end of the wire in question and see if there's a 'leak' anywhere?

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Yes, make a long test lead with anything that conducts so you can get back to the meter... typically one wire in an old mains lead. Croc clips help but aren't essential. Remember to disconnect the ends of the one you're testing from anything else.

Chafing can be intermittent (and I recently learnt that if its seems to be related to engine temperature it's worth looking first near the engine and gearbox).

Send me a Private Mail if you'd like the diagram, which will give the wire colour.

Jonathan

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So I've investigated this further and I'm still no closer to a solution. Here's what I know:

  1. The earth going to the sender is good
  2. The earth going to the gauge is good
  3. Continuity between the end of the wire that goes into the sender and the other end of the same wire that goes into the gauge is good
  4. The tank is full of fuel

In checking continuity I've discovered I had the sender feed and 12v feed going into the gauge the wrong way round (How this produced the same fault as the old gauge when the old gauge was working previously I've no idea).

When swapping these round the needle now slowly makes its way to the empty mark on the gauge. Not resting on the pin but top of the red. It moves in the same was as it used to when it was working turning on the ignition.

It's almost as if the gauge is reading back to front (i.e. Full is showing as empty and vice versa). Is that even possible?

Any other thoughts as I'm stumped?

Tom

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Fuel gauges used to work off the heating effect of the current on a bimetallic strip, which meant the direction of connection didn't matter. But I have no idea how they work now, including yours*. Have you got a serial number and a photo?

Continuity between the end of the wire that goes into the sender and the other end of the same wire that goes into the gauge is good

Check the resistance of that wire (with both ends disconnected) to earth.

Jonathan

* When we've got this sorted let's go back to explaining what happened with your two gauges... perhaps one used heating effect and the other some electronic stuff.

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Here's the new gauge: http://www.demon-tweeks.co.uk/performance/gauges/racetech-fuel-level-gauge-electrical

I'm feeling pretty stupid right now as I've just looked on the Racetech site and there it mentions 'the necessary sender unit' making me think there's no fault, just that I'm missing the right sender. http://www.racetechdesign.com/fuel-level-gauge/4589434900

I'll try the old Catetham gauge in the morning and see if that's working now. Strange thing is the Caterham gauge uses a block with three spades so there's no way that could have been wired up wrong as there's only one way it connects.

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So I've just tested the old Caterham gauge again and it is still stuck on full. It doesn't move no matter how it's connected or even if it's disconnected it still sticks on full. It's making me think it could be broken.

So based on that assumption, and the fact that I now know that the Racetech gauge needs a special sender it's making me think the wiring could be fine.

Would you agree? Any other tests I could do before I order either a racetech sender or a replace,ent Caterham gauge?

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can you take the gauge out of cct and put you MM across the wires that attach to the gauge, then manually operate the float and see if the resistance ranges low to high, mine swings between 4 and 100 ohms ish, mine is top mounted pump and sender in one unit, similar to this , I think you have done this already but if you then check out each wire to earth that will have eliminated both Short and open CCTs. 

http://www.caterhamparts.co.uk/img/p/3445-3419-large.jpg

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Thanks for your suggestion.

I've taken the float out of the tank, disconnected the gauge and then measured the resistance from the wire that comes from the sender and normally connects to the gauge. Ishould say that I have a separate sender and pump.

The resistance goes from 19 at one end to 195 at the other.

Both earths are good.

so what do we think the conclusion here is, that the wiring is good, my Caterham gauge is faulty and my Racetech gauge needs a special sender?

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Those resistance values suggest the sender is working. But there do seem to be different ranges used by different senders and gauges, see this thread, which also suggests how to work around that.

Some presumably common sender ranges are in the calibration table in this Stack manual.

is there any documentation for your new gauge?

While you've got in bits take the opportunity to test the whole lot on the bench without any of the possibly problematic wiring, either feed or earth.

  • Fused 12V supply.
  • Completely new wiring from that through the sender to each gauge in turn, slowly moving the float from end to end.
  • How does each gauge respond, and is the response consistent?

Jonathan

PS: ... so what do we think the conclusion here is, that the wiring is good, my Caterham gauge is faulty and my Racetech gauge needs a special sender?

Possibly, but you may be able work with that sender and some trickery. 

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JK is this the sort of trickery you are talking about , TomPerkins guess at this point  a Phone call to Racetech ( 01491 822000) will answer all questions, I suspect from tests done we can be confident that wiring is OK  (although haven't done any voltage checks at gauge) and the Gauge is just not getting an input it is happy with. 

Tim

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  • 4 years later...

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