Recalibrating the fuel gauge using a Spiyda Gauge Wizard

[John Vine]

You need to measure the resistance values of the sender alone, disconnected from everything else (not just with the ignition off).

Yes indeed, and that is what I did for the readings in Cols 2 and 4 in post #45.  (I'm assuming that there isn't some arcane link between the pump circuit and the sender circuit.) 

But I still don't understand what's behind the behaviour in post #32, although Barry's comment in post #49 does sound logical. 

JV

[John Vine]

Do the fuel pump and sender have completely separate wiring or do they share an earth? I guess even if they have separate wires from the unit they will share an earth at some point.

Andrew, I can send you a PDF of my Assembly Guide if that would help.  It contains a wiring diagram of the R400D main chassis harness.

JV

[John Vine]

You could confirm this my measuring the voltage on the earth connection at the pump relative to the battery -ve. I suspect you will see the small additional voltage on the earth.

Hi Andrew,

I've just done this, and the results are:

 

The connector wiring didn't match my Assembly Guide, however.

R400D:

Sigma:

My car matches the Sigma layout above.  Judging by the gauge of the wires, I've assumed:

• YG168 (actually Yellow on my car) is the pump 12v feed
• B10 is the pump earth
• GB148 is the sender signal (edited to correct)
• SG196 (actually black on my car) is the sender earth

JV

[revilla]

John, I think that pretty much confirms it. All you are seeing is the voltage drops across the earth wiring due to currents flowing. So there's some disagreement about the exact voltage of "earth" at different points. Circuits that need to transmit small signals accurately (crank sensors and the like) wouldn't be wired this way, they would have two wires from the sender all the way to the receiving end with no other currents flowing in them and probably an earthed screen around them (earthed at one end only again to avoid currents flowing). I don't think the small voltage offsets you are seeing are at all unusual or indications of anything wrong. Just limitations of the design and the way it works.

[revilla]

If you wanted to eliminate this error, you could try disconnecting the sender earth at the sender unit and running a separate earth wire from the sender back up to whatever point the Spiyda unit is earthed. This would probably give better results, but it seems like overkill in search of perfection to me ... I think I'd be inclined to just accept that's how it is!

[John Vine]

I think I'd be inclined to just accept that's how it is!

I agree, Andrew.  As I now seem to have a working gauge that displays useful and relevant info, I think I'll call it quits for the time being.  Many thanks to you and Barry for your considerable expertise.

JV

[revilla]

[bjw]

As a footnote - I found a wiring diagram of the R400 which throws a little light. All the grounds from the rear end (pump, sender, all lamps, speed sensor) are commoned at a point buried in the loom in the transmission tunnel ~900mm from the fuel pump connector. The actual ground connection is made via a wire that runs all the way from there to a tag on the chassis near the left headlamp connector !

Cheers,

Barry

[revilla]

Yes, that explains a thing or two!

[John Vine]

If you wanted to eliminate this error, you could try disconnecting the sender earth at the sender unit and running a separate earth wire from the sender back up to whatever point the Spiyda unit is earthed.

Hi Andrew,

This has been bugging me for a while, so I thought I'd do a little test along the lines you suggested.  I was unable to detach the sender earth from the pump connector, so I cut the wire down near the connector instead.

I refitted the Spiyda, and then connected the sender earth wire two ways:

• Direct to the battery -ve
• Direct to the Spiyda earth point (the big earth bolt on the wiper mount)

This is how the gauge reacted (already showing just over "Full" with a brimmed tank):

 

So, establishing a direct earth cured the needle "drop" (from F+ to between 3/4 and F).  Interestingly, the reading increased very slightly with either of the direct earths (engine running).

I've now removed the Spiyda as my gauge is behaving well, having adjusted the float arm.  I might refit it if I pursue a "low fuel" warning lamp. 

JV

 

[Jonathan Kay]

Mmmmmm... data.

Jonathan

[StevehS3]

Some info about my fuel gauge. It's a 2016 310R and the fuel gauge reads just over 3/4 when full and zero when there is about 15L (~ 9 usable) in the tank. The sender unit resembles JV's. My sender and fuel tank have alignment marks (see photo). After removal I found it very similar to JV's except it has what appears to be a shorter float arm. The float itself was already fitted inwards. The float rested part way up the pump similar to JV's before adjustment - this ties-in with the error I am seeing on the gauge and JV's findings. I bent the arm downwards to almost level with the base of the pump/sender assembly. The gauge now reads almost 1/4 tank higher than previously and looks correct with my estimate of fuel in the tank at the time. I need to full it up and take note of readings but I am confident this has solved the problem. Many thanks to JV for his guide. 

[John Vine]

  I've very pleased to have been of help.

For info, this is the Guide mentioned in #62.

JV

[Grumbleweeds]

I thought my fuel gauge was a bit ropey but having read most of this thread I think it's working quite well!

I reset the trip mileage upon fill up as I know I can get at least 250 miles. Simples

[Neil Allen]

Me too