Fuel Gauge Issue

[BigCol]

I disconnected my fuel gauge to get at some cabling behind the dashboard.  Upon reconnection, it read zero.

I have had this happen before and last time as I wasn't sure how much fuel I had, I decided to fill-up.  With the car full, the gauge started working again.

However, I have just done the same and the gauge still says zip.

There are three terminals at the back of gauge.  I have 12V and a good earth on two.  The other I presume comes from the sender.  What should I see on that - a voltage or a resistance (and of what values)?  At the moment I have neither.

Is there a way of testing gauge itself to identify whether the problem is there or with the sender/wiring?

Also, is there a fuse associated with these?  (all other gauges work OK so I suspect that's not the issue)

If relevant, the car is a 2013 Sigma-engined SuperSport.

 

[OliverSedlacek]

Fuel gauges work in one of two slightly different ways. You can test a gauge by disconnecting the sensor wire (gauge shows empty) and shorting the sensor wire to chassis (gauge shows full).

[John Vine]

I don't know what sender your car has, but the sender resistance readings on my 2008 S3 are: 

• FULL: 26 ohms
• EMPTY: (that is, when the misfiring starts) about 250 ohms

(Mind you, my gauge is super-pessimistic, reading 3/4 at full and -1/4 when empty.)

The wires in my 3-way connector are:

• Green = 12V+
• Green/black = sender signal
• Black = earth

Measure the resistance across the Green/black and the Black.

Re your gauge not working, is there a connection to the tacho?

JV

[BigCol]

I tested the gauge earlier and it's OK (as suggested above... power and earth attached and shorted sender terminal to earth and it gave a full-scale deflection)

All fuses are OK.  All other gauges work.  No resistance when measuring between sender and earth with tank full.  So am confident that the issue is definitely on the sender side of things.

Thinking about it, I reckon I managed to short the sender feed to 12V... so what damage might I have done?  (another reason for thinking the issue is related to the sender!!)

Like JV, a chap on PistonHeads says that the sender is linked to the tacho... Wire Y198 on the wiring diagram does just that but how is that connected to the float/sender in the tank?

http://i1366.photobucket.com/albums/r765/BigCol911/Wiring%20Diagram_zpsnyoxxo4j.png

[John Vine]

No resistance when measuring between sender and earth with tank full.

That's ominous, and suggests an open circuit somewhere.  The sender rheostat could be faulty or perhaps the float/arm of the sender is jammed or obstructed. 

JV

[ChrisC]

From the diagrams its not a simple connection between the sender and the gauge.   

Fuel Gauge 
G76     Connected To G74/77/78/79/80/81/82/83/85/171 
            G74 Connected To Instrument Fuse 
B27/28 Connected To B19/22/23/24/25/26/27
             Earth 6mm Eyelet (near wiper motor)
Y198     Connected To Tacho

Fuel Sender / Pump
YG168 Connected To Inertia Switch
            Connected to fuse and fuel pump relay
GB148 Connected To GB149/150 
            GB150 Connected To Tach 
            GB149 Connected To Engine Loom Plug (large) GB52 engine loom
            GB52 Engine Loom Connected To Chassis Loom Plug (small) Fit Blank 
SG198 Connected to Engine Loom Plug (large) BY51
            BY51 Engine Loom connected to Earth eyelet 6mm
B10      Connected To B 2/12/13/14/15/16/17 
            Connected To Earth 6mm Eyelet

So the signal wire for the gauge is Y198, if you connected it to 12v you are more likely to damage the Tacho than the sender.   The sender signal seems to come from GB148 on the pump unit, but it looks like there are possible areas where the signal could be lost, for example in its connection to GB150/149, the connection through the Tacho or via engine loom plugs in some way. 

I would use the engine loom plugs to see if the signal is there.   If it is, and its present at the Tacho on GB150, then the Tacho might not be passing the signal thought to the gauge for some reason.    

The Tacho and the sender pump units are expensive parts so swapping them just because is an expensive way to go, do you have anyone local with a similar spec car that's willing to help? 

Hope this helps

[BigCol]

Very helpful Chris; many thanks for the input - now I know what I am looking for on the wiring diagram I can see the common connections you refer to and provided some tracing I would've struggled with.

JV - agree with your diagnosis and the next stage will be investigation of the physical aspect of the sender unit and whether there is a resistance between GB148 and earth.

If not, that's probably the issue - would that necessitate a new pump to sort out the sender unit?

If it's OK, then the issue is somewhere in between which will brings me to Chris's comments...

The GB148 to GB149 connection goes nowhere on my car as it ends up via the engine loom plug at a blank on the chassis loom plug, so can ignored (?)

The link between GB148 and GB150 on the tacho and then onto Y198 will therefore be the potential problem (if the float etc has a resistance).

As far as I can tell, the tacho is working OK - the car runs OK and I certainly didn't notice a lack of a rev counter when I went out the other day.

 

 

[BigCol]

fuel pump connections:

http://i1366.photobucket.com/albums/r765/BigCol911/e3822917-fa65-4367-85d9-1cdeb9c2695b_zps4vgsov8l.png   http://i1366.photobucket.com/albums/r765/BigCol911/2b4c2cc0-6f45-4d55-be55-0f23f33b3230_zpswfyjlaam.png

On the loom side:  B10 and SG196 are good earths.

On the fuel pump:  B10 to YG168 is a very low resistance (0.8 ohm - low enough be considered a short?) as is SG196 to GB148 (8.8 ohm).  All other combinations are open-circuit.

So, is the 8.8ohm sufficient to be considered a reading of the sender rheostat?  Sounds low but it is a reading!

 

 

 

[John Vine]

So, is the 8.8ohm sufficient to be considered a reading of the sender rheostat?  Sounds low but it is a reading!

I recently spent a couple of months trying to sort out the calibration of my gauge with Caerbont. (I'm still trying.)  I'd initially reported resistances of 26 ohm for full and 240-250 for empty.  The Caerbont tech people replied:

"Your gauge is calibrated to Caterham's specification of 178 ohms at the 'R' chaplet, and 7.8 ohms at the '1' chaplet. This explains why the pointer never gets to full, and reads empty well before the tank is actually empty."

So, your 8.8 ohm reading sounds very reasonable and appears to equate to "full" (or almost).

JV

[ChrisC]

So you now need to check you have 8.8 ohm on GB150 at the Tacho,  if you don't then the connection GB148,149 and 150 connection or the engine loom connection could be causing the problem.  If you do then the Tacho is not passing the signal onto the gauge on Y198.   

On the pump SG196 and B10 are both earths, all be it SG196 is a bit indirect.  YG168 is the pump +ve and GB148 is the fuel level sensor.  0.8 ohm between YG168 and earth (B10 or SG196) is the resistance of the pump and sounds about right to me.   Concentrate on GB148 and getting that 8.8 ohms to gauge to solve your problem.  

Based on what you have posted connection or Tacho is my guess now.  When it stopped working, was that before or after you applied 12v to the Y198 wire?  If it was before it stopped working I think your going to need to try swapping the Tacho out.

[BigCol]

Thanks chaps

It's a relief that it seems that the sender is OK and so I now need to trace where the 8.8 ohms has gone!

The gauge worked OK before my stupidity and the 12V short.  As I'd had a similar issue (in appearance - gauge not working having been disconnected; though no introduction of 12V that time!) before, I thought it was one of those things that could right itself as it did before.

Whilst I want to trace the issue and fix the fault properly, would running a wire from GB148 to the gauge be a solution?

[ChrisC]

I guess.  I can't find any information on the connections and what that Tacho does.  I thought it was doing voltage stabilisation, but that might be a bit old school for the wiring on a 2013 car.   Most of the connections make sense, because the Tacho also has warning lights, but I don't understand the fuel level connection being passed through the Tacho, unless it could have had a fuel warning light at some stage in the design, or for different markets.

[ChrisC]

One final thing, the Tacho is also connected to the CAN signal from the ECU, its possible its putting out the fuel level onto the CAN bus.   I am no expert on CAN, but CA instruments on their web site say they have been working with Caterham on CAN instruments, and I know the 3/4 step gauges are CAN not resistance based.    Time to dig out my MBE lead and dig a bit I think.

[BigCol]

Follow the logic...  

Suspect given the variety of possibilities of configuration of a 7, some of the wiring is of a generic/just-in-case/future-proofing basis.  (I still wonder what the point of the GB149 -> GB52 -> blank connection is - but some strange combo of options might need it)

As you say current cars have the 3/4 sweep dials as opposed to the old 1/4 ones that I have.  It would make sense for them to use different and/or CAN bus-based signals.

Hope to find some time later to root around the back of the tacho!

[ChrisC]

Agreed, I found that strange as well.  

What I find strange is the fuel pump unit not being CAN, i.e. the signal from the fuel level isn't CAN given that it's from a modern car. 

I have the tools to decode and debug the CAN bus, but it would have to be on a trail an error basis, i.e. I won't get the information from Caterham about what channels are being used, the only way to find the fuel signal is take snapshots from the CAN bus, then add fuel, take another snapshot and see what's changing. 

[BigCol]

I am now pretty confident that I have damaged the part of the tacho that either deals with or parses the sender signal.

Is anyone near to west London who has a similar car (2013 Sigma-engined Supersport) who would be willing to swap out their tacho - initially put mine in their car with the expectation that their fuel gauge would then not work and ideally drop theirs in mine in the hope my gauge then starts working?   

​(see here for a thread on another forum which leads me to that conclusion)

[BigCol]

My thanks to the chaps (Ian, James and Dave) at PT Sports Cars in Windsor, who took a known good tacho out of one of their cars and dropped it into mine; having done so, the gauge showed a reading.

Good news - My diagnosis was right!  Bad news, I have trashed part of £180-worth of tacho!    

Caterham have stock, so I should have a replacement this time tomorrow ahead of me driving to Blyton!

[ChrisC]

Why do you rule out repair?  Caerbont do repairs, not sure how much or how long they take, then again it might be a loss of driving time / cost equation your working on.

Glad you sorted it, shame it a £180 part, but thankfully it isn't the pump at £420 

[BigCol]

A repair is not ruled out, in fact assuming it is something simple (compared to the rest of the tachometer) and therefore a cheap repair, I'll get it fixed - either to keep as a spare or sell on.

As you say, the timescales of a repair preclude that as an option right now.