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Buggered fuel gauge or sender or fuse or...

| 25 Apr 2017 21:09 | Edited by: BigCol | #1
On country roads

Earlier today 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, today I did 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.

| 26 Apr 2017 17:47 | #2
On country roads

I tested the gauge earlier and it's OK (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.
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?
A chap on PistonHeads say 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? Sigma Wiring Loom
| 30 Apr 2017 10:42 | #3
Planning a blat
Hi, the chap on PH here . Hopefully you will have resolved this by now, but if not, here's some more info. I went through all this in detail for another friend with a sigma, so familiar with the diagram. If you look for GB150 on the tacho plug it connects to GB148 to the fuel pump plug (sensor) it also connects to the GB149 on the engine loom plug which seems to come out on the next plugs to WY186 which goes to a fuse (sixth up on the diagram). The other sensor wire (SG196) goes to the engine loom then BY51 and then earth. The gauge get its input from Y198, via the tach.

Now all that doesn't work from an electrical sense, so it is likely that the diagram has an error, is incomplete, or simply wrong. The sensor in the pump is a potential divider (see photo) so SG196 on the pump must have a voltage that reflects the fuel level and this must somehow be feed back to the guage. I suspect the GB148 on the pump and from the tach is a stabilized voltage for the sensor.

Good Luck

| 30 Apr 2017 18:33 | #4
On country roads

Thanks chap on Pistonheads (and really enjoy your facebook posts re your Duratec too!)

Via Blatchat (threw lots of mud against various walls hoping some would stick!) we seem to have come to a similar conclusion re what's connected to what.

I am confident the sensor is working OK (correct sort of resistance between relevant pin and earth) as is the gauge (see above) - so it is something between GB148 on the sensor/fuel pump and Y198 on the gauge, which as you say, goes via the tachometer (which possibly has been damaged so isn't doing what it should n passing the resistance on).

Haven't had a chance recently to spend any time the garage looking further but hope to tomorrow or Tuesday.

Reckon that if I remove the tachometer and short GB150 to Y149 on its connector, the fuel gauge should give a reading... do you concur?
| 30 Apr 2017 19:45 | Edited by: DCL | #5
Planning a blat
Having quickly read blatchat, my thoughts are that they are suggesting the sender is a rheostat (variable resitor) but my picture of the pump suggest it is a potential divider (provides a voltage out) that is based on the sender part having three wires connected - white to the bottom seems to be ground (difficult to see on the picture) green to the top will be +Vcc, and orange to the arm will be signal. It could be that the gauge is looking for a rheostat and the the tacho is doing some form of voltage/current conversion (for the ford sender unit to older caterham gauge). Whatever the reason I think the gauge and sender will be quite robust, so some experimentation is probably worthwhile. But why not take the pump out to check the wiring for yourself. It is not too bad a job as long as the tank is not full.
| 30 Apr 2017 20:14 | Edited by: BigCol | #6
On country roads

thanks... tank is full!

Follow your logic...

B10 (white, as you suggest, and black) and SG196 (orange) both go to earth; red goes to YG168 (? behind the pump in the photo) which is a live via the interia switch. The fuel level sender feed is on the green and goes to GB148.

but isn't the green between the orange and white (both earth) so will give a resistance rather than a voltage?

Either way, with the tachometer out and linking GB150 and Y149, I should give me a reading on the gauge...(?)
| 30 Apr 2017 21:50 | Edited by: DCL | #7
Planning a blat
In terms of what Ford intended, I believe the sender signal is the orange wire SG196 and if your tank is full then it would be ~earth as it wipes towards the earth side, but voltage would rise as level drops. Green GB148 is what I believe is a 'voltage' either 12V or possibly a stablised 5V. See diagram.

But it is possible that Caterham are not using it this way and GB148 is held high and the SG196 sweeps it to ground. That would fit my reading of the diagram. In other words the GB148 becomes the signal wire (current, not voltage) with high current flowing when the tank is empty and low current when it is full. This may have been done to work with the type of gauge Caterham are using (old technology).

If that is the case, then the fault may be with the tacho.

This could be a tricky one to fix. But as you suggest, checking that there is continuity in all the wires and looking at possibilities to bypass the taco - not easy without fully understanding what's going on and how it works.

| 1 May 2017 09:20 | Edited by: BigCol | #8
On country roads

See what you mean... I think, as you say, the issue is probably at the tacho, given that the gauge is OK, the earths are good, the sender robust and wiring unlikely suddenly to fail. I am now loathed to try my suggested bypass to the tacho as I may be doing even more harm.

I assume repairing a tacho is unlikely and so if that is the issue, a swap is necessary.

I'll put a call out to see if anyone nearby-ish has a similar car and 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.

I wonder also if Caterham at Crawley could also allow me to perform similar tests; especially as if confirmed they'd get a £186 sale!
| 3 May 2017 12:38 | Edited by: BigCol | #9
On country roads

I rang Caterham yesterday to see if they could help as above but am still waiting for the promised call-back!

So I am very grateful to the chaps (Ian, James and Dave ) at PT Sports Cars in Windsor, who took a known good unit out of one of their cars and dropped it into mine; the gauge then showed a reading.

Good news - the above diagnosis was right, the part of my tacho that parses the fuel level sender signal is FUBAR! 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 see Hedge at Blyton!

Thanks also to DCL for his interpretation of the wiring diagram, the time to explain various bits and sanity-checking my thoughts.
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