Fuel gauge and voltmeter packed up. Any clues

[Chris]

OK so before we start, yes there's fuel in it and yes the alternator is charging.

I jumped in on Friday to go to the AO show and the voltmeter didn't lift. Oh well, I have another meter in the set up so I could see all was well. I thought I was doing rather well on fuel getting there but today on the way home after nearly 200 miles the gauge was still reading completely full. I do not believe it's a stuck float in the tank. I'd say it failed at the same time as the volt meter but because the tank was brimmed, it looked normal. All fuses checked and every other gauge and panel light works on the binacle.

Now I did have the dash out recently to replace a lamp but there isn't one single plug for these two items that I might have left off. Plus I'm sure it was working after the replacement. I've looked at the wiring spaghetti maps but can't see any unique correlation between the two instruments. Anyone got any clues? Again, I feel this is a gauge issue rather than any failure in the vehicle.

Any thoughts chaps?

 

[Ben Stratford]

A similar thing happens in my 60 from time to time, the fuel gauge and temp gauge both max out at the same time out of the blue Chris.
I found that the spadeconnector on the temp sensor gets tarnished. A quick rub with emery cloth and all ok again.
So an electrical connection certainly. No idea how it affects the fuel gauge other than a common earth perhaps.
Look where it was recently disturbed first would be my choice

 

[Richard Turner]

Not terrfically helpful but here is a 2nd hand gauge for £50

https://www.facebook.com/marketplace/item/1660953267329740

 

[Dave2000]

The fuel and volt meter share a common earth Chris, I think the fuel gauge works on the stepper motor principle, so needs an earth as well as a live to move down as well as up.

The earth by coincidence is shared by the bulb cluster that runs along the bottom of the cluster. The printed circuit also folds under the binacle right where the earth meets the bulb cluster.

I am unsure if the volt meter is stepper motor controlled, sharing the same earth, and you mentioned a recent bulb change, possibly related?

Regards

Dave

 

[Dave2000]

Just a quick note, the bulbs that share the earth are on the right of the binacle, difflock lights ect.

Regards

Dave

 

[Chris]

Dave you're a diamond. Just the sort of nugget I was looking for. Need some twiddle time I think.

Thanks for the other suggestions chaps, but I am going to explore this one first I think. It fits the picture.

 

[Chris]

Well and hour of fiddling around and no change so far. I did find a common earth ion the foot well that was really loose but not connected to this issue. A hand find though.
Can't see anything obvious in the binacle. Had it all to bits and checked for any breaks. Oh also found a disconnected speaker cable so that an improvement too. Voltmeter not so bothered but not knowing how much fuel you have is going to be dicey.

 

[Andy]

think Ive got an instrument cluster in the garage if its any help, but it probably different to yours (12valve).

it may not be that you disturbed anything changing a bulb, but that's where I would start looking (as you have), or it could be coincidence, cant offer any useful advice really!

 

[frank rabbets]

I wonder if it's a wiring or rheostat problem at or in the tank. Knowing car wiring it might affect the volt meter too.

 

[Chris]

Had the sender out just now. Tested it and also dropped in a spare that I had. No change. I can't see that the fuel sender being stuffed would affect the voltmeter. I have looked at the earths and the voltmeter says it uses one in the foot well and the tank uses one located floor pan centre???? Errr... Had the cluster out again and in bits, nothing found all back together and everthing seems to work except those two gauges still.

Had a kind offer of a loan binacle to try (North's premier potato merchant) so that's worth half an hour of messing when I get chance. Is is coincidence that I had the cluster out and then they stopped working? Easy to think so I know but nothing actually found on inspection and I'm sure that they were working when I drove it from the unit to home after doing it. I'd have noticed I'm sure.

 

[Dave2000]

Bad luck Chris. Only good thing I guess is the low fuel light is independent of the gauge.

Regards

Dave

 

[Chris]

Not working. I connected the sender up outside of the tank and set it to zero position. Nope.

 

[StarCruiser]

Have you tried taking the bulb out that you changed Chris? Could it be it has somehow broken the track underneath it or shorted to something else? If you identify the earth points, you could try using a wire from a known earth with either a bulb in series or a 1A fuse just in case you hit a wrong track and see if providing the correct earth restores normality.

The sender usually uses a variable resistance to earth so it’s the + that is common.

Are you looking at the diagram on page 33 of the wiring manual?

 

[Chris]

Page 28. I didn't actually change a bulb Rich, it was an LED that's been in there for 2 years and occasionally went off. I just bent the contacts and put it back in,.

 

[StarCruiser]

If you could post up or PM me the page you are looking at I’ll take a look. Interestingly on the page I was looking at there is a group of electronics that I was looking at as potentially the source of the problem.

Is it possible this LED got fitted back the wrong way round?

 

[Chris]

LED won't work if reversed. It's been in there ages but occasionally flicked off. It was the main speedo one. Drove me nuts. I pulled the dash, wiggled it a bit etc and put it back. Been fine since. I also took the speedo out to look for signs of tracking. There was nothing. My needle occasionally goes full scale and I already have a new sender on the T box. So I then put it all back together. This is the page. It says volts and fuel use different earth points

 

[Dave2000]

Not working. I connected the sender up outside of the tank and set it to zero position. Nope.

Chris, when I was looking for an issue I was using a 94 12 valve dash, just seen yours is a 95, sorry.

So is your sender in the tank different as well? On mine the low level light has nothing to do with the float position, it is a sensor on the end of a fixed length rod.

If by chance you have the fixed length rod but no warning light with ignition on, then this may lead you to see if there is an earth between light and gauge, or may at least change your train of thought to look elsewhere?

Regards

Dave

 

[StarCruiser]

“LED won't work if reversed.” Correct, i was just starting at the simple end of things.

“It's been in there ages but occasionally flicked off. It was the main speedo one.” Do you mean the red one in the middle for the turbo?

My focus is on the internal link that goes between the fuel gauge(s) volt meter, turbo, speedo, tacho and buzzer. As this is connected to the ‘turbo’ bit which is shown as a transistor, couple of diodes and resistors and the LED, it may be worth revisiting this area and checking for broken tracks on the pcb sheet, or anything else untoward.

 

[Chris]

Nope it's the general dash illumination lamps. I put in a set of brilliant white LED ones.

Dave the low level light is triggered inside the rheostat I believe. Once the float drops to reserve it closes a circuit for the warning light.

 

[Chris]

Looking at the diagram; the fuel gauge is on white plug C wire 14 Y-R and earth BF Floor Panel Centre
the Voltmeter is on blue plug B wire 13 BR and earth ID Right Kick Panel

So the commonality really is the PCB and I guess the track that supplies power to them. Otherwise the fault would have to be in two different earths and two different plugs. Given that as Rich points out the two gauges are Siamesed then the fault most likely lies on the circuit board tracking. I didn't see anything there when I looked. I am wondering if it's possible to test the gauges by applying a test current with my probe to see if they work at all. I'm getting a bit beyond my skills there.

Certainly sticking Andy's spare console would give a quick response. Thing is I might run out of fuel before I get there ha ha. Glad I have a sub tank!