401 Fuel Gauge or Sender Problem.

[mikebro]

Geoff

See a really good explanation here

Fuel gauge function

Most if not all Smiths fuel gauges from the early to mid sixties were like this. The four resistances of a Wheatstone Bridge are made up of three fixed ones in the gauge and one variable one in the sender. I had to fix the one on my Healey 100 a few years back and it is just the same. I found the best way to check that the sender and gauge are OK is to remove them from the car and wire them up per the diagram in the above link. The case of the gauge and the metalwork of the sender need to be connected together and to the battery positive, assuming your car is still positive earth. The gauge terminal that has the Yellow wire goes to Battery negative. The Green wire goes between the other gauge terminal and the terminal on the sender. If everything works then you have a wiring loom problem. This is also a great way to calibrate the gauge so that empty is empty and full is full. The gauge on my Healey had a break in one of the gauge coils. I repaired it by unravelling one turn of the winding and resoldering it to the post. I then had to recalibrate it by slackening the coil fixing and moving it slightly. My 403 system wasn't working either. In this case the problem was purely mechanical - the sender was completely seized up with metal corrosion. As an aside I have also had to dismantle and rebuild the solenoid that operates the fuel reserve. This was also seized up with corrosion.

Regarding the 405 I've no idea if this is the same. Gauges on the 401 and 403 are of the type I have described. One can tell this type because the needle follows the sender behaviour exactly including when the petrol slops from side to side round corners. Later type gauges have inherent damping - I think these work on some sort of heating effect but I've not had to mess with one of these so don't quote me.

Hope that helps.

Rgds

Mike Brooks
403-1402
Scotland

[PEU186F]

You have checked that the gauge case is earthed, haven't you, this is essential. In my experience if the sender goes open circuit and everything else is ok the guage will show full.

I have found this simple explanation of the way the gauge works very helpful, the way the guage is put together became more sophisticated but the principle remains the same

Fuel Gauge - Working or not?

Roger

[Geoff Kingston]

Roger, Mike, Kevin,
Thank you for the additional help and advice I am getting a better understanding now of how the system works and what we should be checking,I shall be printing all this out this evening for reference in the workshop.
John Harris from Bristol was with me yesterday afternoon and he did check the resistance on the sender unit which he thought was about right for the amount of petrol in the tank and also checked the two spare gauges I have which appear to be fine, he could not a good earth reading off the gauge or the metal frame all the instruments and switches are mounted to.
He suggested we needed to look carefully at the Dashboard earths and put a better earth on the sender unit as well.
This afternoon I think I could be a bit closer to a solution as I have discovered that the car has a bad earth on the main battery terminal, have not been able to get it off yet to clean things up as the bolt is turning and is an awkward position between the battery and tool box, not at all easy to at get from under the dash so I decided a coffee and another read of everyone's thoughts on the matter was a better idea.
Hopefully by the end of the week we will have had another go at it and I might be in a position to report some success.
My R Type Bentley has decided this is a good fault to replicate knowing how keen I am on car electrics!, the gauge on that one has suddenly started to read full all the time so with all the help you have all offered we should be able to fix that as well.
Roger you missed a good show near Newcastle Emlyn yesterday, Brian was there with his 410, I had the 603 so if you had made it 100% of the roadworthy Bristols in Pembrokeshire would have been on show.
Thank you all again,
Geoff.

[pnaleahy]

The Smiths fuel sender relies on (a) electrical connection to the coil by attachment of the cable on the outside of the sender unit, and (b) an earth connection to the WIPERS which are attached to the float rod through a pivot hole in the body of the sender thus reflecting the movement of the float onto the wipers.
The earthig issue which stops these units from operating is that the body of the sender unit eventually oxidises and the pivot hole through which the float rod operates the wipers also oxidises thus preventing the earthed body from making contact with the wipers. If the wipers and not earthed due to the oxidised surface and often worn pivot hole, the sender will not transmit to the guage.
One solution to this problem if a new sender unit is unavailabe, is to positively earth the float rod. This can be achieved by using a small flexible braided copper wire which is clamped to the float rod at one end, say two inches below the sender, and the other end soldered to a brass ring with a tag, and formed to sit under the sender unit. The flexible braided wire needs to be light and loose enough to not impede the float rod. This solution has rectified this issue on my 405.

[Geoff Kingston]

We finally have a working fuel gauge, we got the main earth problem sorted which gave us a good earth to the dash but we still could not get the test lamp illuminated properly off the terminal to the yellow wire only on the adjacent one, we were debating whether the gauge might be wired up incorrectly or defective when we realised that someone in the past had already swapped the position of the yellow wire round and got it wrong, we should have spotted it on the wiring diagram sooner but didn't, because everything looked original and undisturbed we were not expecting that problem , with the yellow wire now back on the correct terminal and ignition on the gauge went straight to full, movement at last!
We quickly confirmed this fault was due to a bad earth to the sender unit, using a temporary connection we had the gauge reading properly but the earth wire in the loom wasn't doing its job, we traced the problem to a break in the earth wire where it had been bolted to the body in front of the rear wheel, with a new terminal, nut and bolt and the mounting area cleaned we finally got the good earth we needed and all is now working as it should.
I am very grateful for everyone's input on this it has been a great help in solving the problem.
Geoff.