401 Fuel Gauge or Sender Problem.

[Geoff Kingston]

One of my least favourite things on cars is their electrics, Mike and I had a frustrating morning yesterday trying to get the fuel gauge to move off empty having reconnected the loom, the job was not helped by a faulty Draper test lamp/probe which having been led astray by the thing myself today is now in the bin.
Using a volt meter I have established without doubt that we had the correct wire and the reading I am getting on the feed going into the gauge is the same as the reading on the connection to the sender, around 12.4 volts , when the feed was connected to the sender nothing happened Mike then ran an earth from the sender unit to the chassis still nothing not even a flicker.
Today with the test meter I observed that if the earth wire is connected to one of the screws on the case the reading at the connection to the power feed into the sender drops to just under 3 volts, disconnect the earth and it goes back up to 12.4v, also with the earth lead off I get a reading of 12.4v anywhere on the case of the sender unit, is this normal or is it indicating there is an internal short in the sender unit.
Geoff.

[p5b401]

Had the same/similar problem.
The sender consists of a wire wound potentiometer and a wiper arm as part of the float arm. The earth return relies on electrical contact that is made where the wiper arm pivots in the case of the sender assy. I determined that to take it all apart to refresh the pivot point would be a hiding to nowhere so shaped a length of light gauge spring wire to wrap around the pivot point with one end screwed to the case and the other end coiled a few times round the float arm on the tank side. The spring wire is not strong enough to have any effect on the float position.
John

[innellan House]

1. I assume fuse between A3 and A4 terminals in voltage regulator is OK (i.e. turn signals, brake lamps, and other instruments all working)
2. If isolated gauge or sender failure....next check fuel gauge is earthed (gauge should illuminate at night when you switch on the lights). Also check it has been correctly rewired if it has been removed.
3. Verify that there is continuity between sender and fuel gauge.
4. And sender earth is good.
5. last possibility is sender wiper is making poor contact (clean the surface of resistor over which the wiper travels. Bend wiper to increase contact with resistor).

regards,
vincent

[Geoff Kingston]

Vincent, John
Thanks for your advice.
The loom at the moment is partially disconnected in that all the light fittings had been removed, the fuel tank had been taken off the car years ago and a replacement recently fitted, a local engineer who did some work on the tank tested the sender and said it was ok.
The control box is new and the fuses are fine, the fuel gauge is original and does not appear to have been disturbed. There is continuity between the gauge and the sender but as I said the voltage readings drop when the earth lead is connected, is that normal?
I would have expected to see something if only a flicker on the gauge, is it worth trying another gauge?
How does the system works as I can't get my head round the single wire between gauge and sender.
Also does anyone know if the same sender unit was used in the 405 as I just removed one from a 405 tank. Is there any way to test it off car, like wise the gauge itself.
We only recently reinstalled the loom to the back of the car through the new drivers sill, we established the feed to the sidelights almost straight away, I should have realised it was downhill all the way after that!
Geoff.

[Kevin H]

Geoff,
The reason the sender unit has only one wire is because it is a rheostat, or variable resistor. It's easy to test the sender unit on the bench by connecting one lead from an ohm meter (or a multimeter set to measure resistance) to the wire coming off the sender unit and connect the other meter lead to the body of the sender unit. When you move the float bar on the sender unit you should see the resistance vary on the ohm meter.

You will need to set the ohm meter range to something like 0-300 ohms.

It's important that you have the correct sender unit and fuel gauge, because they differ. For example one brand/model of sender unit might measure 20 ohms when the tank is empty and 220 ohms when it is full, so naturally this resistance range needs to correspond to the fuel gauge requirements.

Models of sender units vary in their resistance range i.e 10-80 ohms or 40-250 ohms, indeed some can work the opposite way - greater resistance as the fuel level decreases. And of course the fuel gauges themselves can vary to 'match' the sender unit.

If your gauge is a Smiths then the sender resistance range is probably around 20-220 ohms.

As for the connections to the fuel gauge, there should be one wire to battery positive (on a negative earth car) and the other wire goes to the sender unit.

There will probably be two additional wires going to the fuel gauge for illumination, and there may also be a separate earth wire to the body of the gauge, so 4 or 5 wires in total.

[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.

[PEU186F]

Glad it all worked out Geoff, it usually does with patience and application. My 410 was actually on parade at the weekend, see attached photo, for the daughter of a well known local brewery. Calon Lan, sung in a Welsh Chapel, will make most people misty eyed (and I was standing just outside), but the bride wept, the brides mother wept, the brides father wept and I have it on good authority that most of the assembled company did as well.

Considering that I hate cleaning cars I think it polished up quite well, but it doesn't really care for weddings. Last time the condenser went literally as I got it out of the garage and we never got there, this time a brake fluid float decided it was saturated and sank, fortunately just after I got them back to the reception. I have the floats wired up to a latching relay and beeper which makes one hell of a noise, all for a false alarm.

Roger

[Geoff Kingston]

Roger,
The 410 looks great, it seems like I should give you a call when I have electrical problems.
I am taking the LED route with the lighting on the 401 which should be interesting, including the headlights.
For the rear lights I am planning to shift the reversing lights next to the numberplate and double up on the stop and tail lights which hopefully will overcome the need to fit a high level brake light.
If the experiment works the rest of the fleet will have an upgrade and another months pension will be gone!
Geoff.