Fuel Gauge / sender problems.

[IanVFR750]

Hi All,
I currently have a fuel gauge that reads FULL all the time on my Tasmin 200. It has been like that since I bought the car but running out of fuel last week made me think it was time to get it fixed.
The Bible says problem likely to be sender wire shorted to earth but I ran a new wire from gauge to sender and gauge still shows FULL.
I'm assuming gauge is OK as needle drops to empty when ignition is off but rises to FULL again when switched back on.
The sender unit on the tank actually looks very new and shinny - it's the 2 connection one (green wire and black earth) with 6 bolts to tank. I even swapped the two wires over to see if that was the problem but still read FULL.

Are there any tests I can do to establish if the sender is at fault or is a new sender the only option.

If new sender is required can anyone tell me where to source a unit - is it specific to TVRs or is it common to other cars of that era.

Thanks - Ian.

[SEvans]

I can't remember which way round it works but taking the wire off the sender should see the gauge move fully one way then shorting it to earth should see it go fully the other way. It's relying on an earth through the metal tank so maybe don't rely on this for a test and run a wire from a known good earth to the sender wire. If doing any of this makes no difference it sounds like you have a short on the wire from the back of the gauge to the sender, or maybe in the gauge itself. At this point it's going to be a dash out job.
Regards Steve

[Adrian@]

The sender will need to match the gauge version...Stewart Warner/ Smiths/ VDO. A@

[IanVFR750]

It's a Smiths Instruments gauge on the car - do you know if Smiths supply compatible senders ?

[Adrian@]

My thoughts are that your era Smiths gauges are powered at 10V via a voltage stabiliser, which as OE is hot wire earth-out dissipate, (it needs to have an earth) AND, you can only read the output with a mean average multimeter (these days there are solid state versions that output at 10V).
This powers both the fuel and temp, as fuel is eternally dropping and being refilled, not static, it is will be an unreliable 'tell', BUT, (changing temp senders on the engine is common in attempts to cure a VS failing) if the gauge is being powered by the car battery (13.8V which is alternator, power down to 10.8V, which is car running and headlamps and heater fan on 'perhaps') the gauge with haphazardly reads what ever it wants (full) regardless of sender (the battery is lower at start up and builds as it being driven too!).
That all said, IMHO I would go back and look for how it is powered. A@

[SEvans]

If the issue is an incompatible Sender/Gauge maybe something like this would be a solution.

https://www.ebay.co.uk/itm/297334458207?_skw=Fuel+...

[IanVFR750]

Thanks guys for all your suggestions.

I've contacted Caerbont Automotive (Smiths Instruments ) to get them to confirm that the new sender unit they supplied to the previous owner in 2023 ( I have copy invoice and part number ) was the correct one for my car and compatible with the fuel gauge. I await their reply.
I may yet end up trying your ebay interface module Steve.

Thanks again - Ian.

[Adrian@]

I would say that the Ohm controller will only work if you have control over the power input to the gauge.
I am assuming you have a voltage gauge...that would read the variable battery power that the VS is controlling down to 10V, but, if the VS is not working (as I said, it is hot wire and they get tired or the earth is poor) the gauge will always be variable to the power being supplied, minus what the sender is trying to feed back to the gauge. Altbough, IF it is a VS issue it will throw a wobbly you are ATM not seeing in the temp gauge. A@

Edited by Adrian@ on Tuesday 18th August 14:30