7 series 4 the fuel tank needs to be replace.

[Dan Pantages]

I ordered a new tank from Compbrake. https://www.compbrake.com/ It came in 6 days from when I ordered it. It took 2 hours to get the old tank out. The old sending unit was a ball of rust. The new tank came with a new sending unit. I put the new tank in, not hooked up or any of the axel/suspension installed, it took 3 hours to line up the top bolts but once I did, it all fit perfectly. Now, the problem. Before I put it in I didn’t test the sending unit with the gauge in the car, stupid mistake. It’s a Smiths gauge. I turned the key on and the gauge went to the full mark, slowly, the tank is empty. I put a meter across the new sending unit and it reads 009.2 ohm’s. I’m quite sure the sending unit is not jammed in the up position. So, does anyone have an idea what to do. If someone wants to talk about it, I would be glad to phone them.  I don't know how to make this a thumbnail.

I have a new email address. pantages47 at gmail.com

[MV8]

Disconnect the lead to the sender. Turn the key on, note the level indication, then touch the lead to ground watching the gauge. Does it swing? You could also remove the sender from the tank, add a ground clip to the assembly, connect the lead, turn the key on, then swing the float while watching the gauge.

[Dan Pantages]

OK, when the lead is disconnected from the gauge, the gauge reads empty.  The ohm reading of 9.2 is just the sending unit.  When you say "then touch the lead to ground watching the gauge" do you mean the gauge lead?  I would like to avoid removing the sender from the tank as that means removing the tank from the car.  As you can see from the picture of the tank the sender is on top, unlike older 7s the top of the tank is totally covered and over all the suspension and rear axel with no way to get to it.  Actually this is the view of the tank without all the rest uninstalled.  At this point I am quite sure I will have to remove the tank.  Hope springs eternal, hoping the float is stuck in the up position. 

 

Please keep coming with questions/ideas.  Without these, there are no answers.

[EdWills]

Hi Dan.

 

Diagrams at erareplicas.com. Choose 'Smiths fuel gauge troubleshooting'.

see: smiths-intruments.co.uk. (Caerbont of Wales).    This site lists all of the sender part numbers and the resistance of each.  From memory, Smiths use a 3 wire set-up from the sender, but one site also mentions 2 wire units.  Era replicas has wiring for the Smiths gauge and also Nisonger. 

 

Google 'Smiths fuel sending wiring', and a lot of diagrams come up.  Good luck with sorting out the problem. Will

[MV8]

I was referring to watching the gauge while touching the lead to ground (briefly) but I think that maybe your tank and sender are fine (guessing 10/80 empty/full), just not compatible with your specific Smiths fuel gauge (guessing 240/20 empty/full). I would ask the tank supplier for more info on the sender full-empty ohms and what gauge it is supposed to work with. You could also test your gauge with a potentiometer (like an old car stereo volume control). They typically have three leads, cost about $10 and come listed for different total resistance values. You can use this to determine what the gauge needs resistance-wise to read full and empty. You may need to replace the gauge . That sender is not like the oem S4 sender in how it fits the tank. Not a problem, but it tells me it may be a universal sender and ohm swing. It is also possible to calibrate the new sender to work with the old gauge.

Edited August 6, 2024 by MV8

[MV8]

'60's Chrysler Tech video but the fundamentals apply:

 

 

[Dan Pantages]

The tank is in and the car is on the road.  The sender that is in the tank is 10 to 180ohms.  10 being empty and 180 being full.  I found a replacement gauge that is a copy of the Smiths gauge that works in this range.