Fuel tank sender unit

[53lake]

On my '68 Traveller I've replaced both the fuel tank and the sender unit, as both were in bad shape, the sender unit was completely seized up.

Also have a new wiring harness installed, hopefully with all the right connections made. Now I can't get a response from the fuel gauge. I took the sender unit out of the tank, turned on the ignition, and manually moved the float, but got no movement on the gauge.

There seems to be only the one hot wire to the sender rheostat. The unit is insulated from the tank by the cork gasket, and the tank is insulated from the body by 3M sealant. So how should the sender be earthed? I don't recall seeing any separate earth connection when I dismantled it.

I'd try connecting a separate earth wire from under one of the fixing screws to an existing earth connection, but with gasoline vapors and safety in mind I'd rather know the correct way. Hope someone can help. Thanks.

BTW there was a neoprene O ring attached to the sender float when I got it. I noticed that after sitting in ethanol-free gasoline for only one month, it has swollen, and was ready to slip off the float into the tank when I lifted the sender unit out. Luckily it didn't. I don't think the material is compatible with gasoline. I guess the only purpose of the O ring is to cushion the float from resting on the tank bottom when empty. If that's the case, I'll leave it off.

IMG_3808.jpg (556.15 KiB) Viewed 1003 times

[ManyMinors]

The cork gasket won't insulate the sender as it is fitted to the tank by metal screws - and so is the tank to the car body, so these complete the circuit.
I haven't ever seen an O ring like that in your picture so I don't think that is standard. I would do without it.
The other thing which can cause the fuel gauge to read incorrectly is the voltage stabilizer which is fitted to the rear of the speedometer. That might be worth checking out if you still have troubles.

[Sleeper]

If you have good eyesight and/or long arms you can switch on the ignition and hold the gauge wire to earth and watch the reaction on the gauge....this will prove/disprove all the wiring....

John ;-)

[geoberni]

The Sender circuit is completed from being bolted down to the tank.
It is possible that if you have got a brand new tank, the paint is insulating it from the car body, despite the number of bolts holding it down.
The Sender is basically a variable connection to Earth for the gauge, so the metal plate of the Sender is bolted to the tank, the tank is bolted to the bodywork.

You can check the Sender by just rigging up a wire from the plate to a good connection on the bodywork, obviously with the Ign on.

Assuming it's OK, then make sure you have good metal to metal connection (through the bolts to bypass the cork gasket) from Sender to Tank to Body.
From what you've said, it sounds like you may have been over generous with 3M sealant.....

the tank is insulated from the body by 3M sealant.

That's a really bad idea . If you have successfully insulated the tank from the bodywork, then you have provided a way to get a potential difference between the tank and it's flammable vapour contents and the bodywork, thus allowing for Static electricity to build up in the right conditions. A fuel tank should always have a bonding connection to the vehicle (whatever it may be) to prevent static.

I know you are in the USA; vehicle static at Gas Stations has given some interesting CCTV fires over the years, especially in dry, low humidity, States....
This one for example, the guy filling cans in the back of his pickup, so no bonding connection at all, because he's likely got a plastic liner in the truck!
Keep the sound down, the music is a bit loud.
https://youtu.be/1tYO4jvnJHw

[53lake]

Thanks for the prompt replies, y'all !

OK, so point 1 is that I will check the wiring as John suggests. Did not think of that.

Point 2, DUH, now I realize the cork gasket doesn't insulate as the screws will make the connection. But by the same token, surely the screws holding the tank down would also bypass the 3M sealant ? When I took the old tank out, it was thoroughly sealed around the rim with dumdum presumably to stop water ingress, so I just renewed the sealant. The tank screws are self tapping, so they have thoroughly chewed into the body metal. Maybe it's the paint under the heads. Perhaps adding some toothed washers under some of the screw heads will help.

Anyway thanks - I will check all this. In any case I'll probably add an earth wire from one of the sender screws to the body to make sure it's properly grounded. I really hope it's not the voltage stabiliser, I'd hate to have to take the speedo out again.

Thanks geoberni for the video, quite entertaining . I just purchased a car fire extinguisher to complete the restoration...my wife rolled her eyes at that. I know she trusts my work though

[geoberni]

The tank screws are self tapping, so they have thoroughly chewed into the body metal. Maybe it's the paint under the heads.

If the tank is held in by self tappers, then I would say that's a pretty good call.
I've not had my tank out, so I just assumed they were bolts.

What I would do is get a multimeter and check a couple of the screw heads to a known good earth point in the boot (trunk).
If they are a good connection, because as you say they have "thoroughly chewed into the body metal", then I'd take them out clean the area under the heads to give bare metal on the tank, and refit them.
That will give you a good contact, then if you want to, just paint over those screw heads to protect the area you've cleaned off.

[ManyMinors]

Thanks for the prompt replies, y'all !



I'd hate to have to take the speedo out again.

It isn't very difficult. Just a matter of loosening two screws and pulling it out.....

[53lake]

Problem solved!
I checked the sender and wiring back to the dash were OK by earthing the sender body, and raising the float arm to the max. Gauge moved slowly to max.
Tank full, and sender back in, no response on gauge. I loosened some of the screws holding the tank down, scratched the paint off under them, and reinstalled them with inside tooth serrated washers under the heads. Screwed back down, and bingo, gauge moved to the max.
Looks like the new paint on the tank was the culprit.
Thanks again for the help!

[ManyMinors]

Excellent! Generally speaking, problems on a Morris Minor are relatively simple to sort out. That is one of the attractions of owning them