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-   -   Need help with gas gauge (http://www.clubcobra.com/forums/era-speak-bob-putnam/140238-need-help-gas-gauge.html)

RockBit 02-17-2018 08:17 AM
Quote:
Originally Posted by Bender (Post 1438671)
Hi Chris.
I had a similar problem recently. If you have fibre or nylon washers on the bolts. The sender does not earth to the tank through the bolts. You need to connect an earth from the chassis directly to the sender unit base. Some sender units have an earth wire spade soldered to the sender unit base. Some don't. The earth wire if connected to the sender base by the bolts. You need the wire/ terminal to have direct contact with the sender base. ie no washers between the wire and the sender base.
Hope this helps and you can be out enjoying your ride.
Bender
There are no fiber or nylon washers, but there is some sort of thin gasket sealer under the metal washers and on the bolt threads. This bothered me, so I am pretty sure I checked the continuity from the installed sender base to the tank with my meter. Even so, I will try this. It will be a few days before I can get back to it.

Thanks for your help.

tboneheller 02-19-2018 08:09 AM
Remove the sending unit. Attach gauge wire and ground using long jumper wires. Place unit in a bucket of water so that you can actually see that the float floats.

RockBit 02-19-2018 09:22 AM
Quote:
Originally Posted by tboneheller (Post 1438869)
Remove the sending unit. Attach gauge wire and ground using long jumper wires. Place unit in a bucket of water so that you can actually see that the float floats.
I will make sure the float floats when I get back on this in a couple of days. Thanks for the suggestion.

RockBit 02-21-2018 11:42 AM
I had abandoned the fuel gauge trouble shooting and decided to work on an oil leak at the back of the engine. I tore it down to the bare block and couldn't tell where the leak was. I cleaned the block, added dye to the oil, put the flywheel/bell housing/starter back on. With the car on jack stands, I started it and watched the gauges for it to come up to temperature.

I noticed the fuel gauge was reading just below 1/2 tank. It has 10 gallons in it. This is the first time I have ever seen it register anything at all. After I shut the engine off, It went to Empty. Turned the key back on, it went back to just below 1/2.

I don't know why it started working, but I'll take it!

Thanks for everyone's help and suggestions.

CCKW353 02-21-2018 12:04 PM
great job.... hahaha. take any win you can get, how you can get it!

DanEC 02-21-2018 04:19 PM
Gremlins - I blame them for any electrical mysteries. Except for a switch or two we don't have any real Lucas electrical - so Gremlins get my vote.

Good luck with the oil leak.

RockBit 02-21-2018 07:31 PM
Quote:
Originally Posted by DanEC (Post 1439053)

Good luck with the oil leak.
Thanks Dan. Based on the UV dye, it is the side seal on one side of the rear main bearing cap. I'll fix it.

monster7 02-22-2018 02:45 AM
You can try to put e specific resistance (e.g. 100Ohm) in between. Just put one side of the resistor in the earth plug and the other side to the chassis somewhere (earth). If the gauge is not showing a fuel level somewhere between fuel and empty, you have some problems with earth.
PS: my Autometer gauges have a hole in the case with a very small poti behind. This is to adjust the resistance for calibration. I do not know the Smith gauges, but maybe you have something similar?

RockBit 02-22-2018 08:52 AM
Quote:
Originally Posted by monster7 (Post 1439103)
You can try to put e specific resistance (e.g. 100Ohm) in between. Just put one side of the resistor in the earth plug and the other side to the chassis somewhere (earth). If the gauge is not showing a fuel level somewhere between fuel and empty, you have some problems with earth.
PS: my Autometer gauges have a hole in the case with a very small poti behind. This is to adjust the resistance for calibration. I do not know the Smith gauges, but maybe you have something similar?
My Smiths gauge does not have any adjustment that I am aware of. It is working now. Thanks for the suggestion.

Dominik 06-04-2018 11:59 PM
Thanks guys, very interesting to read. And thanks for pointing - yet, again - to the ERA website for help! :-)

Calibrating the fuel tank sensor to instrument is a daunting task. I get to it shortly for a customer car: Jaguar XJ6 instrument (200 Ohm empty - 20 Ohm full) with unknown sensor...
Seems like a standard aftermarket sensor (240-33) could work okay. I still have to make the tank.

30 years later I am tempted to rate a well built car by the accuracy of the fuel gage!

RockBit 06-05-2018 02:28 PM
My fuel gauge continues to work, and I am still adjusting to it. When the tank is brimming full, the gauge is just short of the full mark.

I am still resetting the tripometer as a backup. The other day my wife and I went on a 106 mile drive after filling the tank. I usually get 8 mpg around town, but the 106 were all highway miles. When I got home the gauge read just over half, so I thought it was wrong. When I filled up it took 8.8 gallons (19 gallon tank) so the fuel gauge was spot on! Most encouraging.

If I ever have the tank out again I'm going to solder an extra ground wire to the top of the sender. I think poor grounding was my root problem originally.


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