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Fuel gage

PostPosted: Mon May 10, 2021 4:34 pm
by Lotus 50
I bought my Plus 2 in 1984. Back then it would run out of gas at 1/4. Now the gage goes over full and the low fuel light comes on at 1/2 tank indicated. 12V into a 6V sender?

Re: Fuel gage

PostPosted: Mon May 10, 2021 6:06 pm
by mbell
Check the voltage regulator, I believe the tank sender should be 10v from the voltage regulator. It is a bi metal type regulator originally so may not show as 10V on a modern multi meter, if you see a solid 12+v it probably not working, if you see the voltage moving around or dropping to zero it probably is working.

Re: Fuel gage

PostPosted: Tue May 11, 2021 1:10 am
by Lotus 50
I expect the voltage regulator is wrong - it’s from a Ford Pinto and has been there since I bought the car. Can I drop the voltage just to the sending unit?

Re: Fuel gage

PostPosted: Tue May 11, 2021 3:34 am
by mbell
How is the engine temperature? I believe it's is also powered from the voltage regulator so would get similar issue with that gauge if voltage related.

I'd check the voltage at the sender, with it disconnect there will be no current flow so should read around 10v or float about.

Another alternative is to pull the sender from the tank and measure it resistance across the range. This will also let you exercise it that might dislodge any crud that could be increasing it's resistance.

Also check the float, mine had cracked and taken on fuel, which could cause the behavior you describe. I was able to get a new float for a old jeep for a few dollars that's an exact fit.

Re: Fuel gage

PostPosted: Tue May 11, 2021 8:36 am
by alan.barker
Sorry but it's a "Voltage Stabiliser" many times fitted to back of Speedo (to earth it) and it's small.
Lots of past posts about this :wink:

Re: Fuel gage

PostPosted: Tue May 11, 2021 11:02 am
by Andy8421
The original Smiths voltage stabiliser/stabilizer/regulator is a piece of junk.

For the technically inclined - An element heated a bimetallic strip that opened and closed a set of contacts to give an 'average' of 10V at the output to supply the instruments with a stable reference voltage. If the battery voltage changed, the ratio of on / off changed and the average output voltage remained the same. Similar to a modern switch mode supply. Except it wasn't very good, and the average output voltage wandered around all over the place. Since the gauges weren't much good either, it didn't really matter.

Measuring the output voltage of a working stabiliser won't give you 10V. If your meter's response is fast, you should be able to see the output switch back and forth between 0V and battery voltage. If it is stuck at 0V or battery voltage, then the stabiliser is probably a dud.

Interestingly, the original regulator had 'TOP' and an arrow marked on it. I can only assume that gravity changed the way the bimetallic strip flicked back and forth and altered the average output. I believe the modern replacements use a semiconductor linear regulator, and although the electrons don't care which way they are up, the case still has 'TOP' marked on it.