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  #1  
Old 09-17-2020
D. Troit D. Troit is offline
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Location: Victoria
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Default Erratic Fuel Gauge.

On a short 30 mile trip into town today I noticed that my fuel gauge needle would occasionally spike up to FULL, and then return immediately back to normal. It did this several times while driving. Never saw it do this while stopped.
I'm thinking it may be a sending unit issue, or the wire shorting to ground.
I believe the sending unit works on variable resistance to ground (-), Yes?
For the gauge to spike to full, would that be zero ohms to ground, OR max ohms to ground?
Or, none of the above?
Anyone else have this issue and solved it?
Thanks for any advice.
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Robert Murdock
Victoria, TX
1987 PT38
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  #2  
Old 09-17-2020
tintent tintent is offline
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I think it works the other way 0 ohms = empty a whole batch to fill it just like the tank
so possibly an open circuit or the sending unit
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Mathew Dondero
Eliot, Maine
92 SP 36
mrmotors1@comcast.net
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  #3  
Old 09-17-2020
D. Troit D. Troit is offline
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So spiking to full would indicate an open circuit, or no connection at all. That could be the sending unit, or a bad connection anywhere between there and the gauge.
Hmmm.
I wonder if there is any way to access the wires on the sending unit without dropping the tank?
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Robert Murdock
Victoria, TX
1987 PT38
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  #4  
Old 09-17-2020
cinerama3 cinerama3 is offline
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Sender should be putting out 33 to 128 ohms, IIRC.
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Michael Kraus
Indianapolis, IN
'90 WB 40
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  #5  
Old 09-17-2020
Tom and Erin Tom and Erin is offline
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Replaced mine this year. I used a WEMA SSL25 and if my notes are accurate my gauge used 240 ohms empty, 33 full. 300 gallon tank is 27” deep. Sender requires space between it and bottom of the tank. I figure a 2 inch pad of fuel gauge empty is a good thing. 1983 PT35
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Tom and Erin Sitterley
SeaTac, WA
1983 PT35 "Miss Adventure'r"
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  #6  
Old 09-17-2020
D. Troit D. Troit is offline
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Quote:
Originally Posted by cinerama3 View Post
Sender should be putting out 33 to 128 ohms, IIRC.
Would that be 33 ohms for empty, 128 ohms for full?
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Robert Murdock
Victoria, TX
1987 PT38
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  #7  
Old 09-17-2020
D. Troit D. Troit is offline
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Quote:
Originally Posted by Tom and Erin View Post
Replaced mine this year. I used a WEMA SSL25 and if my notes are accurate my gauge used 240 ohms empty, 33 full. 300 gallon tank is 27” deep. Sender requires space between it and bottom of the tank. I figure a 2 inch pad of fuel gauge empty is a good thing. 1983 PT35
So, low # ohms = full, higher ohms = empty.
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Robert Murdock
Victoria, TX
1987 PT38
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  #8  
Old 09-17-2020
cinerama3 cinerama3 is offline
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Tom is correct. 33 to 240. And I just did this job. Sheesh!
As he said, 240 empty, 33 full.
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Michael Kraus
Indianapolis, IN
'90 WB 40
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  #9  
Old 09-17-2020
D. Troit D. Troit is offline
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OK, So if my gauge needle pegs to full, then I have a pretty good short to ground (zero ohms).
I'm not too sure that a malfunctioning sending unit would do that. Most bad sending units I've dealt with over the years were open circuit.
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Robert Murdock
Victoria, TX
1987 PT38
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  #10  
Old 09-17-2020
D. Troit D. Troit is offline
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If I take the wires off the sending unit at the tank and tape them up, my gauge should read empty. Correct?
And if I go for a drive with the wires disconnected and then the fuel gauge continues to occasionally peg to full, wouldn't that indicate a short to ground?
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Robert Murdock
Victoria, TX
1987 PT38
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