Fuel gauge short: summary

[Simon Montagu <simon.montagu@domain.elided>]

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About three weeks ago (AD 7/610) I posted a message asking for advice
on fixing a short in the fuel gauge of my 71 Spider. As promised, the
following summarizes what I've found out and includes some of fixes
suggested by fellow Digesti. Around ten responded with suggestions: many
thanks to you all.

* First, it appears that problem is common;
* The most likely cause is a poor ground in the fuel sender unit/gas tank.
  Gauge and Low Fuel Warning Light both grounded through the Sender.
  Sender unit is grounded to the gas tank, which in turn is grounded to
  the car;
* Dirty/corroded/loose sheet metal screws grounding the tank break
  the circuit, resulting in the erratic gauge.

The test for a poor ground is simple enough:
* Pull one wire off the sender. Attach alligator clips to end of wire and to
  a good ground. If you got the gauge lead ("violet" in my Spider), then
  the gauge should peg to full. If you got the low fuel light (black/white)
  the low fuel light will illuminate.
  **NB: I've read that you don't want to keep these circuits in this state
  for long. You may burn out the gauge!

With this info in hand, the fixes suggested by fellow Digesti make a lot of
sense. The following suggestion from Simon Phillips was fairly typical:

>I would recommend attaching a separate ground wire from the base of the
>fuel sender across to the edge of the fuel tank and into the body somewhere.

Carson Damm also suggested the following method for testing the continuity
of the sender unit itself:

>A sending unit is pretty straight forward to test. You have to take it out
>of the tank and you will need a multimeter. What you want to know is that
>the resistance changes smoothly as you move the float up and down.

Thanks again to everyone that responded.

Simon.
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Content-Type: text/enriched; charset="us-ascii"

About three weeks ago (AD 7/610) I posted a message asking for advice

on fixing a short in the fuel gauge of my 71 Spider. As promised, the 

following summarizes what I've found out and includes some of fixes 

suggested by fellow Digesti. Around ten responded with suggestions:
many 

thanks to you all. 


* First, it appears that problem is common; 

* The most likely cause is a poor ground in the fuel sender unit/gas
tank.  

  Gauge and Low Fuel Warning Light both grounded through the Sender. 

  Sender unit is grounded to the gas tank, which in turn is grounded to


  the car;

* Dirty/corroded/loose sheet metal screws grounding the tank break 

  the circuit, resulting in the erratic gauge.


The test for a poor ground is simple enough:

* Pull one wire off the sender. Attach alligator clips to end of wire
and to

  a good ground. If you got the gauge lead ("violet" in my Spider),
then

  the gauge should peg to full. If you got the low fuel light
(black/white)

  the low fuel light will illuminate. 

  **NB: I've read that you don't want to keep these circuits in this
state 

  for long. You may burn out the gauge!


With this info in hand, the fixes suggested by fellow Digesti make a
lot of 

sense. The following suggestion from Simon Phillips was fairly
typical:


>I would recommend attaching a separate ground wire from the base of
the 

>fuel sender across to the edge of the fuel tank and into the body
somewhere. 


Carson Damm also suggested the following method for testing the
continuity 

of the sender unit itself:


>A sending unit is pretty straight forward to test. You have to take it
out 

>of the tank and you will need a multimeter. What you want to know is
that 

>the resistance changes smoothly as you move the float up and down. 


Thanks again to everyone that responded.


Simon.

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End of alfa-digest V7 #667
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