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Ohms Help!!

Discussion in 'Early CJ5 and CJ6 Tech' started by tgregg, Aug 3, 2005.

  1. Aug 3, 2005
    tgregg

    tgregg Member

    Oak Hills, CA...
    Joined:
    Apr 14, 2005
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    529
    I'm trying to trouble shoot my fuel gauge and I don't know anything about ohms except they are a measure of resistance.
    My sending unit reads 95 from center post to grnd with a full tank.
    What is it telling me?
    Thanks
     
    tgregg, Aug 3, 2005
    #1
  2. Aug 3, 2005
    hudsonhawk

    hudsonhawk Well-Known Member

    North Texas...
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    hudsonhawk, Aug 3, 2005
    #2
  3. Aug 3, 2005
    hudsonhawk

    hudsonhawk Well-Known Member

    North Texas...
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    hudsonhawk, Aug 3, 2005
    #3
  4. Aug 3, 2005
    tgregg

    tgregg Member

    Oak Hills, CA...
    Joined:
    Apr 14, 2005
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    With an ohmmeter check the resistance between the round sender post (pink wire) and ground.
    It should be:
    * 73 ohms Empty
    * 23 ohms at 1/2 tank
    * 10 ohms Full

    This is the chart I found but I don't understand enough to use it.
    A direct connection with no resistance is 1. depending on the scale. On the sending unit it is .95 again depending on the scale.
     
    tgregg, Aug 3, 2005
    #4
  5. Aug 3, 2005
    Boyink

    Boyink Super Moderator Staff Member

    Tulsa, OK
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    Sep 20, 2002
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    6,197
    Technically a direct connection should be 0 ohms (no resistance). Any amount of wire to connect the two leads will have *some* resistance, so that's probably where you're getting your .95/1 reading.

    Do you have the sending unit out of the tank? It's much easier to understand/explain if you can see it work.
     
    Boyink, Aug 3, 2005
    #5
  6. Aug 3, 2005
    tgregg

    tgregg Member

    Oak Hills, CA...
    Joined:
    Apr 14, 2005
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    529
    Mike,
    Thanks. No the sending unit is still in the tank. I have a new one coming and with the new fuel gauge I should have a working gauge at least for the under the seat tank. I also have a 15 gallon rear tank but as far as I know it doesn;t have a sending unit in it. I may replace it one day and then I'll find out!!
     
    tgregg, Aug 3, 2005
    #6
  7. Aug 3, 2005
    Boyink

    Boyink Super Moderator Staff Member

    Tulsa, OK
    Joined:
    Sep 20, 2002
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    6,197
    OK - basically the sending unit has a float that always stays on top of the gas.

    As gas level changes, float level changes.

    The float is attached to an arm.

    The arm has a "wiper" that rides on a small little coil of wire. One end of the coil is connected to your sending unit wire.

    This coil is the resistor, and depending on where the wiper is contacting the coil the resistance is different. More coil between the wiper and the wire means more resistance.

    The lower the float drops, the wiper moves closer to the non-connected end of the coil, making the resistance closer to the 73 ohms.

    When the tank is full, the wiper is contacting the opposite end of the coil, closer to the pickup wire, less coil means less resistance or closer to the 10 ohms.

    Your gas gauge is basically just an ohmmeter that reads from 73 ohms to 10 ohms.

    If your ohmmeter shows the same reading with tank empty and tank full, then either the tank isn't grounded or the sending unit is bad. I noted on mine the ground wire off the sending unit looked easy to break, so I zip-tied it to the pickup fuel line as a strain relief.

    Not that I think it helped...I didn't get a working gas gauge first shot either...still have to do some trouble shooting. Then again, never have had a working one in 18 years of Jeep ownership...;)
     
    Boyink, Aug 3, 2005
    #7
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