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Technical Electrical help... gauges driving me crazy

Discussion in 'The Hokey Ass Message Board' started by dmac620, Sep 1, 2020.

  1. dmac620
    Joined: Sep 16, 2007
    Posts: 357

    dmac620
    Member

    I recently picked up a 1959 Ford Ranch wagon. I do not have a working fuel or temperature gauge. When I purchased it, the fuel gauge would read 1/4 tank no matter what level of fuel was in the tank, full to drained and completely empty. When I would start the car, the temperature gauge would immediately climb to the “H” and pin itself to the right side. I tested the constant voltage regulator on the back of the gauge panel and found that both terminals on the regulator were reading just over 13 volts when the car was running. Clearly, the regulator was not dropping the feed to the gauges down to the desired 5 volt range. I swapped out the regulator and now I get zero readings on both the fuel and temperature gauges. I tested the terminals on the renew regulator and found 13.2 volts going in and 5.06 volts going out to the gauges. I then tested the volts on each power terminal going into each gauge and at normal engine operating temperature. The power feed going into the temp gauge reads 5.03 volts and the terminal on the back of the gauge that is fed by the sender wire is reading 3.6 volts. Seems like the resistor in the sender is doing its job. The power feed going into the fuel gauge reads 5.03 volts and the sender terminal reads 4.7 volts. (Tank was filled approximately 25 miles ago so it is nearly full)

    Additional tests I did.... I removed each gauge from the panel and tested it independently by attaching the positive side of a 3 volt battery to one terminal and the negative side to the other terminal. When I did this to the fuel gauge it read 3/4 full. When done to the temp gauge, it read just under the hot “H” mark. Therefore, both gauges are working. I removed the wire going to the temp sending unit on the engine and grounded it and the temp gauge immediately climbed to the “H”. I swapped two different temp sending units into the engine block and reattached the wire and both rendered no reading on the gauge. Lastly, I removed the sender line going into the fuel gauge while it is running and nothing happens to the gauge, still reads completely empty.

    I’m at wits end.... not sure where to look now to diagnose. A ground somewhere? I found a separate ground wire run from the back of the gauge panel to the bottom of dash that was there when I purchased the car. I undid it and saw no change at the gauges.
    Any advice is GREATLY appreciated.
     
  2. 52HardTop
    Joined: Jun 21, 2007
    Posts: 915

    52HardTop
    Member

    I would say the wire from the temp sending unit may be grounded somewhere. The wire to the fuel gauge sending unit could be broken? Each of those wires are negative connections controlling their respective gauges by being grounded. I would say, there should be no positive or voltage going to those sending units.
     
  3. Sounds like the grounds between the sending units and the rest of the car are bad. Try grounding the temp wire to the sensor body, if it still doesn't work check the resistance between the sensor body and another ground. If you've used teflon tape on the sensor, it may not be grounding. Could be the same sort problem at the fuel tank.

    Also check voltage on the sensor wire to ground at the sensor. You should see the same voltage you registered on the sending unit side when testing just the gauge within a tenth volt or so. If it's much lower, the problem is in the wire.
     
  4. 52HardTop
    Joined: Jun 21, 2007
    Posts: 915

    52HardTop
    Member

    Unless it's different with a Ford, the wire for the temp gauge going to the sending unit should be a negative connection. I would disconnect that wire from the back of the gauge and see if the gauge drops down from the Hot reading. There should be a resistance between that wire and the engine block being controlled by the temp sending unit related to the engine temp. The same with the wire to the fuel gauge sending unit in the tank. That too is another negative connection that is regulated by the changes in the resistance of the sending unit. Again controlled by the level of the fuel in the tank. There would be no positive voltages present at those sending units.
     

  5. squirrel
    Joined: Sep 23, 2004
    Posts: 46,872

    squirrel
    Member

    First thing I would do, after reassembling it all, is remove the sender wires from the gages, and measure the resistance from each wire, to ground at the gauge panel. Tell us what you find.
     
  6. 1ton
    Joined: Dec 3, 2010
    Posts: 399

    1ton
    Member

    Just for grins, make damn sure that you have good body grounds. Body to frame. Body to engine block. If your body is totally ungrounded current could be going through the temp gauge wire. And without good body ground the fuel gauge may not be getting enough "oommph" from what little ground may be present.
     
  7. dmac620
    Joined: Sep 16, 2007
    Posts: 357

    dmac620
    Member

    Thanks for all the input. I plan on going through each suggestion out in the garage tomorrow. I will update the post.
     
  8. Funny thing I did with the Dodge that the gauge always read hot - and I could remove the radiator cap with not boil over - I shot the motor with one of those lazer gun temp gauges and it was a cool 180.....installed a good under dash water temp gauge. Fuel tank is another issue.......just bought some new yard sticks at Home Depot for my bangers....
     
  9. jaracer
    Joined: Oct 4, 2008
    Posts: 501

    jaracer
    Member

    Your car has thermal/electric gauges. The CVR supplies them with either a pulsing voltage (original CVR) or around 5 volts if it is a solid state unit. The sending units complete the gauge circuit to ground through their resistance. You can check gauge operation by momentarily grounding the sending unit wire. This should cause the gauge to go to full scale. If it does, the CVR, gauge and the wiring to the sending unit are good. It sounds like you did this with the temp gauge and it did go to hot like it should. That would point to a bad sending unit, but you say you tried other sending units. Are you sure you are using the correct units? If they are correct, I would suspect a poor ground at the sending unit. With everything connected and the key on, check the voltage at the sending unit terminal. It should be the same as the voltage at the sending unit side of the gauge. Then move your positive voltmeter lead to the base of the sending unit,(leave the negative lead connected to ground) it should read less than 0.1 volt. Any reading above that means you have a poor ground at the sending unit.
     
  10. 54vicky
    Joined: Dec 13, 2011
    Posts: 1,457

    54vicky
    Member

    sometimes simple things just work :D:D:D
     
  11. 54vicky
    Joined: Dec 13, 2011
    Posts: 1,457

    54vicky
    Member

    it sounds like you have a bad temp sender on engine as I am pretty sure they have a bimetal strip that reacts to temp changes if contact are stuck possibly from the bad constant volt regulator?it may register hot when current is sent to it.as for fuel gauge I do not see where you eliminated the sender in tank.possibly the float bad (it happens)pin hole leak.but as has been posted you will need a ground from gauge to chassis the thing is you need to know that where you try to ground is indeed a good ground.if you ground the wire from the fuel gauge it should go to full assuming you have a good ground but we all know what assumptions can do good luck
     
  12. dmac620
    Joined: Sep 16, 2007
    Posts: 357

    dmac620
    Member

    measured 46 ohm at the temp sender wire and 121 ohm at the fuel gauge sender wire.
     
  13. dmac620
    Joined: Sep 16, 2007
    Posts: 357

    dmac620
    Member

    I just grounded the fuel sender wire where it connects to the sender in the tank. Fuel gauge immediately went to full. Removed the ground and it dropped to empty.
     
  14. dmac620
    Joined: Sep 16, 2007
    Posts: 357

    dmac620
    Member

    I just grounded the temp sender sender wire where it would connect to the sender in the engine block. Temp gauge immediately went to full hot.
     
  15. squirrel
    Joined: Sep 23, 2004
    Posts: 46,872

    squirrel
    Member

    Starting with the fuel gauge...the resistor in the sender should provide 73 ohms when empty, and 10 ohms when full (ohms to ground, that is). Since you have 121 ohms, it thinks the tank is past empty. I would next poke around the sender with the ohm meter. First, find a good ground on the frame or body, and measure the resistance from that good ground, to the sender housing (the plate that is screwed to the tank). See if it's zero ohms, or close. If that checks out, then with the sender wire disconnected, measure the resistance from ground to the sender wire terminal, at the sending unit. If it reads more than 73 ohms (empty tank) then you definitely have a bad sending unit.
     
    dmac620 likes this.
  16. jaracer
    Joined: Oct 4, 2008
    Posts: 501

    jaracer
    Member

    Gauges and wiring are good, your problem is either the senders or the ground for the senders. You can check this with a voltmeter as I described earlier. You have to have the circuits live to do this.
     

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