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Technical 49 Mercury temp

Discussion in 'Traditional Hot Rods' started by Marv 49, Oct 8, 2020.

  1. Marv 49
    Joined: Jun 6, 2020
    Posts: 6

    Marv 49

    IMG_3726.JPG

    I am trying to determine if the temp gauge in my grandpas 49 Merc is accurate or not. It wants to run with the needle about 3/4 the way hot. This is after about 5 miles of driving. I have shot the heads and fittings around the temp sender and high temp switch. They consistently show about 170ish. The gauge also doesn’t go all the way to cold when I first start the car. It wants to set about 1/4 mark. My question, is this pointing towards the gauge or the sender? I would like some knowledge on trouble shooting for the gauge and sender. Thanks!


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  2. tubman
    Joined: May 16, 2007
    Posts: 5,458

    tubman
    ALLIANCE MEMBER

    My '51 Ford runs a little over 1/2 on the gauge (slightly lower than you show) when fully warmed up. I have a mechanical gauge that shows 170 at the same time. I think that what you are experiencing is normal. I think the difference between "slightly over" and a "bunch over" is due to age and the variation between gauges.

    I think that modern cars all read lower on the gauge (if they have one) because, with their highly pressurised systems, they have the capability to run a lot hotter before boiling.
     
  3. Caprice89
    Joined: Dec 30, 2014
    Posts: 266

    Caprice89
    Member

    If you disconnect the sender, does it still starts at 1/4? If so, then it is probably the gauge. If not, it could be the sender or bad ground of the engine block.
    My 2 cents.

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  4. Marv 49
    Joined: Jun 6, 2020
    Posts: 6

    Marv 49

    Some correction and updates:

    When I originally posted, it was from work at lunch and I was going off memory. The gauge only goes to the half way mark when cool with key on engine off (69 degrees as measure with a temp gun at the sender).
    As far as testing the gauge with the sender wire off, that creates an open circuit which results in the gauge remaining at the H.
    I did clean up the ground from the motor to the battery as well as cleaned up the wires from sender to high temp switch, and to gauge.
    Next item checked, with the sender wire still off I am getting 5.4 VDC between sender wire and ground. With the wire landed and the key on, I have 4.2 VDC at the sender. Across the gauge I have 10.7 ohms and the sender measures 151 at 69 degrees.
    My biggest question now is, does anyone happen to know the resistance of a new sender or a relatively new one, as well as the resistance of the temp gauge with the key on engine off?
    I know this is a lot of information and I probably over complicated the whole thing, but sometimes I also figure my messes out when I have to explain my process. Thanks for the help!


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  5. tubman
    Joined: May 16, 2007
    Posts: 5,458

    tubman
    ALLIANCE MEMBER

    If your grandpa's car is original, it uses the "King-Seely" type of gauges. These operate differently than the gauges in other cars, and do NOT operate by resistance. I would suggest that you do some research on the principles of operation of "King-Seely" gauges before you get much further into this.
     
  6. My '40 pickup uses a stock King Seeley temp gauge and I had trouble relying on it. I added an aftermarket SW mechanical temp gauge and it was very accurate. It showed me how inaccurate the original gauge was. I keep the stock gauge for nostalgia but I rely on the mechanical gauge. Some guys use two gauges, one for each side because the flathead uses two completely separate cooling systems.
     

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