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Technical Testing gauges and sending units

Discussion in 'The Hokey Ass Message Board' started by REBEL43, Oct 28, 2020.

  1. REBEL43
    Joined: Feb 17, 2007
    Posts: 705

    REBEL43
    ALLIANCE MEMBER
    from TENNESSEE

    Hey guys, Jeremy @ Rebel Wire here again. Just wanted to post about another thing I've ran into lately. So again, this is on a customer's 55 Chevy that I'm rewiring in my "spare time". So I get to the fuel gauge and it's reading wrong. Actually it ran out of gas while I'm backing it in and out of my carport to work on it, so I know it's empty. I first figured fuel sending unit (maybe wrong sender/ohm range...who knows really). Of course I had already powered and grounded the gauge to make sure it moves from empty to full...full range of motion...figured the gauge is okay. So I pull the sender out of the tank. From everything I can find all tri-5 Chevy's are 0-30ohm senders. I test this one, which looks to be new, and it's about 0-31 ohm, I call that good. I take my little resistance box and connect it between ground and the gauge and the stock gauge wants about 70-80 ohms to get to full. Even pull it out of the cluster and connect it to the sender. That 30 ohms just won't get it past about 1/2 a tank or so. Through all my tests, now I'm comfortable ordering a new fuel gauge and going from there. For my own piece of mind though, I am waiting until it comes in, to redo my bench tests with the new gauge and same sender.

    This is the stock gauge, adding resistance to see what it wants to get to full.
    https://fb.watch/1psNgSY0nR/

    This was with the gauge out, bench testing it with the stock sender
    https://fb.watch/1psYo02hUU/

    I even did a similar test on the temp gauge (stock 235 engine) the 55 mechanical gauge had been swapped to a 56 electric gauge. So I needed to cap off the small hole for the capillary tube and I "T'd" into the lower thermostat housing to add in the electric sender for the 56 gauge. Through testing the gauge I knew that I needed a sender in the range of about 330ohm-60ohm (don't remember the exact range) but that's close and it was nowhere near what the sender the customer brought me was. It was around 100-19ohm. So I could have spent time adapting that sender, only to have the temp gauge read wrong as well. It's not pretty, but I can post pictures of the brass fittings I used to mount the 56 sender in the 55 if anyone wants.
    Thanks,
    Jeremy
     
  2. olscrounger
    Joined: Feb 23, 2008
    Posts: 4,368

    olscrounger
    ALLIANCE MEMBER

    On the temp gauges for trifives and early corvettes. the new/repo senders all seem to be off. I find old ones at swap meets or ad resistors to get them correct-don't recall right off but knew the OHM reading at temp in my pocket on normal day-carried small OHM meter to check. -Have a few stashed away.
     
    REBEL43 likes this.
  3. REBEL43
    Joined: Feb 17, 2007
    Posts: 705

    REBEL43
    ALLIANCE MEMBER
    from TENNESSEE

    This one checked in spec so far, well, what I did was use my resistance box to find out what it wanted for Cold and Hot and just found a sender in that range. I found some numbers that are supposed to be good on the Stovebolt forum. My plan is to leave the aftermarket mechanical gauge in for right now to compare to the 56 electric and this sender. If it's good I'll pull the mechanical out and cap off the old hole. Actually I just remembered I had pictures on my phone, borderline OCD when it comes to numbers, haha. This sender was a BWD crossover from a Echlin TS6469. 100deg Cold at 368ohms-220deg @ 80ohms. 1/2 pipe with the button (nail head looking) terminal. The fuel sender should be in Friday and I'll run it and see. Got it plumbed in already. I'll let you know how it goes though.

    This is where I've put the electric sender in this old 235 for now. I read you can get the housing off of a 261 and it's got 2 tapped holes, but this guy is going to small block this car pretty soon anyway. So he said T it in.
    image1 (2).jpeg
     
    Last edited: Oct 28, 2020
  4. 325w
    Joined: Feb 18, 2008
    Posts: 5,652

    325w
    ALLIANCE MEMBER

    Used to add resistors tied together to get the temp gauges that read high back down as close to middle as possible. Add the total of resistance and then buy one resistor to put in the wire and shrink it over. Electirical guy told us it a gauge not an instrument. Gauge will be close.
     
    olscrounger likes this.

  5. blowby
    Joined: Dec 27, 2012
    Posts: 7,957

    blowby
    Member
    from Nicasio Ca

    Bump. My '56 Vette gauge isn't working. When it turn the key on it moves from below empty to empty, so it must have power. If I ground the other side terminal (brown wire going to tank), without disconnecting the brown wire, should it read full?
     
  6. REBEL43
    Joined: Feb 17, 2007
    Posts: 705

    REBEL43
    ALLIANCE MEMBER
    from TENNESSEE

    Is it just a 2 wire gauge? I wasn't sure if the Vette gauge was something different or not. Just saw this and hadn't looked for a diagram yet. Basically when I was working on the 55 Chevy, I grounded the gauge (cluster/gauge flange) and powered it up and with full ground to the sender post, it sat at E. When I moved the float on the sender and gave it 30 ohms of resistance to ground...it would move to full. So 0 ohms or straight ground at the sender post (12v at the power post of the gauge) was Empty and 30 ohms resistance to ground was Full. So if it works like this 55 gauge I was working on... It sounds like you're getting full power and ground to get you to empty and your float or sender is never giving any resistance to make it move toward Full. You could take a meter and check resistance to ground on your sender wire or at the sender and see what you've got.
     
  7. blowby
    Joined: Dec 27, 2012
    Posts: 7,957

    blowby
    Member
    from Nicasio Ca

    I just finished messing with it. My wiring diagram shows 3 wires I think, hard to tell. Anyway I disconnected the brown wire to the sending unit and it goes to full when I turn the key on, so I suspect either a grounded brown wire or the sending unit isn't working. Thanks for the reply. 20210719_155918(2).jpg
     
  8. REBEL43
    Joined: Feb 17, 2007
    Posts: 705

    REBEL43
    ALLIANCE MEMBER
    from TENNESSEE

    No problem, the float may have a hole in it and it's sitting on the bottom of the tank (showing empty). Looks like it is a 3 wire, but the 3rd wire is just a jumper wire to power the temp gauge. I think you're on the right track though, finding out why your sender wire is giving the gauge full ground.
    Thanks,
    Jeremy
     

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