'68 Riv Temp Gauge Inop

Discussion in 'The Bench' started by Gary Anderson, Dec 1, 2019.

  1. Gary Anderson

    Gary Anderson Well-Known Member

    There's so much wrong and missing from this car I really don't know where to begin to find the source of this problem. The coolant temp gauge rests at around 1/3 all the time - doesn't seem to move. Common issue? Easily fixed? All fuses are good, AFAIK; some gauges have lights, other's don't. Speedo works, clock is dead, fuel gauge works, gen and oil lights seem to work properly.

    Provided that the temp sending unit is good, how would I go about checking the gauge? If I decide to slit my wrists and pull the dash panel out, what do I do first? If the gauge is dead, is it dead forever?

    Engine notes: 430 has been rebuilt using Edelbrock intake and carb, and they splurged on some chrome valve covers and accessory pulleys so I tend to think they didn't go cheap on the rebuild. I think they installed a HP cam because it's a little "lopey" but can't tell for sure yet, and no way to check at this point. I don't even know if they put a thermostat in the water neck. TVS has no vacuum lines going to or from it. Heater valve in the rear of the intake has a single vacuum port on it, but the entire heating hose system has been disconnected and blocked off.

    Worst case scenario, I just buy an aftermarket temp gauge and sending unit, but I'd rather not.
  2. Briz

    Briz Platinum Level Contributor

    Thought the gauge on my 68 was bad or mis-reading also. sits around 1/4 - 1/3 all the time when at operating temp. Installed a mechanical gauge and it shows 190* . Changed the sending unit and there was no change. When the Riv got hot the mech Gauge said 225 and the dash said 1/2. I just accept the 1/4 - 1/3 as normal. Its all about learning the norm for your car. Oh, and you will not get to the back of that cluster from under the dash.
    Gary Anderson likes this.
  3. Steve Craig

    Steve Craig Gold Level Contributor

    Disconnect the wire at the sender, start the car. Put a 12V DC tester across the output of the sender & the manifold or any other suitable ground. As the temp increases you should see a change in voltage from the sender. This will indicate whether the sender is working or not.
    Chassis Manual will tell you which bulbs go to the individual instruments. My '71 has 2-3-4 different styles, depending on which instrument. Wording in the Chassis Manualor Dealer Service Bulletins stated the brightest area was right in the middle for the speedo.
    Gary Anderson likes this.
  4. Steve Craig

    Steve Craig Gold Level Contributor

    On second thought I've got it wrong!
    Believe the gauge is wired in series with a + 12V source going to the sender & then to ground(-). Pull the wire & you should see 12V measured to ground.
    The resistance will change in the sender as temperature changes, allowing more or less current to flow. The change in current flow will affect the deflection you see on the gauge. A high resistance at cold will restrict current flow & a lower resistance at warm/warmer will allow more current flow.
    Put your tester in the OHMS position & it will change as the temp. changes.
  5. Gary Anderson

    Gary Anderson Well-Known Member

    Thanks, Steve. I'll give it a try when I get the DS exhaust manifold back from the machine shop and back on the car. (Still learning to use my multi-meter properly...)
  6. Gary Anderson

    Gary Anderson Well-Known Member

    Can you give me some kind of linear explanation of this? 12V goes from the gauge directly to the sender (which is grounded to the engine)...?
    OR, is the sender a dual-prong, and 12V goes to one prong and then the other prong goes to the gauge...?
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I would think 12 volts goes to the gauge and one wire goes to the sender. The sender varies the resistance according to temperature with short to ground being one end of the sender needle travel.
    1973gs and Gary Anderson like this.
  8. Gary Anderson

    Gary Anderson Well-Known Member

    I haven't yet looked at the sender in the engine, but based on a few quick searches, this is the correct sender for a gauge: single pin on top. Does this give any of you guys a better idea?

  9. Gary Anderson

    Gary Anderson Well-Known Member

    Here's a snap of the wiring schematic from the Chassis Service Manual. On the gauge icon, "C" is a dk green wire that goes directly to the sending unit (which is then grounded to the block). "H" is a brown wire circuit that is connected to other gauges (including the FULL side of the fuel gauge, which is working), and then to the voltage regulator, so I suspect it's the 12V circuit. I think the unmarked pin is for the light bulb.

  10. Steve Craig

    Steve Craig Gold Level Contributor

    Need to think in this, without a manual I'm guessing. I do know the sending unit will vary the resistance in the circuit. Just not sure how the instrument is wired internally.
    Gary Anderson likes this.
  11. buick64203

    buick64203 Just plum crazy Staff Member

    Cant you just ground the sender wire and see if the gauge goes to cold? With it disconnected it should go to full hot
  12. Gary Anderson

    Gary Anderson Well-Known Member

    Yes, I'll try this tomorrow. So far, the gauge hasn't moved at all while driving - neither 'cold' nor 'hot', but 1/3 away from 'cold' and just sitting in that position even with the engine cold and key on. I'll see if grounding the wire makes the needle jump at all. Thanks for the suggestion.
  13. Gary Anderson

    Gary Anderson Well-Known Member

    No luck with grounding the signal wire @ the manifold. The sender shows it's making good ground, but I should be getting voltage in the sending wire from the gauge, right? (Yes, all tests with key on.) Currently I'm getting zero volts from the wire - so something seems shorted out between the gauge and what's exposed of the wire out of the front harness.
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Why are you checking for voltage in the sender wire? Hook up an OHMMETER between the sender and ground. look for a change in resistance as the temperature changes.
    Gary Anderson likes this.
  15. Gary Anderson

    Gary Anderson Well-Known Member

    Isn't the gauge/sender wire supposed to be sending 12V to the sending unit? That's what the schematic shows.

    I'll try the ohmmeter tomorrow.
  16. telriv

    telriv Well-Known Member

    There is NOT any voltage at the green wire attached to the sending unit. It works via resistance supplied by the sending unit. I would use a jumper wire from the green wire to the negative side of the battery to make ABSOLUTELY sure you have a good ground. The temp. gauge should react one way or the other. Full hot or full cold. It could be something as easy as when someone installed a new temp. sensor they used Teflon tape which could be causing a bad ground OR the wrong sending unit.
    IF you get no movement on the gauge grounded the problem is between the green wire & the inside of the car. This could be a bad connection at the bulkhead connector harness from corrosion which connects the outside wiring to the inside wiring. Could even be a bad temp. gauge.
    Do the easy stuff 1st. then it's just a matter of trouble shooting. One step at a time.
    AGAIN ground the green wire to the negative side of the battery.
  17. Gary Anderson

    Gary Anderson Well-Known Member

    Okay, so...

    I put a single ground lead from battery (-) to the green wire unconnected from the sender. The gauge moved to halfway between middle and 'hot'. Tried different wire connection configurations (wire plugged onto sender, not plugged on, with and without direct ground, etc) and got pretty much the same response. The needle never went 100% to 'hot' or 'cold'.

    But here's something weird: with the car running, headlights on (and all dash lights working except the one in the temp gauge), turning the dimmer knob moves the needle in the temp gauge. First of all, the dimmer only works on-and-off (not gradually) but when I turned it to 'brighter' (counter-clockwise) and the dash lights came on, the temp gauge needle moved toward 'hot' as well. Turned the dimmer down/off and the temp needle went back down to just past center. Seems I have a dash grounding issue, and maybe a sender issue as well.

    And finally, as the engine warmed up, it had no effect on the temp gauge at all. The only thing that moved the temp needle was turning the lights on and off with the dimmer set to 'bright'.

    And yeah, I don't know why I was thinking the green wire would carry 12V; that would close the circuit and melt some things...

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