'68 Riv Temp Gauge Inop

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.

 

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.

 

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.

 

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.

 

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...)

 

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...?

 

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?

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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...