67 Wacky Water Temp Gauge

[Bill Becker]

My water temp gauge decided to go south just a few days from heading to Hershey.
When I turned on the ignition (engine cold), the needle s**** way past 250 to about the 4:00 position and stayed there.

Gauge and sending unit were synched not too long ago by a well known shop (can't reach).

Here is what I know:

Tested the sending unit at about 80 degrees - reads 600 ohms
Have continuity in the wire back at the terminal at the clip, same reading.
When connecting the two terminals at the clip, still reads 600 ohms
When ignition is turned on - no resistance. Shows 12.6V

Start motor and warm up, still no resistance, shows 13.6V
When ignition is off, then you can watch the resistance go down (250 and below)

While ignition is off, needle will move slightly to about 3:00, but goes back to about 4:00 when key is on.

Won't pass ops in this condition, any suggestions?
Thanks,
Bill Becker

[Dan Tewell]

Re: 67 Wacky Water Temp Gauge

Bill, This may sound strange but here is how I fix mine when it does this. First take the lead off the sending unit, then turn the ignition switch to the on position. This should force the needle to the far left (cold) position. Now turn the ignition off and reconnect the lead to the sending unit.It should work properly now. I don't know what causes it do do this, but this seems make it work again.
Dan

[Bill Becker]

Wishful thinking

Sorry Dan,

Didn't do it.

Thanks,
Bill

[Ed Jennings]

Re: 67 Wacky Water Temp Gauge

From what you describe, it looks like your gauge may have died.

[John H.]

Re: 67 Wacky Water Temp Gauge

Bill -

I'm no electrician, but it sounds like the thermistor unit in the sender has gone south (has resistance with no voltage on it, but goes to zero when power is applied). Maybe Jack Humphrey will chime in - he knows the innards of those things better than I do.

[Jack H.]

Chime....

Temp gauge saw dead short to ground to drive it off-scale/hot. Situation will continue until you find the source of the fault (most likely nicked insulation on the sender lead wire).

The temp gauge has a pretty cheap mechanism -- internal movement is NOT a high-$$$ jeweled bearing setup. You can PEG 'em so hard they get stuck at one end of the scale. Plus, needle bearing can 'swing' out of its dimple mount. If this has happened, the odds to recover opertion in-dash are slim.

What can work (50-50 shot), is to gently tap on the back of the instrument cluster when the engine is cold (or FORCE a cold reading by substituting a 500-600 ohm, 1/2-watt, resistor for the temp sender to ground) and see if the ammeter's needle bearing will naturally free itself and swing back to COLD....

[Bill Becker]

Hi Jack...

Don't say that in an airport!

Is there supposed to be resistance with the ignition on? When ignition is on the meter goes to zero. Gauge can't work with no resistance - right?

I gotta believe the problem is not in the sending unit. With leads connected to the gauge end of the wires, you can actually watch the resistance go down as the engine warms up after the key is off.

Bill

[Bill Becker]

Drinks are on me...

Tried bypassing the sending unit lead wire, no change
Tried a new lead wire with spliced in 600 ohm resistors, no change
Tried a borrowed temp gauge out of a '64 cluster, just got more confused
Tried getting down on my knees and praying, no change

Finally, thinking I was going to have to replace the gauge, I took a 3/8, 6" ratchet extension and tapped the backside as per your suggestion, - Bingo! The needle popped back to 100 degrees.

Jack, I'm buying you the coldest, tallest beer I can find in Hershey.

Thanks for the tip,
Bill

[Jack H.]

Re: Hi Jack...

VOM (volt-ohm meter) measures resistance by using a clamped/controlled voltage source (battery you install in the meter) and reading the amount of current the meter itself expends into the load under test (meter then translates its own outward current flow into equivalent DC impedance or resistance).

I don't think there's a meter made who's operator's manual doesn't tell you NOT to attempt to measure resistance in an active circuit! Current already flowing in the circuit under test is going to, at best, give erroneous readings because the meter will 'think' all the current it sees flowing is coming from it's internal battery!!!!! At worst, you're in a race contest between the meter's internal protective fuse blowing (meter is essesntially DEAD now that it's been abused) and things melting down in your hands...potentially lethal!

The logic that says you can measure the impedance of your temp sender with a VOM set to resistance mode while the circuit is active can be scaled.... Hey, let's measure the DC impedance of the local power generating station and the wiring from it to your home!!!!!

We'll simply switch the meter to resistance mode and JAM the test probes into the AC wall outlet in your home.... I guarantee you such abuse of of an electrical test tool 'could' result in immediate CATASTROPHY--do NOT attempt such a foolish move!!!!!!!!!!!!!!!!! That's akin to getting a feel for brake horsepower by reving your BB up to speed and grabbing the flywheel with your bare hands........

If you want to measure the DC impedance characteristic of your temp sender while it's in-circuit/actively powered, do it the right way. Set the VOM to voltage mode and look across the temp sender to see how much voltage is being dropped there. Now, flip the VOM to DC current mode, and connect it in series between the wiring harness and the temp sender terminal contact to measure how much current is flowing through it.

Then, using Ohm's law (V=IR) work backwards (R=V/R) and compute the equivalent resistance. And, yes, some devices WILL exhibit different resistance values based on how much current is actually flowing through them (heating effect) because they're not really linear devices. You can measure one resistance value given a static test with a VOM versus another value with the device under actual dynamic load (the second method mentioned). But, if the non-linearity characteistics of the device are small/negligible, the difference between static and dynamic measurement techniques will also be small/negligible.

[Bill Becker]

Re: Hi Jack...

OK, TWO beers.

:-)

[Jack H.]

Save the brew....

there's always the 2004 Regional in Breckenridge, CO to comtemplate.... BTW, that's EXACTLY what this forum was intended to be -> folks helping folks (not selling parts, shooting the breeze about politics, Etc.). If a suggestion I made helped 'ya; hey even a blind squirrel finds nuts now and then!