Temp Gauge Testing

[azblackhemi]

Any way to accurately bench test this gauge?

 

[Big_John]

Depending on the year, 5 volt power supply with a decade resistor box is the ultimate answer, but to see if it works, a 5 volt source and then ground the "s" terminal will do it.

The 5 volt power supply can be sourced from an old desktop computer. Look on YouTube for how to make one. Taking it further, the decade box is a little specialized, but I found this a while back and it looks pretty good. Under $30!

Gauge Tester

 

[Vaanth]

Any way to accurately bench test this gauge?
View attachment 744588View attachment 744589

The gauge can be bench tested with a 5 volt DC power source and select resistance values or the Miller C-3826 gauge tester.

Set up a circuit as shown in the following diagram:

To check cardinal points of the gauge, do one of the following:

Using resistors

Point resistances can be made using fixed resistors or a combination thereof, variable resistors, or decade resistance boxes.

To check points of the gauge, use the following resistance values:

Chrysler Specification:
Cold = 72 Ω
Middle = 22 Ω
Hot = 9 Ω

Miller Special Tool Specification (used by Chrysler for testing):
Cold = 75 Ω +/-5% -> 71.2 - 75.8 Ω
Middle = 22 Ω +/-5% -> 20.9 - 23.1 Ω
Hot = 10 Ω +/-5% -> 9.5 - 10.5 Ω

Gauge position tolerance at these points is listed in the service manual, but in general, observe the following:

1. With the 9 Ω resistor, observe "Hot or H" plus or minus 3/32" on the gauge.
2. With the 22 Ω resistor, observe the gauge to slowly advance to ½ position.
3. With the 72 Ω resistor, observe "Cold or C" plus or minus 3/32" on the gauge.

Using the Miller C-3826 tester:

The Miller C-3826 tester can be used to test these points. If using the Miller tester, substitute it in place of the resistor on the diagram.

Gauge position tolerance at the test points is listed in the service manual, but in general, observe the following:

1. With the gauge tester on "H,", observe "Hot or H" plus or minus 3/32" on the gauge.
2. With the gauge tester on "M", observe the gauge to slowly advance to ½ position.
3. With the gauge tester on "L", observe "Cold or C" plus or minus 3/32" on the gauge.

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Note regarding test lead resistance when measuring low resistance values:

Minor test lead resistance is not much of a concern at higher resistances, but when measuring low resistances, such as the ground to ground continuity, low resistance of sending units, or resistors, account for the resistance of the test leads connected to the measuring device. This can be done by shorting the leads together, noting the resistance, and subtracting it from the measurement readings, or use the meter's relative setting (if equipped) similarly with the lead shorting.

Some meters have a function to measure conductance and convert accordingly for low resistance values. Others, typically lab or some bench type meters can use a four wire ohms measurement to overcome any test lead effects.

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[Boydsdodge]

I used 3 AA batteries inline with a couple of test leads

 

[72RoadRunnerGTX]

Speaking of the Miller C-3826

Building a Miller C-3826 clone is fairly easy.

 

[Gerald Morris]

If you seek a variable resistor of the same range, get a fuel sender, wire it to this gauge, and run the float up and down. The resistances will be the same as your temp gauge, so you should see the needle deflect over the same values. After all, those fuel senders were just crude pots. 4 Rechargeable NiCD C or AA cells will get you 4.8 VDC, when charged. A USB power supply will give you 5 VDC, but get one with some decent ampacity, lest you burn it out on that paleolithic sort of load!

 

[HWYCRZR]

My home built tester. A pot from Amazon and an old fiberglass electrical box.
Use with a 5 volt power supply.
10 Ohm High or full. 23 Ohms Med or 1/2 full. 74 Ohms Low or empty.

 

[CBODY67]

When I first saw that Miller Tools "factory tool", I was amazed that it had that much variability as to the needle position on the gauge scale. Such variability can display the amount of variance in production items, though. Still, I consider the gauges to be generally accurate. An IR heat gun can determine that for the temp gauge, but even the IR heat guns have their own production tolerances.

In other forums, I have seen posts about "Is my car overheating? The temp gauge readds ____."

In reality . . . when the fuel gauge gets to 1/2 and 1/4 full, just refill the tank and see how much it takes. The IR heat gun can read coolant temperatures at the radiator inlet hose or thermostat housing. IF the engine knocks, not enough oil, but notice where the gauge runs at 55mph or so and then consult the FSM for specs. Seems that the middle mark in the temp gauge is approximately the opening of the thermostat used OEM, which can mean about 200*F. Get comfortable with the readings on YOUR car, though. No special tools really needed.

Just some thoughts and observations,
CBODY67

 

[Gerald Morris]

When I first saw that Miller Tools "factory tool", I was amazed that it had that much variability as to the needle position on the gauge scale. Such variability can display the amount of variance in production items, though. Still, I consider the gauges to be generally accurate. An IR heat gun can determine that for the temp gauge, but even the IR heat guns have their own production tolerances.

Miller made some cool specialty tools, for sure!

In other forums, I have seen posts about "Is my car overheating? The temp gauge readds ____."

In reality . . . when the fuel gauge gets to 1/2 and 1/4 full, just refill the tank and see how much it takes.

Soooo right! The gauge and sender combo in Gertrude right now still reads close enough to be useful. When full, the gauge pegs to the far right, burying the needle. Half volume shows about 5/8 full on the gauge. "E" means, "get gas NOW, as I'm sucking dust!" Mind you, I try to avoid testing "E"....

The IR heat gun can read coolant temperatures at the radiator inlet hose or thermostat housing. IF the engine knocks, not enough oil, but notice where the gauge runs at 55mph or so and then consult the FSM for specs.

All Gertie came with was her idiot lights, which work tolerably well too, BUT, I obsess on numbers, so I run my little gauge cluster. I still use that old Stewart Warner oil gauge, which I got for that engine 9.5 yrs ago. Oil pressure has to be < 4 psi before the idjit light comes on. I make it a point NOT to test that case often! Once per decade is fine for me. Aside from the ONE GOOD capillary tube temperature gauge, made by Clark Bros. USA, stupidly CUT by a ham fisted Chebbie Chimp with a State Certificate, I've found Autogauge to be a distant second for quality capillary tube temp gauges, and am using one now. Modern Stewart Warner has twice proven to be short-lived, overpriced TRASH, Hecho in Mexico. Both of those failed in just a few months, and at that I swore "Never Again." IF/When the Autogauge unit fails, I now have at LONG last, some good Clark Brothers replacements.

Seems that the middle mark in the temp gauge is approximately the opening of the thermostat used OEM, which can mean about 200*F.

I've yet to get an Old Mopar with a factory gauge. I WANT TO!

Get comfortable with the readings on YOUR car, though. No special tools really needed.

Just a working brain w a few pre-frontal lobes still firing..... Sage Advice.

 

[azblackhemi]

Thanks for all the info and technical advise. It's painfully obvious that I spent way to much time learning body & paint and nowhere near enough time on electrical!

 

[Gerald Morris]

Thanks for all the info and technical advise. It's painfully obvious that I spent way to much time learning body & paint and nowhere near enough time on electrical!

Don't belittle yourself. Paint & body SELLS CARS, while electrical keeps customers happy w what they bought. I need to pick your brain a bit on the matter of a bent hood. Nasty crease near the front edge, causing the sheet metal to bend down near 90 degrees on the driver side. UGLY, though the hood still functions well enough.