The temperature gauge in my 69 Barracuda doesn’t work, as the Queen once said recollections vary about whether it ever did. (I’ve had the car about 7 months).
I ordered a replacement from Rockauto, I popped out the old one and grounded the lead to the block to check that the gauge was responding which it did - needle swung up pretty fast and slowly returned to rest. So I went ahead and fitted the new sender, the coolant was up to the top of the hole so I’m sure it’s sat in the coolant, but still no joy at the gauge.
So not sure what’s happening here, the lead to the gauge and the gauge and voltage regulator would appear good based on testing it to ground? I guess I could just be unlucky and have a bad sender out of the box, but the one that came out looked pretty new too so I’m wondering if there is another issue?
OK so this is a bit left field but here goes.
Is the sender unit a resistor whose impedance is changed by temperature? If so is it correct to assume that the circuit runs from gauge to coolant to block to ground? In other words the conductivity of the coolant grounds the sender? I only ask because Evans waterless coolant is propylene glycol which is non conductive?
I know it’s a bit of a stretch but I’m almost positive that the gauge worked until I changed over to waterless.
The electrical conductivity of the coolant is not involved with the type of sending unit with which you are dealing. The thermal conductivity of the coolant, and its surface tension, can affect response time of the sending unit to a negligible degree. The coolant itself is not part of the circuit path, which flows as shown in the following:
Battery Positive (+) --> Voltage Limiter --> Gauge --> Sending Unit --> Engine Manifold/block --> Battery Negative (-)
The temperature sending unit in question is a negative temperature coefficient resistor. The impedance of the unit is essentially and wholly a direct current function of resistance only.
Replacement components can be bad, and modern units have a much worse record than older aftermarket and original units. For testing the sending unit in operation, the following cardinal points and resistance values are of interest:
- Ambient temperature at about 25 degrees Celsius, before running engine, gauge needle at lowest point, sending unit resistance = 360 Ω
- Engine at ~49 degrees Celsius - Needle moves to Cold mark on the temperature gauge, sending unit resistance = 72 Ω
- Engine at ~88 degrees Celsius - Needle moves to middle on the temperature gauge, sending unit resistance = 22 Ω (approximate range of ~18 to ~26 Ω)
- Engine at ~110 degrees Celsius - Needle moves to Hot mark on the temperature gauge, sending unit resistance = 9 Ω
Note that some gauges have intermediate marks above Cold and before Hot and others have actual numeric markings from 120 to 170 to 230 to 250 degrees Fahrenheit.
Also note that Chrysler revised the actual numeric equivalence to the hot and cold marks a couple of times, but the resistance values of the sending units and their correspondence to the positions on the gauges themselves are essentially the same with a slight variance for peak temperature. This correspondence is true for the temperature sending units as well as those for fuel level and oil pressure which operate with most of the King-Seeley Constant Voltage type gauges Chrysler used from about 1960 through 1989 on domestic rear wheel drive vehicles.
When checking the resistance of the temperature sending unit, probe the unit terminal post and body to check the unit itself. Probe the terminal post to manifold to check body to ground, and further check terminal post to the battery negative (-) terminal for a more end to end post to ground test. These tests should show the same effective resistance. Direct continuity between the body and the points can also be tested directly by probing between the unit body and the ground points. The resistance should be effectively zero.
You tested the wiring to gauge set up by shorting the terminal lead to ground and watching the gauge response. This confirms the circuit is operational, and although not a fully thorough check, usually sufficient. Other functional checks can be made by using a voltmeter between the sending unit post (connected) and ground or a test light between the sending unit post and wiring terminal. With the key on, watch for cycling voltage on the voltmeter (an analog meter is better in this case), or for the flashing of the test light as the voltage limiter cycles. As mentioned, a quick check of the sending unit as it warms can be performed with the test light leads between the unit post and +12 volts at the battery or other point and watch for the intensity to increase as the resistance decreases as the engine warms.
A quick test lamp can be made from an 1156 bulb, a couple of wires, and a couple of clips. For testing between the sending unit and sender wire, a 6 volt bulb, such as 1129, better matches for the voltage limiter output. Test lights, are quick and simple tools, but are more qualitative than quantitative. You have a meter which can do the quantitative checks as well.
Although it sounds like your gauge and associated wiring is okay, if desired to check cardinal points of the gauges, use the following resistance values:
Chrysler Specification:
- Cold = 72 Ω
- Middle = 22 Ω
- Hot = 9 Ω
Gauge position tolerance at these points is listed in the service manual.
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
The Miller C-3826 tester can be used to test these points, or the point resistances can be made using fixed resistors or a combination thereof, variable resistors, decade resistance boxes.
