In what ways are mechanical and electrical temperature gauges distinct, and how can you tell which one is more accurate?

A while ago, in a post about radiator troubleshooting, we briefly touched on the distinction between mechanical and electrical coolant temperature gauges.

But since this is an important topic, we'll spend a little more time on it, and we also figured it would be useful to demonstrate how to check the accuracy of both electronic and mechanical coolant temperature gauges. In that case, let's start at the beginning.

Methods of Operation for a Mechanical Water Temperature Gauge

This Summit Racing gauge kit has been used in multiple vehicles and is still functioning perfectly. (Image/OnAllCylinders)

To begin, a mechanical water temperature gauge is actually a pressure gauge because it measures variations in pressure caused by a gas's response to changes in temperature. Built on a concept known as a Bourdon Gauge, but we'll spare you the technical details and get right to the point.

If you take a look at the back of a mechanical temperature gauge, you will probably find a sealed tube that is attached there permanently. There's a gas or fluid inside that tube, and as the temperature inside the tube rises and falls, the gas or fluid expands and contracts. The probe threads into your engine's water jacket or other cooling passage, and the tube connects to the gauge.

The gas reacts as the engine heats and cools, and the gauge's internal mechanism reads the pressure changes to display the engine's temperature.

Benefits and Drawbacks of a Mechanical Coolant Temperature Gauge

In particular, take note of the black capillary tube that is permanently fastened to the back, and the metallic probe that threads into your engine. (Image/OnAllCylinders)

A mechanical gauge's main benefit is that it doesn't need batteries or electricity (aside from a backlight, of course, for nighttime viewing). Which means you can forget about making sure the gauge is connected to the right sending unit.

And because it doesn't need power, it will continue to provide a precise reading even when the key is taken out of the ignition; this is useful if you want to monitor the rate of heat loss when the engine is turned off.

To install a mechanical temperature gauge, simply thread the probe through the instrument panel and into the engine compartment, where it can be secured in a coolant port.

However, a mechanical temperature gauge's installation is often cited as a significant disadvantage. The capillary tube must be expertly directed through the engine and dashboard, with no sharp turns or kinks. If the gauge's tube is damaged in any way, it's usually more cost-effective to replace it than to attempt repair.

An additional issue that may arise with a mechanical gauge is its probe. As a result of its larger size, it may not be possible to plug it into a life vest's electrical sending unit. Therefore, many people prefer to install the probe into a manifold or water neck, which provides additional depth for the probe to reach the coolant. (Image/OnAllCylinders)

The ambient temperature can affect the gauge reading, so you should keep the tube away from sources of heat like the blazing hot header tubes. However, the length of the tube is fixed, so if the gauge and the location where you want to install the probe are not within its range, you will need to either replace the gauge or relocate the probe.

A guide to the inner workings of a digital water thermometer

This is a common temperature gauge sending unit used with electric coolants. Though they have different appearances, most perform similarly. The left-hand threaded section serves as the wire connection to the gauge, the center section threads into the engine, and the right-hand cylindrical section projects into the coolant passage. (Image/OnAllCylinders)

The sending unit and the gauge are the two main components of an electrical water temperature gauge, and they are linked by a cable. Using the image above as a guide, thread the sending unit into the engine's water jacket or other coolant passage, and connect the top terminal to the gauge inside the vehicle using the corresponding hookup wire.

A thermally-sensitive electrical resistor (or "thermistor" for short) is housed inside the transmitter. Basically, the thermistor's electrical resistance shifts as the temperature does around the sending unit. The temperature is determined by the gauge's interpretation of the varying resistance values.

Advantages and Disadvantages of an Electronic Coolant Temperature Gauge

In our test, the black clip at the sensor's threaded location in the engine serves as chassis/common ground, and the red clip would normally connect to the back of the gauge. (Image/OnAllCylinders)

The primary benefit of an electrical coolant temperature gauge is that it eliminates the primary drawback of a mechanical gauge's capillary tube. This is because the wire can be routed around corners, through tight spaces, and even near other heat sources. (Original Equipment Manufacturers are fond of electric gauges because they simplify the car design process. )

The flip side, however, is that there are a few drawbacks to think about. Your electrical connections must be free of debris and in good working order for the gauge to function properly. The gauge readings may be affected by corrosion, buildup, or loose wires, and you might not even notice it.

Because the sending units frequently use a chassis/engine ground, inconsistencies in readings can occur if there is poor grounding or continuity between the engine and the gauge.

Gauges and sending units aren't always interchangeable, so getting the wrong one could result in erroneous readings or even the gauge not functioning at all.

Mechanical Coolant Temperature Gauge: How to Perform a Test

The probe is first placed in a water-filled coffee can on top of a stove. To keep the capillary tube away from the gas flame, we propped up the tea kettle on a spoon. (Image/OnAllCylinders)

Once you've removed the mechanical temperature gauge from the vehicle, testing it is a simple task. The temperature gauge's probe is placed in a pot of hot water on the stove, and the rest of the capillary tube is placed safely away from the burner.

A boil in the water occurs at about 212 degrees Fahrenheit. (Image/OnAllCylinders)

When you turn on the heat, the water temperature should cause the needle on your gauge to move. takes off Water is considered to be boiling when the thermometer reaches a temperature of 212 degrees Fahrenheit. To be sure of the temperature, you could also submerge a second, trusted thermometer and compare the two readings as the water warms and cools. change

If there is a difference, or if the gauge doesn't read at all, it's probably broken.

In order to check for binding in the gauge movement, we lowered some ice into the boiling water and watched the needle's descent. (Image/OnAllCylinders)

Tutorial on Checking the Accuracy of a Temperature Sending Unit for Electrical Coolants

While it may seem daunting at first, testing the sending unit of an electrical water temperature gauge is actually quite simple for those who are familiar with electricity.

For the sake of this article, we will only be testing the sending unit, and even then, we will be doing so at will (you'll see what we mean in a moment).

We rigged up a holder for the sending unit of the water temperature gauge to keep it safely in the watering can. Keep the two ends of the clip leads apart and out of the water. (Image/OnAllCylinders)

Because sending units can vary depending on the gauge manufacturer (as alluded to above), it's hard to attribute a precise resistance reading to a known temperature. For this purpose, we will merely demonstrate how to take resistance readings, after which you can evaluate your findings in light of the gauge manufacturer's specifications.

It's possible to find a website that has already done the research for you in comparing resistance and temperature.

With the thermistor in the sending unit clipped to our resistance-reading multimeter, we observe a reading of about 214 ohms at about room temperature. (Image/OnAllCylinders)

Take resistance readings (in ohms) with a high-quality multimeter for this test. An additional thermometer will be useful for keeping track of resistance readings at various temperatures and comparing them to the manufacturer's specifications.

You can test the sending unit by attaching test leads to it as shown in the image above (or to its individual terminals, if your unit is different from the standard design). Please keep them from getting wet. Put some water in a pot on the stove, clip your leads to your multimeter probes, and place the sending unit in a jig (we used one of those "helping hands" parts holders).

Almost instantly after the gas was turned on, the resistance dropped. As the water reached a boil, the resistance dropped to well below 30 ohms. (Image/OnAllCylinders)

Similar to before, increasing the temperature of the water should cause the resistance to decrease. rise Again, these values won't mean much without the manufacturer specifications for your particular gauge (for example, 30 ohms = 200 degrees F). If that's the case, you should keep track of the temperature and the corresponding resistance readings in a chart.

This test can demonstrate, even without knowledge of the manufacturer's specifications, that the resistance readings are fluctuating and that the thermistor inside the sending unit has not completely failed electrically.

This highlights the significance of using the proper sending unit for the electrical gauge.

A Coolant Temperature Gauge: Mechanical or Electronic?

The correct response is that it is context-dependent. With proper maintenance, both designs have demonstrated precision and dependability. It might be simpler to install a mechanical gauge if there's enough room, but an electrical one might be necessary if the cabin is too small.

Is it either/or, as the meme suggests, or both?

Mechanical water temperature readings are displayed on a separate gauge in a custom instrument cluster. Be aware of the temperature gauge located in the stock Jeep CJ's big speedometer bezel. The two gauges are linked, so we can quickly check the stock C-H gauge and then check the mechanical gauge for more accurate readings if necessary. (Image/OnAllCylinders)

We were able to keep the Jeep CJ's original electrical gauge hooked up for convenience and added a second gauge in a second panel for added safety and security. We simply adapted a later model 4 cylinder engine to its 258 inline six. The stock sending unit threads into the back of the cylinder head, as usual, but we fabricated a water neck for our 0L engine that included a port for our mechanical gauge.

If you're handy, you could add a similar setup by tapping a hole for the additional probe, but a sender fitting adapter in the radiator hose might be a simpler option.

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