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Water Temp Gauge Reads High

Sometimes there comes a time when you want to check the accuracy of your temp gauge, or maybe you simply do not believe the temperature that is being displayed. We are going to start off with one of today’s biggest misunderstandings when it comes to checking water temperature…. Comparing to the ever-growing, popular temperature gun (laser pointer, infrared gun, etc.). These are quite the handy little devises, that work very well for checking the surface temperature of nearly anything, ranging from your header pipe, to your favorite cold drink. It can also be used to compare a temperature reading with, with your gauge, HOWEVER….. This is to be used as a basic comparison, not as an exact. Here is why: Your temperature gun measures surface temperature, and has no magical ability to measure the coolant temp through the aluminum, or cast iron component. Surface temp will always be cooler than the coolant inside the engine, or otherwise known as engine “core temp”. The surface area of what you are measuring has a job to do, which is to dissipate heat. A typical temperature difference would be between 5 and 25 degrees, between the gauge in question and the temperature gun. This depends on the efficiency of the component being shot with the temperature gun to dissipate heat. So, if your gauge reads 195 and you point & shoot and get a reading of 185, you can safely assume that you are seeing a reasonable, and realistic comparison. So, when is it a good idea to trust the temperature gun? If you suspect the gauge of reading low, then that changes everything. Here is an example: If the gauge is reading 160, but you suspect the engine is running hotter, you may take temperature samples with your temperature gun from the surface of the thermostat housing, intake manifold (where a water jacket is present), and sender location. If you get a reading of 195 with your temperature gun, while the gauge reads 160, then knowing now that the surface temperature is lower than core temp, you can safely assume that the gauge is indeed wrong. Your core temp may be as much as 220, or as low as 200, but still much more than what the gauge shows. Keep in mind, that if we, or any other manufacturer, as well as the OEM’s were able to record surface temp to give you an accurate temperature reading, we would all save a lot of money, and make installing a gauge much less messy, and easier to do. There would be no potential coolant leaks, or loss, and far less risk of burning yourself while working on a hot engine. No holes to drill, and no plugged holes to locate, and no adapters needed. Unfortunately, we want to know core temp, so that is what we measure and trust for our engine management systems, and for our gauges to know how hot the engine is. So, to re-cap, it is ok to use a temperature gum to make a basic comparison, as long as you understand, that when everything is working properly, the gauge will likely always read higher since the gauge measures core temp, and the temperature gun measures surface temp.

Checking the accuracy of a mechanical temp gauge: A mechanical temperature gauge will have a thick black tube attached to the rear of the gauge with a sensing probe on the other end. This is called a capillary tube. It is charged with a flammable, ether gas, therefore you do NOT want to test the gauges sensing probe with a flame.

The proper method to test one of these gauges is in a pot of hot water. Set the sensing probe fully into the water, and monitor the water temperature with the gauge and also with a known good & accurate thermometer. Of course use extreme caution when dealing with any hot fluids. Also, knowing that water boils at 212 degrees F (at sea level) may give you an indicator as to the accuracy of the gauge in question.

Mechanical temperature gauges do not utilize a replaceable sender, and are typically not repairable. Therefore, do not attempt to remove the capillary tube or sensing bulb for any reason.

Checking the accuracy of a short sweep temperature gauge (one that uses a single terminal sender): This gauge typically has 3 terminals on the rear of the gauge (not including the light connections). It sends a signal out to the sender from the “S” terminal, and it is the senders job to vary this signals resistance to ground. With this said, for a sender and gauge to operate, then sender must be threaded into something that is grounded since the ground path for the sender is actually the sender mounting threads. The gauge itself really is nothing more than a fine tuned & calibrated ohm meter. To check the gauge, you will need a controlled and measurable resistance to ground source. Many shops that deal with automotive electrical repair have what is called a gauge tester, which is really a switchboard with different resistance values. The Auto Meter 100 to 250 degree, short sweep temperature gauge uses a specification of 1123 ohms of resistance to ground to = 100 degrees F, and 65 ohms of resistance to ground to = 250 degrees F. You may contact our Service Department for further specs.

If you do not have access to a gauge tester, you may always start with the sender. To check the sender, warm the engine up to a stable temperature. Have an ohm meter ready, as well as the tool(s) needed to remove the wire off of the sender. With the engine running/idling, notice what the gauge is reading, and write this down the reading. With the engine still idling, remove the wire off of the sender, then hook the positive wire of your ohm meter to the sender where the sender wire was. Now hook the negative wire of your ohm meter to ground. What resistance do you measure?

I will give you some examples to help you determine what to do next.

Let’s say that the gauge read 210 degrees, and then you took a measurement of 123 ohms. This would mean that the gauge was reading exactly as it should. And if you thought the engine was running hotter than that, or colder than that, either the sender is the problem (remember the gauge was reading perfectly based on the information from the sender), or the engine really is running that temperature.

Another example for you: You suspect the gauge is reading hotter than actual, and you measure the resistance of the sender again. You measure 253 ohms, and the gauge reads 210 degrees. In this case, the sender is at 170 degrees (based on the resistance measurement), and the gauge is not responding the way it should. In this case, either the ground to the gauge itself is insufficient (which will cause a too high reading), or the gauge is out of calibration.

Never trust how a ground looks. If your gauge shows higher than normal and the sender tests where it should, then run a new dedicated ground from the threaded portion of the gauge ground terminal to an engine ground and see if the problem persists.

If this seems like too much, or is confusing, we will be happy to test your gauge for you if you send it into us. Please contact our Service Department for further assistance

Checking the accuracy of a full sweep electric temperature gauge (one that uses a 2-pin plug-in connector): This gauge would typically have a 10-pin, plug in connector on the rear of the gauge with (depending on model #) 4 or 5 wires, and a 2-wire/2-pin plug in connector. This gauge design is similar in how a short sweep version operates, based on variable resistance, but yet different.

The gauge supplies a 5v signal on the gray wire to the sender. The sender then (with variable resistance) attenuates the voltage back to the gauge, and the gauge then responds to this returned voltage.

The easiest test would involve using an ohm meter. Run the vehicle until the gauge levels off at a steady temperature, and then with the engine still running, unplug the sender connector. Using your ohm meter, measure across the two sending unit terminals. Depending on the actual temperature, the range can be anywhere from 85,256 ohms at 5 degrees F, to 309 ohms at 266 degrees F. With this said, you may have to adjust the range on your meter, depending on the temperature, however if you do this with a warmed up engine, you can start off expecting a reading (on average) of about 2,000 ohms or less.

Here is an example: If you measure 1,574 ohms, and the gauge was reading 167 degrees F when you plugged it back in, the gauge was then responding properly to the resistance reading of the sender. If you had the same resistance measurement, but the gauge was reading 210 degrees F, then the gauge is not responding properly.

Once you have the reading, plug the gauge connector back in and see what the gauge reads. You do not want to shut the engine off until you are done with the tests, otherwise the engine will heat soak and rise in temperature before it begins to cool off, and this can cause erroneous readings that do not compare properly. Contact us to see what your resistance measurement should = in temperature.

While the gauge is unplugged from the sender, it is not uncommon for it to register temperature in the 110 to 140 degrees F range.

If the resistance measured matched the gauge reading, the problem will likely either be the sender, or that the engine really is running the temperature that your gauge is displaying.

If the resistance measured does not match the gauge, the problem will likely be the gauge.

For further assistance on this, please contact our Service Department for more information