I recently got a 1991 Mercury Tracer (1.9 Liter) for about nothing. When I got it, the temperature gauge didn't work. The temperature gauge is the little gauge in the dashboard of your car that has a needle pointing to either 'C' (cold), 'H' (hot) or (hopefully) somewhere in between. The problem with my temperature gauge is that no matter how long the car ran, the needle never moved off of 'C'.
So here's how I solved the problem. A friend and I don't know a whole lot about cars, just enough to be dangerous. So what the heck, let's give it a shot. We believe that if something doesn't work, you can't hurt anything by trying to fix it. You have to at least give it a shot!
Keep in mind, this is an old car. Most new cars probably work considerably differently, but it shouldn't be too hard to extend this to your car.
Here's what the gauge is for:
The temperature gauge monitors the temperature of your engine. Specifically, it measures the temperature of the liquid in your engine, which is most likely a mixture of anti-freeze and water (but will be referred to simply as water). This water circulates around your engine to keep it cool. The engine heats up the water, then it runs down a radiator to cool the water back off again. The gauge is supposed to show 'C' (or even below) when the car first starts. Then, depending on factors such as outside temperature and how you are driving the car, the needle will begin to move toward the 'H' as the engine (and the water) begin to heat up. The gauge should begin to rise after a few minutes of the car being on, and should stop moving somewhere between the 'C' and the 'H'. If the needle gets towards the 'H', you could have serious problems. If the needle stays on the 'C' (like mine did), your gauge isn't working correctly.
Here's how it works:
The physical gauge in my car has three wires running to it: one wire is the hot (red) wire (to the battery), one wire is the ground (black) wire (to the battery) and one is the variable "sensor" wire (which is ground in my car). The first two wires simply provide the power to operate the gauge (move the needle) and don't actually have anything to do with the temperature monitoring itself. The sensor wire runs to a "temperature switch" (which is a stupid name, but that's what it's called). A more reasonable name for it would be "temperature sender" "engine temperature sensor" or something similar. Here's where things get a little technical, and you should note that your car may be significantly different from my car.
The temperature switch is a thermistor (a resistor whose resistance varies with temperature). This little guy is a hunk of metal (it has not electronics) that screws into the engine (mine is on the side, not in the engine block, while other cars may have it directly into the block). The "inside" face of the "screw" is flat, and connects to a ground inside the tube that it screws into (you can't see it). Then, the third wire from the temp gauge connects to the other side (the outside) of the temperature switch. Temperature switches come in many forms, and some cars may have an explicit ground wire (aka, more than one wire connects to the "outside"). The idea of the thermistor is based on Ohm's Law which states (inaccurately, as described in my other blog) that V=IR. The voltage from the battery ground to the gauge is V = 12 volts. When the engine (or more accurately, the water in it) is cold, the thermistor has high resistance (R), which, following the equation, means low current (I). As the water gets hotter, it runs past the flat "inside" surface of the temperature switch, which heats up the switch itself, which, due to its properties, lowers the resistance in the metal, thus providing more current (a better connection to ground). More current makes the needle move towards the 'H' on the gauge in your dash.
Next up: how to troubleshoot
Before tearing apart your dash, it might be a good idea to check a couple of other things first.
1. Check your thermostat. Engines run best at a certain temperature. The thermostat controls the circulation of the water in the engine. When the engine is colder than optimal, the thermostat closes, which doesn't allow water to circulate and cool. It gets hotter and warms up the engine. When the engine is hotter than optimal, the thermostat opens up and lets more water circulate to the radiator to cool the engine. Since it is a mechanical part, it can fail. Old thermostats "failed closed" which means that when they stopped working, they would close up. This spells major problems if and when a thermostat fails because it can't cool your engine, and your engine overheats (that's bad for it). Good thermostats today "fail open". This means that if and when your thermostat fails, it will "over-cool" the engine. While this makes it run at less-than-optimal efficiency, it won't cause any immediate damage to your engine. In fact, some people simply remove a broken thermostat and don't put a new one in, since it isn't strictly necessary. If you had a "fail open" thermostat, it is at least theoretically possible that your engine is so cool that the gauge really is reporting an accurate temperature when it sits on the 'C'. You would probably expect though, that after running the engine for some time, it should at least move a little from its resting spot. But it's possible.
So the first thing we did was to replace the thermostat. We actually discovered that someone had previously removed the thermostat and hadn't replaced it. With an operating thermostat, the car should get hot enough to move the needle to the middle of the temperature range. In my case, that didn't help.
2. The next thing we checked is the temperature switch (thermistor). These can go bad over time. When they do, the internal resistance may be constant, or may be out of the expected range. Either of these problems might cause my gauge to sit on 'C'. There isn't anything complicated here, we simply got a new temperature switch. If you replace yours, you should do it when the car is not hot, because it is likely that some water (anti-freeze, water mix) will spill out. In my car, about 1/4 cup spilled out when the temperature switch was removed. There is also a heated debate about whether teflon tape should be used around the threads of the temperature switch. The theory is that the tape could interfere with the connection to ground, thus increasing the apparent resistance. We chose to tape the new switch. Unfortunately, even this new switch didn't fix the gauge.
We tested the connection to ground by using a voltmeter. We put one side on the red battery terminal (hot) and one side on the "outside" connection of the switch. When the car was hot, we observed current (which meant that the wire from the battery/ground to the switch itself was okay).
Before removing the gauge, you can test if it functions by grounding the sensor wire. You should see the needle quickly move toward the 'H' on the gauge. This means that the gauge is physically operational.
3. Finally, we investigated the gauge itself. This required removing the entire dashboard.
Practical notes on how to remove the dashboard from a 91 Mercury Tracer:
You will probably have problems getting the dashboard around the emergency flasher switch. Unhook the casing around the steering wheel. At about 8 o'clock (facing the steering wheel) is a screw which will loosen the turn signal/wiper/flasher switch assembly. Rotate the assembly so the flasher switch is at an angle. Then carefully proceed to remove the dash.
To remove the instrument panel (after you get the dashboard off), you will need to unclip three wire bundles and remove the speedometer cable from the back. You can reach the speedometer cable from under the steering wheel.
Once you get the temperature gauge out, you can trace where the wires connect in the bundles around the edges. The gauge plugs into pins "B", "S", and "G". "B" is the "hot" battery wire, "G" is the battery ground, and "S" is for the temperature sensor/sender/switch. First test to make sure that the B and G wires have 12 volts. Then test to make sure the S wire isn't broken by testing connectivity between the end that connects to the temperature switch and the end that the gauge attaches to.
After testing all of these things, we couldn't figure out why the gauge still wouldn't come off of cold. We had ruled out:
-open thermostat
-broken thermistor
-broken wires (both power and sensor)
-broken gauge
About the only possible thing left was that the gauge operated on a different current range than the switch was providing. This seems rather unlikely, considering that we had a brand new switch, and we couldn't imagine how the gauge could become "uncalibrated". In the end, we decided to get yet another temperature switch, since we didn't know how to change the resistance of the gauge itself even if we wanted to. After looking online, we discovered that there are actually two different temperature switches. One is for if your car only has an idiot light (a light indicating that your engine is too hot), and one if for if your car has a temperature gauge (like mine). When we got the first new gauge, no one asked us which one we wanted, or even mentioned that there were two. These two thermistors could have very different properties and apparently, the light thermistor works at a significantly high resistance/lower current than the gauge thermistor. After getting the appropriate thermistor, the temperature gauge worked!
The ultimate problem:
When we got the car, the temp gauge didn't work. Either the thermistor was bad, or someone already tried to replace it with the wrong one. Either way, it didn't work. Then we replaced that thermistor with either a bad gauge thermistor (out of the box) or (more likely) the wrong kind of thermistor. Finally, because we had ruled out practically everything else, we decided to get yet another thermistor, and happened to discover that there are two different types. Basically, we got lucky.
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temperature gauge doesn't work
temperature gauge stays on cold
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