How To Diagnose Gauge Problems
Don't always assume your Mustang is really overheating; it could just be the gauge
Mustang Monthly contributor and former staffer Jim smart produced an excellent article on gauges in the May 2010 issue and we felt it was time to pdate Jim's work into the Fox Mustang era with this story. Read Jim's original story, Ford Mustang Instrument Panel Troubleshooting.
Chances are there are more than a few of our readers who are driving around in their vintage or Fox Mustangs with a gauge or two not working. Like most electrical issues, we've found that owners tend to "get by" without because they don't understand how the circuitry works or how to fix it. Be it a lazy oil pressure gauge or an inoperative fuel level gauge, the main theory of operation is the same for most gauges. The gauge circuit is provided a specific voltage and the ground side of the circuit passes through a variable resistor, such as the oil pressure sending unit or the fuel level sender. These variable resistors control the ground-high current flow to ground makes a gauge read high, lower current flow to ground makes the gauge read low.
Don't take the expensive route and simply throw parts at the problem. Instead, take the time to properly diagnose the troubled circuit. With the use of a shop manual, wiring diagrams, and the wealth of information available at your fingertips on the Internet, finding the problem and properly repairing it is certainly within the realm of possibility for even the "greenest" of shade tree types.
More times than we care to remember, we've been asked to look at a gauge issue and found it to be nothing more than a basic wiring problem (lack of ground, loose wire/connector, broken wire, etc.). Having a quality wiring diagram will help for instances such as these as you can trace the wire circuit routing, verify wire color, etc. and visually inspect the wiring from the gauge to the sender. Examples of wiring issues we've seen over the years include frayed wiring touching the engine (causing a short to ground and the effected gauge to peg high), loose bulkhead connectors at the firewall, and broken wiring at the gauge cluster. Don't forget that the voltage provided to the gauges is circuit protected as well. First thing to always check is your fuses!
If your wiring physically checks out fine, then you've just narrowed down your gauge problem to either the sending unit, the gauge itself, or the instrument voltage regulator (if equipped). Sending units can easily be checked for proper operation with an ohm meter, either in place or off the car. For example, the water temperature sending unit can be tested for the proper ohm rating with the engine cold and then running at operating temp (thermostat open), or you can remove the sending unit from the car and test it in a pan of boiling water. The same can be said for a gauge-style oil pressure sending unit, using regulated compressed air to test the sender's resistance. The fuel level sender is actually the easiest; simply remove it from the fuel tank and move the float by hand.
Gauges themselves rarely fail, but if they do, testing is easy using an ohm meter. Gauges like the fuel level, oil pressure, and water temperature will show continuity across their connections if they're in working order. Otherwise, a bad gauge will show no continuity or be "open" when checked with an ohm meter. Lastly, there's the instrument voltage regulator or IVR, which takes battery voltage and reduces it to a 5-volt signal to the gauges. Original IVRs use a bi-metallic strip that opens and closes a set of contact points to regulate the voltage down to what the gauges need by pulsing the voltage. However, you can now use updated solid-state electronic IVRs that are much more accurate.
Checking the IVR is simple with a volt-meter. The input terminal should show battery voltage and the output terminal should show approximately five volts. You can also quickly check the IVR by removing a sending unit wire and testing the sender wire with a simple test light and watch for the test light to pulse.
Check out the captions for more testing tips and what to look for when your gauges don't work as they should.
While we briefly touched on how to test gauge sending units in our introduction, there are far too many years and engine options to cover all sending units and their ohm ratings. However, all hope is not lost. We've bookmarked the following digital catalog on our computer- www.wellsve.com/ecatalog.html- and it has helped us diagnose sending units on many vehicles we've worked on. Simply launch the catalog and use the drop-down menus to configure your year, make, and model (and engine if necessary), then click on "sensors" or "switches" depending on what you're looking for. Scroll through to find the part you need info for, such as a coolant temperature sensor, and select the part number. You'll find temperature-to-ohm rating info under the part's detail tabs. This information can then be used to verify the sending unit on your vehicle is working right or even if it's the right one for your car.