Engaging Your Gauges

| April 07, 2005

This amber malfunction light on the Mack electronic dash shows there is a stored blink code that may indicate a sensor malfunction.

You’re bowling along Interstate 10 late in the day and you glance at your gauges. Yep, you’ve got enough diesel to get all the way to the terminal, your oil temperature and air pressure are fine and the axle lube gauge says everything there is A-OK too.

Or maybe you’re about to pull out at the start of the day and your trailer air pressure gauge says there’s something seriously amiss.

But what happens when these valuable readouts that you rely on have a problem?

The operation of traditional mechanical and electrical gauges is fairly simple, so finding the trouble with them isn’t normally very complicated. A little knowledge will carry you through straightforward gauge repair.

Standard fuel and temperature gauges
A thermistor is a device whose electrical resistance goes up and down when the temperature goes up and down. These are used to measure engine coolant, oil temperature, and transmission and axle lube temperatures. A variable resistor changes resistance in response to rotation – in the case of a fuel gauge, when the fuel in the tank moves a float up and down.

If either of these types of gauges are on trucks built prior to the advent of the new electronic dashboards, the first step is to check out the sending unit. But how do you tell whether the problem is the gauge, the sending unit or the wiring?

On models with a single wire to the sending unit, Don Fetherolf, a supervisor of service training at the North American Institute run by Volvo and Mack to train technicians and customers, recommends first grounding the signal wire to the chassis. “Anything like a temperature gauge that uses a thermistor, or a fuel gauge that uses a variable resistor, will go to the maximum temperature reading or ‘full’ position when you ground the wire from the sensor,” Fetherolf says.

On more recent models, there is both a signal wire and a ground wire headed back toward the cab. The unit is grounded to the chassis through the second wire, rather than to the tank. On these, you could just jumper between the two connections. When the signal line is grounded or the connections jumpered, turn on the ignition switch and see what the gauge does. If it now goes to the full position, this means the gauge and wiring are okay. The sending unit is very likely at fault. But before replacing it, make sure the connections are clean and tight so you know it’s the sending unit itself that’s not carrying current for the gauge.

You can easily confirm that the problem lies in the sending unit with an ohmmeter. Turn off the ignition switch. Remove the sending unit and its gasket from the tank by disconnecting the wiring and then removing the attaching screws. Connect the ohmmeter across the two connectors, or from the single connector to the metal body of the unit where it is mounted onto the tank. Watch the resistance – if it rises continuously as the arm is moved from the full position down to the empty position, the unit is working.

What if the gauge with the problem is a temperature gauge for the engine, transmission or an axle and the sending unit a thermistor? If the unit has a single connection, just ground that to the engine or a clean spot on the frame. Most newer sensors have a two-prong plug. In these cases, disconnect the two-prong electrical connector that carries two wires back to the dash. Then put your jumper wire across the two prongs or apertures in the connector going to the dash. If the gauge now goes to full, you know the problem is in the sending unit itself and that it should be replaced.

By installing a jumper wire across this two-prong coolant temperature sensor connector, you can test the coolant temperature gauge.

As in the case of the fuel-tank gauge, you can confirm the sensor problem. Connect your multimeter to the sensor itself with the controls set to read ohms. If it has two prongs or apertures, connect across the two. If it has a single connector and is grounded, then connect the multimeter to the single prong and a ground right near the unit. The resistance in the case of a Mack is 700 ohms at room temperature. Check your manual for the right figure for your particular vehicle. As the engine or drivetrain unit warms up, the resistance will decrease. Operate the truck till the components are warm, then repeat the resistance test. If it remains constant, or if there is zero or infinite resistance, replace the sensor.

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