Engaging Your Gauges

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.

If the gauge does not respond when you jumper the connections, there is a problem in the wiring or the gauge itself. You will have to find damage in the wiring or determine that the gauge itself is defective. This is a job for the relatively experienced technician. Especially for those with older trucks likely to have deteriorated wiring, the best procedure might be to just take the vehicle to a competent repair shop.

If you feel you want to at least attempt to look further, the first step would be to remove the gauge from the dashboard. Remove the mounting screws and remove the bezel (the cover with the gauge glasses in it). Then gently pry the gauge itself out of the dash panel. Now, run one jumper wire from a 12-volt source to the positive terminal on the back of the gauge. Run another from the negative terminal to a good ground. If the gauge is good, it will jump to the top of the scale; if the needle does not move all the way up, replace the gauge.

If you’ve found the gauge and sensor are good, you could then take the truck to the repair shop and tell them to find and repair the wiring problem. You’ve eliminated the possibility that the problem is the sending unit or the gauge itself, making the job easier for them and saving you money.

Standard oil and air pressure gauges
In the case of pre-electronic oil pressure and air pressure gauges, small lines still carry the fluid itself right to the gauge. On most trucks, air gauges still operate this way even with a dash that is otherwise electronic. Make sure to shut the engine off and drain all air pressure out of the air system. Reclose air system drains. For an oil pressure gauge, unscrew the collar with a wrench and pull the line out of the gauge. For air pressure gauges, unscrew the collar or, on those that don’t have flats for a wrench, simply push it toward the gauge with a screwdriver and hold to release the air line. Then remove or slide out the air line.

If testing for oil pressure, put the line into a bucket or can. Then start the engine and see if oil/air comes out of the line. If it does, you know the gauge itself is defective. If not, you’ll need to replace or unclog the line or look for a poor connection to the air system or engine block.

Testing speed and tachometer sensors

On most trucks, the speed sensor is on the rear of the transmission, while the tachometer sensor is on the flywheel housing. You can easily check a sensor by disconnecting its leads and testing its resistance with your multimeter set to read ohms. Check the manual for the specified resistance range. Put the two multimeter leads across the two prongs on the sensor plug. If the resistance is not within range, the sensor is bad. You can also do a simple sensor test to confirm the problem. Remove the sensor by loosening the jam nut and unscrewing it. Set your meter to measure AC volts and then measure between the two prongs while passing a metal object like a wrench the specified (by your OEM manual) distance from the end of the sensor. If it is working, your meter will react as the wrench passes in front of the sensor.

Note that sensors must be properly adjusted to work right. If you need to replace a sensor or adjust one that has come loose, they are normally turned in until they touch the part whose speed is being measured and then turned out a specified amount. You then snug up the jam nut to hold the adjustment.

Make sure all connections are clean and tight. Use a quality electrical insulating paste on the connections or risk a citation.

Diagnosing Electronic Dash Instruments

The latest electronic dash instruments are often “multiplexed.” This means they may be wired together in series with a common cable, like Christmas tree lights.

Even where each gauge has a separate connector, as in Macks with V-Mac electronics, the data comes to the dash on a single cable and is later broken up and routed to each gauge. On Peterbilts there is an Interface Module that receives all the inputs (including air system pressure) and then sends the information to the gauges on a single cable. Either arrangement incorporates what is called a “data bus.” On a data bus, the gauges receive their sensor information in pulse form. The pulse signals are coded so each gauge or separate gauge circuit reads the signal intended for it and ignores the rest. This kind of technology means diagnosis can be complex.

Knowing this, the designers did some things to make life easier for the trucker with a gauge problem. For one thing, the gauges are designed so they run through a standard test sequence right after you turn on the ignition switch. If the gauge passes this, it’s okay, and if it doesn’t, it needs to be replaced.

Your tractor may have its own way of checking, so consult your OEM manual. For example, on Macks, the test can also be initiated by pressing and holding the mode button, which is located on the right side below the speedometer. Hold the button for longer than two seconds and then release it. The hand on every gauge will then travel through its entire range, first sweeping all the way to the right, then sweeping all the way to the left and finally returning to its normal reading. On Peterbilt 387s, at power-up the 2-inch gauges position their pointers at the 12 o’clock position for five seconds and light their LEDs. After five seconds, the LEDs go out and the needles return to their normal readings. The Speedometer Integrated Message Center will first display a software version number and then position the speedometer needle at the12 o’clock position for two seconds. After that, the needle goes to its normal zero position, and shortly thereafter displays the odometer reading.

Late-model trucks with electronically linked gauges universally provide such a power-up or manually activated test sequence so you can always be sure whether or not a gauge itself is operating properly. Just note that if any gauge does not react at all, you must make sure it has power and ground before deciding it’s defective.