The weighting game
Onboard scales show your gross, and axle totals as you load.
When it comes to what you weigh, there will be times you have no doubt your rig is legal. You have an experienced eye, and you know what you have on board and where it is. It’s way under and all in the right places.
But weight and weight distribution are not things to gamble with. So what happens when your certainty begins to weaken as a load gets heavier and you know you must be 100 percent certain? You know you’re going to have to make a run to the scales and back. Unless, of course, you have onboard scales.
A typical 80,000-pound rig with a single front axle and drive and trailer tandems will have 12,000 pounds on the front axle and 34,000 on each tandem.
Onboard scales give the driver an instant indication of weight distribution so he or she won’t have to drive back and forth to a scale to check and redistribute it.
How onboard scales work
The pressure inside air springs is controlled by a leveling valve. The valve’s purpose is to adjust the firmness of the spring to the load so the trailer will always ride smoothly.
As you load a trailer, the leveling valve will add enough air to restore the trailer’s ride height. So there will be a consistent relationship between the pressure in the air bags and the weight on each axle.
Onboard scales measure the air pressure in the air springs on each axle group with an electronic sensor. The sensor converts the pressure into an electronic signal. It then sends the signal to a ComLink (Air-Weigh) or a hand-held reader (TruckWeight).
The digital processor in the ComLink or reader has calculated the relationship between air pressure and weight for each axle. Instead of sending a signal representing the air pressure, it mathematically converts the pressure to a digital reading of the axle’s weight.
Air-Weigh tractor system installation
Air-Weigh, of Eugene, Ore., has been making an onboard scale system that can give a driver accurate axle weights on the dashboard for some time. We visited Dennis Trucking in Philadelphia, where Steve Spriggs, customer support and training manager for Air-Weigh, installed a system on a Mack CH600 tractor.
The first component in the system is the sensor. The sensor’s signal is transmitted by wire to a ComLink, which is a type of processor called a “reader board.” Another wire then conducts a different signal, representing the axle weights, to an Air-Weigh gauge installed in the dash.
Spriggs only took about an hour to install the system, even though he was unfamiliar with the tractor model. He says that where you need to run an air line all the way from the suspension up to the dash, you should add another half hour.
- Find a place for mounting the system’s digital gauge. In this case, Spriggs found a round orifice in the Mack’s dash that was perfect and removed the rubber insert. The gauge needs a hole that is about 2 1/16-inch in diameter.
- To complete the installation, remove the two bracket attaching nuts from the threaded studs, remove the bracket and put the gauge into position. Make sure it is level. Then slide the bracket onto the studs and install and tighten the two nuts.
- Install the ComLink behind the dash. Spriggs chose a location on the right side, behind a panel that was easily popped out from the bottom. Remove two covers from two adhesive coating strips on the back of the ComLink. Place the ComLink against the rear of the dash in a position that would make it easy to plug the connections in, and press with about 15 pounds of pressure to activate the adhesive.
- Drill tiny holes through the screw holes in the ComLink’s mounting holes, and then install self-tapping screws to help hold it in place.
- Plug in the cable that connects the ComLink to the display. It’s easy to do this correctly because the plugs can be installed in only one direction.
- This truck has a pressure gauge for the air suspension located right in the main part of the dash, where the speedometer and tach are. Spriggs first removed the attaching screws in the corners and then pulled the panel out. He then dumped air pressure from the suspension, cut the line going to the gauge and installed a tee provided in the Air-Weigh kit between the two open ends of the air line. He then connected the air line for the sensor to the open end of the tee, ran the line over to the dash opening on the right, installed ties as necessary to hold the line in place, and then replaced the left side dash panel.
- The sensor was then located right behind the dash cover on the right, and its output wire was connected to the ComLink.
- The ComLink requires 12-volt power from the truck’s electrical system. Connect it to a post that is powered up only when the ignition switch is on by using an eyelet included in the installation kit. Spriggs found a spare connection post that was not used for any accessories behind the right side of the dash.
- Stabilize all wiring with zip ties so it won’t shift around and develop frayed insulation. Replace dash panels.
Air-Weigh trailer system installation
The Air-Weigh system for trailers consists of a scale module, an air suspension pressure sensor, a sensor cable and a power cable. Since the cables are of fixed length, Spriggs first laid everything out. He picked a location for the module in relation to where he was going to get air pressure so he knew all the cables and the air line would reach.
- First dump the trailer air. The system requires one sensor for each separate air suspension height control valve. Each sensor must be connected where it can read the pressure used to suspend that particular axle or group of axles. Spriggs says if the particular axle you’re working on does not purge, you should carefully loosen an air line connection and allow the pressure to be slowly dumped out.
- This was a tandem-axle trailer with a single height control valve, so only one sensor was required. The kit comes with two types of plug-in breakout connectors. These plug in between the ABS power cable and the ABS module, connecting with the two standard types of plugs. They provide what amounts to an electrical tee so the power line from the module can be simply plugged in to get power via the ABS power supply. Spriggs found where the ABS ECU plugs into the trailer harness and unplugged it. He determined which type of breakout connector was needed and then plugged in both plugs of the circuit used to carry ABS power at the ECU. He then plugged the power line for the scale into the open plug on the breakout connector.
- Next connect the sensor near one of the air bags. After unscrewing the original connection, carefully clean off the threads. Wrap Teflon tape on the existing threads where needed, and install the brass tee supplied by Air-Weigh (The Air-Weigh tee’s threads already have Teflon tape on them.) Connect the tee between the air bag and the incoming line, and then connect the sensor to the long end of the tee.
- Route the harness carrying the power line for the module and the cable for the sensor through an existing grommet. Spriggs tied this harness to the existing trailer harness at numerous points to keep it stable.
- Decide where to mount the module, then drill through the trailer’s support beam for the mounting screws. First position the mounting bracket and mark the four hole locations. Then use a much smaller drill than required for the final boltholes to make the drilling of those holes much easier in the tough metal. Finally, drill the four holes with a drill a little smaller than the screws.
- Put the mounting bracket in position and install the four attaching screws.
- Plug in the module. Position it on the mounting bracket and install the two mounting screws. The finished product allows the driver to read the rear axle weight while loading.
- Restore air pressure to the trailer air springs and check on the module which automatically goes into a diagnostic mode, to see that the sensor produces a reading.
The trailer module sends a signal representing the digital weight reading into its power cable. The dash display recognizes and reads the signal on its own power line, since all the vehicle’s electrical wiring is interconnected once the ignition switch is turned on.
The final step is calibration of the trailer module and tractor ComLink. Air-Weigh provides standard calibrations for a few common air suspensions, and if your tractor and trailer have examples of these, you can just input that information at this time. However, individual calibration of the vehicle is a bit more accurate, so this rig was calibrated.
This involved taking it empty to a truck scale and weighing each axle with the truck level and brakes off. The axle weights were then input into the dash display and trailer module with an indication that this was the empty weight.
Next, the trailer was fully loaded. The weights were again input, indicating that this was the loaded weight. The ComLink contains some sophisticated software that allows inclusion of the front axle weights with the calibrations. The reader board then calculates front axle weight, too.
TruckWeight system installation
TruckWeight, Inc. of Halifax, Nova Scotia, Canada, recently entered the on-board scale market with the Smart Scale system. It employs wireless transmission of the signal to a hand-held reader to simplify installation. Both the sensor unit and reader are powered by AA alkaline batteries, rather than the vehicle electrical system.
We visited K.L. Harring Transportation in Bethel, Pa., where President Keith Harring and Tommy Vajdic, the maintenance shop manager, assisted Peter Panagapko, president of TruckWeight, in doing the installation.
- First unscrew the top of the sensor. Install two AA alkaline lithium batteries. Panagapko recommends coating the O-ring with a little petroleum jelly to help preserve it before replacing the top.
- Next, find a mounting location that will protect the sensor from road debris or tire failures, yet allow it to transmit its signal directly to the hand-held reader. An ideal
- Once you have picked a location for the sensor, hold the sensor there in the best orientation and have someone test for a signal to the hand-held unit. Do this while sitting in the cab or, if it will be used during loading, where the driver would stand. If the signal is poor, shift the location as necessary.
- Panagapko and Vajdic found a perfect location on the tractor crossmember that was already drilled. The best location on the trailer, however, needed to be drilled.
- The bolt is extra long to help increase the number of potential mounting locations, and you can get a longer bolt if you need it. You can remove a washer, if necessary, if the crossmember is too thick.
- The next step is to rotate the sensor to produce the ideal orientation. This must be done because the transmitter’s signal is most powerful in one direction. That direction is indicated by a larger + than the one used to show battery polarity. The sensor should be mounted vertically, and the big + should face the cab if the reader will be used there most of the time. You’ll need to face the + to the appropriate side if the driver will read the weights while loading. Loosen the clamp that holds the bolt in place. Then remove the nut and washer from the end of the mounting bolt, place the bolt through the hole in the crossmember and hold the sensor in its mounted position. Rotate the sensor until the + is facing in the right direction, and tighten the mounting clamp’s bolt and nut around the sensor.
- Screw on the top of the sensor and snug it down gently. Then put the sensor in position with the bolt through the crossmember, and install and snug up the nut on the other side.
- Connect the sensor air line to the air suspension. This trailer provided a typical connection point. After dumping all trailer suspension air pressure, Panagapko cut the air line going to the air bag. He then installed the tee onto the open end of the incoming line, and inserted the lines going to the airbag and sensor into the other two openings. These are simple push fittings – all you have to do is force the line securely into the open end of the tee. If the line does not lock securely on its own, pull the sprung collar out of the tee with your fingers to make it hold.
- To connect to the suspension on the tractor, Vajdic found an unused hole with a plug in it in a suspension air line. After dumping air pressure, he installed a fitting that allowed him to connect a 3/8-inch line to run toward the tractor sensor. While he had materials available to convert this line to the smaller size normally used, Panagapko says you can also substitute a 3/8-inch or 1/4-inch fitting on the bottom of the sensor.
- He then installed the other end of the line into the fitting on the bottom of the sensor.
- The final step in the process is, as with the Air-Weigh system, calibration. Panagapko pointed out that the setting of the fifth wheel should be recorded. This is the setting that should be used if front axle weight is critical, because the system accurately estimates front axle weight from the tandem axle readings. It can’t do this accurately if the fifth wheel is moved to a different position. Weigh the rig empty and then loaded, and input all three axle weights for either situation. Then remove a lock pin from the back of the reader so the calibrations cannot be accidentally changed. The reader then shows the weights for all three axles.
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