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Federal Highway Administration Research and Technology
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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-05-153
Date: December 2006

Long-Term Pavement Performance Program Falling Weight Deflectometer Maintenance Manual

Chapter 5. Electronics


There are four proximity switches (figure 128):

  • Weight height (WH) proximity switch sensor is activated by close presence of any of the falling weight stops (located in a rail on the drop weight). The (red) WH LED must turn on at each passing of a falling height stop.
  • Trigger (TRG) proximity switch sensor is activated when the drop weight is less than 6 to 12 mm (0.25 to 0.50 inch) from its lower, resting position. The (green) TRG LED must be on when the weight is low and it must turn off when the weight has been raised 6 to 12 mm (0.25 to 0.50 inch).
  • Plate low (PL) proximity switch sensor is deactivated when the falling weight subassembly is no longer suspended from the tow shafts of the side cylinders. This is normally when the plate has come to rest on the ground, but it may occur if the subassembly is prevented from being lowered, such as if the transport locks have are still locked. The (yellow) PL LED must turn OFF when the plate is low.
  • Plate high (PH) proximity switch sensor is activated only when the plate (i.e., the falling weight assembly) is close to its highest position. The (green) PH LED must be on when the plate is high. It also must be on when the subassembly is resting on the transport locks in either or both sides.

Figure 128. Four types of proximity switches.

Photograph shows four types of proximity switches. Arrows indicate the locations of the P H proximity switch, P L proximity switch on the underside of strike plate, W H proximity switch, and T R G proximity switch.


Check electrical connections and clean annually (figure 129). Refer to the manufacturers wiring schematics and owner’s manual for further information on the wiring diagram.

Figure 129. Electronics connection box.

Photograph shows a typical electronics connection box. Refer to the specific manufacturers wiring schematics and owners manual for more information on a specific wiring diagram.


Annually remove the system processor (figure 130) and clean all internal components. Periodically check connections in the rear of the processor (figure 131) to make sure they are secure and no strain is put on the cables. Also, check that the processor is securely mounted to the vehicle. Refer to the manufacturer’s owner’s manual for repairs and maintenance beyond the routine.

Figure 130. System processor.

Photograph shows a system processor mounted behind the seat of the F W D vehicle.

Figure 131. Rear view of vertically mounted system processor.

Photograph shows the rear view of a vertically mounted system processor. Check connections and make sure cables are routed correctly so there is no strain on them.


The vehicle-mounted proximity switch is a distance-measuring instrument (DMI) (figure 132). It should be checked monthly to make sure the correct air gap is used. Stainless switches can have a sensor gap of up to 8 mm (0.315 inch). The yellow plastic switches can have a gap of up to 5 mm (0.197 inch).

Monthly check the wiring and switches to make sure there is no strain on the cable. Visually check the switch for damage and replace if necessary. Recalibrate after repairs are made.

Figure 132. Vehicle-mounted proximity switches.

Photograph shows vehicle-mounted proximity switches used for the distance-measuring device.


Monthly clean the power supply connection between the vehicle and trailer (figure 133). Also, check that the connection is snug to make good contact between the two plugs (figure 134).

Figure 133. Vehicle charge plug.

Photograph shows a vehicle charge plug that connects the power supply between the vehicle and the trailer.

Figure 134. Trailer-to-vehicle charge plug.

Close-up photograph shows a trailer-to-vehicle charge plug that connects the power supply between the vehicle and the trailer.


The power control board is located behind the panel shown in figure 135. Remove the front panel and inspect all connections, making sure they are tight. Clean all connections annually.

Figure 135. Power control box (board behind front panel).

Photograph shows a power control box that is located behind the front panel.


Figure 136 shows the trailer fuse block. Two more fuse blocks are located inside the vehicle. All three fuse blocks should be inspected and cleaned monthly.

Figure 136. Trailer fuse block.

Photograph shows a trailer fuse block, one of three fuse blocks open parenthesis two others are inside the vehicle close parenthesis that need to be cleaned and inspected monthly.


The electronics battery is located inside the vehicle (figure 137). Check that it is securely mounted to the vehicle floorboard, and check and clean the connections monthly. Also, check the water level inside the battery. If water needs to be added, use distilled water.

Figure 137. Electronics battery inside the vehicle.

Photograph shows an electronics battery located inside the F W D vehicle securely mounted to the vehicle floorboard.
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