DSP-15 User Manual
Page 22 of 23
DSP15_MAN_C
Detect LED Intermittently Comes On / Stays On Without a Vehicle Present
This type of symptom is usually caused by one of three issues: physical issues with the loop, electrical
interference, moving objects in proximity to the loop.
Physical Issues with the Loop
– There are many ways in which a loop installation can go bad. The insulation of
the loop wire can fail. This can be due to the loop wire being exposed in the saw lot, damage to the wire
insulation during loop installation, physical stressing of the wire due to movement (crossing of expansion joints
or asphalt that has slowly moved or deformed), wires moving in the saw slot due to poor loop sealant
encapsulation, foreign objects embedded in the saw slot, and poor electrical connections in the loop circuit.
The best way to check for any of these issues is to use a megohmmeter (commonly referred to as a megger).
Disconnect the loop wires in question from the vehicle detector and any other electronic equipment. Connect
one lead of the megger to one end of the loop wire and the other lead to earth ground. Measure the resistance.
For accurate measurements the ground and the loop should be wet or at least damp (use a hose or a bucket of
water if needed to get the area wet). The reading should be at least 100 megohms. If it is less than 50 megohms
the insulation is compromised and the loop circuit has to be replaced. Between 50 and 100 megohms, the loop
may or may not work properly and reliably.
Electrical Interference
– There are several possible sources of electrical interference: loop cross-talk, power
lines, electric motors, and insufficient twisting of the loop wires, just to name a few.
Other loops in the area that are connected to a different detector are prone to cross-talk (when the magnetic
fields from different loops interfere with each other). Adjusting the loop frequency of one or both of the loops
in the same area will usually allow you to find a setting that both loops will work reliably with.
Anything that uses electricity is a possible source for electrical interference depending on its proximity to the
loop and the amount of energy being used. If you believe the loop is experiencing electrical interference, turn
off the device believed to be the source of the interference and see if the problem goes away. Sometimes this is
not possible and more technical means are needed to help identify the source. Call Technical Support in this
case.
If the electrical interference is occurring in the wire from the loop to the detector, additional twisting should
help mitigate the issue.
Moving Objects in Proximity to the Loop
– Objects that can move and are metallic or somehow electrically
conductive, may cause detection issues.
A common issue is movement of a slide gate or gate arm in close proximity to a loop. The best solution would
be to move the detection area further away from the moving gate. We recommend that all loops should be at
least 4 feet from a slide gate. Try lowering the sensitivity one level so that the desired vehicles are still detected,
but not the moving gate. NOTE: Do not lower the sensitivity too much or vehicles will no longer be detected.
Another possibility is metal objects in close proximity to the loop. Utility manhole covers are objects that may
move slightly when vehicle tires drive over them, especially if the vehicle turns while a tire in on the cover. Most
