DSP-19 User Manual
Page 23 of 24
DSP19_MAN_A
Power LED Flashes Off Every Two Seconds
These flashes indicates that the detector has had a failure, but is currently working correctly. There are three
types of failures that the detector will remember:
Flashes Remembered Failure
1
Open Loop Failure
2
Shorted Loop Failure
3
Large Inductance Change failure
Intermittent failures are usually open loop failures. Any splices in the loop wire should be redone. If there are
any wire nuts used in the loop circuit, remove them, and replace with a crimp connection or preferably, a soldered
connection. The open loop fault could also be a fatigued point in the loop wire. This can occur at locations where
the loop wires cross an expansion joint in the road surface. Any place where the loop wires must move, even if
only a very tiny amount, can cause wire fatigue. The actual failure point may be very difficult to find. Often the
loop must just be replaced if the issue persists but cannot be found.
It is possible for the intermittent failure to be a shorted loop fault. One possible source of this type of fault is a
foreign object being embedded in the loop saw cut and damaging the wire. Another is that the loop wire has been
damaged where it enters or exits a conduit or junction box, or that a conduit that the loop wire is in has been
damaged (crushed, kinked, bent, cut, etc.).
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.
