DSP-50 User Manual
Page 25 of 27
DSP50_MAN_A
If a loop is connected to the correct pins of the detector, disconnect the loop and using an ohmmeter, check the
resistance of the loop circuit. If the resistance is below 0.2 ohms there is a short in the loop circuit. The
resistance will typically be 0.5 ohms to 1.5 ohms.
If the resistance is above 0.2 ohms, the loop inductance should be checked. This is done using an inductance
meter. The inductance of the loop should be more than 20 microhenries. If the loop inductance is less than 20
microhenries, the loop was probably not wound correctly and only has one turn in it. In this case the loop must
be replaced. Other possibilities include a foreign object embedded in the saw clot and shorting some or all of
the wires, or failed wire insulation due to the wire being exposed or the wrong type of wire being used. Very
small loops may also have a low inductance value if sufficient turns were not added. Contact technical support
for help with very small loops.
If you do not have a meter capable of measuring resistance and inductance but do have another operating
detector in the same box, you can skip to this step. Swap the loops between a working detector and a failing
detector. If the problem follows the loop the loop is the problem. If it stays in the same detector, replace the
detector.
Power LED Shows Two Quick Flashes Once Every Two Seconds
This flash rate indicates that the channel has had a failure of some type, but is currently working correctly.
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.
