🚗 Loop Detector Troubleshooting

Guide to testing and diagnosing vehicle detection loop systems.

How Loop Detectors Work

Loop detectors sense vehicles by detecting changes in inductance when a metal object (vehicle) passes over the wire loop embedded in pavement. The detector sends an oscillating signal through the loop and monitors for frequency changes.

System Components

Loop (wire in pavement): 3-5 turns of wire in a saw-cut slot
Lead-in cable: Twisted pair from loop to detector
Loop detector: Electronics that sense inductance changes
Output relay: Signals gate controller when vehicle detected

Testing Procedures

Test 1: Check Detector Power

Locate loop detector unit (usually in control panel)
Check power LED - should be lit
If no LED, measure input voltage:
- Most detectors: 12-24V DC or 120V AC
- Check detector label for specifications
No power = check breaker, wiring, power supply

Test 2: Measure Loop Inductance

Most detectors have digital display showing inductance
Typical reading: 50-300 microhenries (μH)
Note the reading with no vehicle present (baseline)
Drive vehicle over loop
Reading should change by 0.2-0.3% (detect LED should light)
No change = loop problem or sensitivity too low

Test 3: Check Loop Resistance

Disconnect loop wires from detector
Set multimeter to ohms (Ω)
Measure resistance across loop wires
Typical reading: 2-8 ohms
Results:
- 0 ohms = shorted loop (broken insulation)
- Infinite ohms = open loop (broken wire)
- Normal resistance = loop is intact

Test 4: Check Output Relay

Locate detector output terminals (usually labeled)
Connect voltmeter across output
No vehicle: Should read 0V (normally open)
Vehicle present: Should read voltage (relay closed)
OR use continuity tester to check contact closure

Adjusting Sensitivity

Locate sensitivity adjustment (pot or DIP switches)
Park vehicle over loop (or use metal plate)
Slowly increase sensitivity until detect LED lights
Increase slightly more for safety margin
Test with different vehicle types (small cars, motorcycles)
Adjust to detect smallest vehicle reliably

⚠️ Don't Over-Adjust: Too high sensitivity causes false detections from:

Rebar in concrete
Nearby vehicles
Shopping carts
Temperature changes

Common Problems & Solutions

Loop Detects But Won't Clear

Vehicle still over loop (expected behavior)
Sensitivity too high (detects rebar or adjacent lane)
Failed detector (stuck relay)
Solution: Lower sensitivity, test relay, replace detector

Loop Won't Detect Vehicles

Sensitivity too low
Loop damaged or open circuit
Loop depth too deep in pavement
Detector failed
Solution: Increase sensitivity, test loop resistance, check detector

Intermittent Detection

Loose connection at detector terminals
Water in lead-in cable or junction box
Loop partially damaged (intermittent short)
Solution: Check all connections, seal junction box, test loop

💡 Pro Tip: When sawing new loops, use loop sealant to fill the slot. This prevents water intrusion which is the #1 cause of loop failure. Twisted pair lead-in cables reduce electrical interference.

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