9.3 Diagnosing Dark Signals, Flashing Operation, and Cabinet Faults
Key Takeaways
- A Red Fail names a channel whose red, yellow, and green inputs are all inactive, which points at a burned-out indication, open or shorted field wiring, a failed load switch output, or a bad transfer relay.
- A Dual Indication fault means two signal indications on the same channel are active at once, which points at a shorted load switch triac or crossed field wiring.
- A Clearance Fail means the yellow change plus red clearance interval was cut short, which points at controller programming, pre-emption programming, or an overlap with zero clearance times.
- A Port 1 Fail is a TS 2-only fault indicating the monitor stopped receiving Type 0 messages from the controller for 300 milliseconds, pointing at SDLC cabling, a bus interface unit, or the controller.
- A generator whose output voltage or frequency falls outside the monitor's window will hold the intersection in flash, and a shift of about two hertz is enough to do it.
9.3 Diagnosing Dark Signals, Flashing Operation, and Cabinet Faults
Three complaints account for the majority of unscheduled signal calls: the intersection is dark, the intersection is flashing, or one indication is out. Each has a decision tree, and each tree starts with an observation rather than a component.
Where Is the Flash Coming From?
Flash has four distinct sources, and they are distinguished before any tool comes out of the truck.
| Source | Evidence |
|---|---|
| Monitor fault (MMU/CMU trip) | A fault indicator is latched on the monitor and its output relay is in the fault position |
| Manual / police flash | The police panel AUTO/FLASH switch is in FLASH; monitor shows no fault; on many cabinets a Local Flash Status indicator is active |
| Automatic / programmed flash | Monitor clean, police switch in AUTO; the controller's time-of-day scheduler or a flash event is commanding it |
| Start-up flash | Occurs only after a power restoration, held for the programmed minimum flash duration (typically 6 to 16 seconds) before the first green |
If the monitor's fault indicators are dark and its output relay is normal, the monitor did not cause the flash — go to the police panel and the controller's flash programming. This single branch saves more field time than any other habit.
Fault-to-Cause Mapping
The monitor names the failure class. This table is what an experienced technician carries in their head.
| Fault Indicator | What the Monitor Detected | Where to Look |
|---|---|---|
| Conflict | Two channels not programmed as permissive were simultaneously active | Load switch shorted; crossed field wiring; controller output error; wrong program card |
| Red Fail | All signal inputs on a channel went inactive with red monitoring enabled | Burned-out or failed indication; open or shorted field wiring; output side of a load switch open; bad flash transfer relay |
| Dual Indication | Two indications active simultaneously on the same channel | Shorted load switch triac; crossed or shorted field wires |
| Clearance Fail (Min Yellow / Y+R) | Yellow change plus red clearance was cut short | Controller timing programming; pre-emption programming; an overlap for a right turn with zero clearance |
| CVM / Watchdog | Controller voltage monitor input dropped, or the external watchdog input stopped changing state | Controller found its own fault; police flash switch; in TS 2, check frame faults. Typically non-latching — restores when 24 V DC returns |
| 24V-1 / 24V-2 | A 24 V DC monitor input fell below threshold | Cabinet power supply in TS 2; the controller itself in TS 1 |
| Port 1 Fail | No valid Type 0 message from the controller for 300 ms | TS 2 only. SDLC cabling, a bus interface unit, or the controller |
| Diagnostic / Program Card | Internal self-test failed or the program card is absent or ajar | Reseat the program card first; then suspect the monitor |
| Field Check Fail | Controller's commanded output does not match the field status the monitor senses | TS 2 only. Isolates whether the controller or the load bay and field wiring caused a fault |
| Local Flash Status | The cabinet's own flash circuit is active | Police door switch; controller time-of-day flash event |
| Type Fault | The Type Select input does not match the monitor's configuration | Wrong monitor type installed, or Type Select miswired. Read only at power-up or reset |
| Recurrent Pulse | Repeating short pulses accumulated into a fault | A flickering load switch output that individually never exceeds the normal fault window |
Frame Faults in a TS 2 Cabinet
When a TS 2 controller reports a frame fault, the fault number identifies the device class that stopped responding on the SDLC bus. Frame faults in the low range correspond to the MMU, a middle range corresponds to the terminals and facilities BIUs, and a higher range corresponds to the detector racks. That grouping turns a generic communication error into a specific device to inspect — check the SDLC cable to that device, reseat it, and confirm its power indicator.
The Dark Signal Decision Tree
Whole intersection dark is a power problem, not a signal problem:
- Are other services on the block dark? → utility outage. Establish alternate traffic control and notify the utility.
- Is the service disconnect open or the main breaker tripped? → find out why before resetting. A breaker that trips again immediately is protecting against a real fault.
- Is a UPS installed? → the batteries may be exhausted, or the bypass switch may have been left in the wrong position after a previous visit.
- Is the cabinet powered but all field circuits dead? → look at the flash transfer relays, the main signal breaker, and the field terminal feed.
One head or one indication dark is a circuit problem, traced from the field back:
- Is the module actually failed? An LED module with a failed driver is dark; one with partial array loss is dim but lit.
- Is there voltage at the head? Measure at the head splice during the interval when that indication should be on.
- Voltage at the head but no light → the module.
- No voltage at the head, voltage at the field terminal → open field conductor between the terminal strip and the head.
- No voltage at the field terminal, load switch commanded on → failed load switch output, failed transfer relay contact, or an open between load switch and terminal.
- Load switch not commanded on → controller output, BIU, or a programming problem.
This trace is a half-split done with a meter, and it converges in three or four measurements.
Line Voltage and Generator Conditions
A class of faults that looks like equipment failure and is not:
- AC line brownout. The monitor's power fail function drops out below 89 Vrms and restores above 98 Vrms. Line voltage sagging under the dropout threshold suspends monitoring and drops the intersection to flash. A TS 1 assembly running on main power below about 90 volts behaves the same way. Repeated brownout events point at the utility service, not at the cabinet.
- Generator power. When an intersection is fed from a portable generator during an outage, output that is too high or too low will push the intersection in and out of flash repeatedly. A generator whose output frequency drifts by roughly two hertz from nominal will hold the monitor in flash, typically presenting as a Red Enable / Red Fail condition. The remedy is a different generator, not a different monitor, and the failing unit's identification should be recorded so it can be removed from service.
- Voltage sag at the far end of a long field run. A signal head at the end of a long circuit may be receiving materially less voltage than the terminal strip. That has to be measured at the head, not inferred from the cabinet.
The Order That Saves Time
- Read the intersection from the street before opening the cabinet.
- Read the monitor's latched faults and channel status before resetting.
- Check the police panel switches, including the door switch, before assuming an electronics fault.
- Read the event logs — monitor and controller — before clearing them.
- Then measure, half-splitting from the field terminal strip outward or from the monitor's channel status inward.
- Verify across multiple cycles and confirm nothing new latched.
A monitor latches a Dual Indication fault on channel 4. What does this specifically mean?
An intersection is being fed by a portable generator during a utility outage and keeps cycling in and out of flash. Monitor faults show a Red Enable or Red Fail condition. What is the most likely cause?
A single 12-inch red indication is dark. The technician measures 120 volts AC at the head splice during the red interval. What does this establish?
A Port 1 Fail indicator is illuminated on the signal monitor. What does this tell the technician?