Troubleshooting the Installation 5-5
Troubleshooting Overview
Each fan tray incorporates individual fans that draw +24 VDC from a DC-DC converter
on the CSC that operates under control of its onboard MBus module. Both fan trays
should begin to operate about two seconds after application of system power.
The fan trays incorporate a variable-speed feature, enabling the fans to run at a slower
speed (when the internal chassis temperature remains within the normal operating
range) or to run at a higher speed (when the internal temperature of the router exceeds
a specified threshold). In the latter case, the voltage being delivered to the fans is
increased, causing them to run at maximum speed to force a greater volume of air
through the router.
In a noisy, air-conditioned environment, it may be difficult to hear whether or not the
fan trays are operating. The first indication of a fan failure comes from the side-by -side
fan tray status LEDs on the CSC faceplate (see Figure1-14).
If the LED on the left is on (amber), a fan failure has occurred in the card cage fan tray .
If the LED on the right is on (amber), a fan failure has occurred in the power supply fan
tray.
Each fan in a fan tray is monitored separately for failure. A failed fan is not shut off in
the usual sense; instead, a current-limiting feature in the faulty fan prevents it from
interfering with the operation of other fans. The most common fan failure is that the fan
just stops running, but continues to draw current.
If a fan in the card cage fan tray or the power supply fan tray fails, the CSC increases
the voltage being delivered to the fans, causing them to run at maximum speed to
compensate for the failed fan.
You can make a gross assessment of fan tray operation by placing your hand near the
exhaust vents at the top rear of the router enclosure. A reduced flow of air from the card
cage exhaust vent or the power supply exhaust vent ma y in di cate t hat one or more fans
on a given fan tray have failed. However, this method of checking fan operation is
neither definitive nor reliable.
If you determine that a fan tray is not operating properly, you should immediately
consider replacing the faulty fan tray to guard against an overheating condition in the
router that could approach the shutdown threshold.
No operating adjustments can be made for either fan tray. An individual fan is either
operating normally or it is not operating at all.