ATA 33-51, Emergency Lights

L1 door "EXIT" sign not coming on during emergency light test.

Boeing 727, 737, 757 and 767's use battery packs for powering the emergency lights. Two packs are used on 737's. Both signage and floor track power come from these units. Typical output voltage is 6 VDC.



There are fuses shown on the unit's schematic manual. I've never opened one up to view them and I'm not sure they're line replacable.



On this aircraft, pin 9 of the forward pack is used for the L1 door exit sign. When shot for continuity to ground (this would verify power input wiring, light assembly, and ground wiring) a high resistance over 250 ohms was noted.



To isolate the actual problem, the power input wire was disconnected from the light assembly. Shooting the assembly's input wire to ground verified that both the light and ground wire were functional.


This only left the power wire from the pack to light as suspect. With the pin pushed out of plug and examined (wiggled and tugged on), the high resistance remained. At this point, the ceiling panel was accessed and the wire replaced (total length 2.5 feet). No evidence of wire damage on the old wire was noted.



6 VDC does not have much punch to it. 270 ohms was probably enough to drop that voltage to 0 at the light assembly.

737-400 WDM 33-51-12

That wire was nice enough to show the high resistance while unplugged. Could you have found that wire under a load while looking at voltage drop? Enjoyed the training Mark!

You would somehow have to place meter (on voltage) right at light input. I didn't do that.

Have you ever opened up one of those power supplies? I'd like to know if those fuses are replaceable.

No, I have never opened one of those boxes. If you could, next time see what the voltage drop is when troubleshooting a circuit. Thanks Mark. Very curious.