Working on a Boeing 767-200 pilot reported LH WXR would not pass the test mode when selected to LH side.
They found that when selected to LH side that LH WXR RT would be stuck into a boot loop, just showing the main screen and as soon as it finished or almost finished it would do it over and over again no way to BITE the box.
I started on this project late the other guys spent about a week trying to track down the issue.
With the LRU having these weird issues it smelled like a wiring issue to me.
So when i started to work on this based on the last guys diagnosis I started at the wave guide S/W. I found out that with LH WXR selected whenever you unplug the waveguide S/W that the box will boot up and you can perform the BITE test but it just shows Wave Guide S/W since its disconnected so no help from the box nothing in the fault memory to help either. So what I decided to do next was make a jumper harness and try to isolate some of the wiring to see if i can find out what is causing the problem from the wave guide plug.
After trying many different configurations of wiring I find that the only time the fault on the box is present is when PIN A and C are both connected. Next thing I decided to do was just straight jumper pin A-C when connected the box went back into its reboot cycle. This told me that wave guide its self is for sure good because issue still present when bypassed. So what I have decided is that this is most probably an issue coming from the A wire.
So at this point i rang out the wiring from the controller to waveguide and to the RT and from the wave guide to RT rack all checked good. I also double checked the wire from controller to wave guide and waveguide to RT rack with megger just to be sure. To this point everything had checked good. So next thing i did was ring out all the data bus and control bus wiring to the RT. Im thinking that maybe im to hung up on the A wire and maybe when the signal to activate comes in from the controller something else in the box is changing, the symptoms on the box are similar to ones you might see that have shorts to shield or ground. Everything checks good, at this point i would have changed the WXR control panel but already done so I had to dig deeper.
After checking the system Schematic manual I found out that wire a goes to the controller then back to the RT when in LH mode and to ground when in RH mode. The signal coming in comes from the antenna drive, so next stop was the ATT.
When i got to the antenna i unplugged the LH output from the ATT drive went back to the LRU and finally the box had stop rebooting.
Next thing i did to verify everything down stream was to plug back into the att drive and then swap the output connectors at the bulkhead. I would have completely changed the harness over but due to the back shell it didnt have clearance to fit past the wave guide.
After swapping the output plugs LH side is passing the test and RH side failed the test. So has to be the antenna drive or the wiring harness from bulkhead to antenna. I decided to ring out the wiring harness from the bulkead to the radar and everything checked good. Did learn something new though a lettered 55 pin plug never have come across a plug labled like this before so luckily i was able to find a diagram so I could read it.
Ring out checked good, radar drive replaced and finally DMI cleared. Probably wasted a lot of time ringing wires but the problem at the box was so odd that it had me thrown off. You would think that the LRU would be able to interrogate and identify a hardware problem in the antenna without having this rebooting issue. My only thought is when we removed the old drive assy we found some corrosion on the very small wiring so possibly something short within the assy that is sending a bad signal to box causing the rebooting issue.