The images are pretty poor, but hopefully the explanation of system operation will make sense.
We recently had a MD-11 with the #1 engine showing amber "U/L" for the thrust reverser in-transit, but no green "REV" at full deployment.
First of all, this system uses nothing but grounds for the logic gates in the DEU's. (Another nice thing is that these thrust reverser systems are hydraulically driven instead of pneumatically.)
The full deploy (REV) indication is derived from position switches mounted at the forward end of the top left and right actuators. The ground signal is wired in series. Both actuators must show full deploy before the ground is seen on pin 12F of the DEU's. I'm guessing that these switches do not flip the contacts until the actuator is near or at full travel.
The grounds for the in-transit or unlock (U/L) indication go through the relaxed contacts of the upper actuator switches. Any one (or all), of the remaining mid/lower actuators can provide a ground. Again, a guess, but the mid and lower actuators must flip almost immediately upon travel in the deploy direction. The Hydraulic Control Unit (HCU) also has a pressure switch that can supply a ground.
The indications show up in the EPR section of the engine display.
We popped the left cowl first. I removed the plug off of the left upper actuator and grounded pin 1 (with the T/R deployed), we had the "REV" indication. Just to make sure, I verified the hard ground was present on pin 2. The upper left switch was at fault and it was replaced.
Awesome info Mark! We normally just have time to defer the T/R on the flight line; however, This info is great if I have to troubleshoot, thanks! We mostly see sheared flex shafts on our CF6s, and have to perform the torque check of the other shafts before deferring the system.
Yes we have the Pratts as well which never seem to give us problems, since their hydraulic. Have problems with Pratt's T/R blocker door seals which lift into the air stream, causing us to deactivate the T/R- bummer!!