location of surface deice timer

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      I am away from my maintenance manuals for a few months and my surface (wing/tail boots) deice timer seems to have gone bad. I need to know the location of the timer so I can rebuild or replace it. So if someone has a maintenance manual close by, I would appreciate knowing the timer location. This is for the wing deice boots, not the prop boots.
      Thanks Dick Welsh


      Hi Dick,

      Looks like under the pilots seat, located near the stall horn relay and resistor etc. part 133 on attached




        Andrew, thank you for the pics. That is just what I needed.


        On my 421 its is by the pilots left knee. Start shopping for a replacement, mine was quite expensive.


          I found the problem with the surface deice system. It was a broken wire at the timer plug. In the process I believe I acquired another timer that appears to be for a 421 and is slightly different then my 340’s timer although it look the same internally with the same color coded wires. The mounting is slightly different. So if someone need a surface deice timer, I will have a spare for sale.

          In the process I also learned quite a bit about the system. The green light that comes on when you engage the surface deice system, only stays lit for 6 seconds and is connected to the tail boots valve even though the rest of the system is still operating for another 18 seconds and running the wing boots and valves. According to Tony Saxton, Cessna put the green light in the tail system because the pilot can’t see the tail boots operating, and this supposedly assures the pilot that the tail boots are operating. The system shuts down after 24 seconds, and the switch must be reengaged to start the sequence again. Even though the engage switch is a switch/circuit breaker combo, there is another circuit breaker labeled surface deice on the lower circuit breaker panel. The spring loaded engage switch has a spring loaded down position which is labeled “Reset” on the schematic, but I don’t know what it does. I originally thought that the down position (reset) was for engaging vacuum to suck down the boots when the sequence was over, but I don’t see how that can happen after going through the schematic.

          Maybe someone knows about the down position on the switch.


          It must be like your house circuit brakers. If the switch is automaticaly turned off, you have to push down to “reset it” and then to the on position. Otherwise you will not be able to turn the switch on.
          “Reset” means to reactivate the protection.

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