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Thread: EGT & CHT readings are 'too good'...

  1. #1

    EGT & CHT readings are 'too uniform'...

    I have a dual AFS-3500 system in my RV-7A. I have always had a problem where the EGTs and CHTs for cylinders 1&2 appear to be in 'sync', for lack of a better word, and cylinders 3&4 are in sync. For example:

    EGT1: 1310 degF
    EGT2: 1310
    EGT3: 1375
    EGT4: 1375

    The exact same thing happens on the CHTs, where 1&2 always read the same, and 3&4 always read the same!

    For clarity, the temps appear to be reading correctly, so the sensors are working correctly, but at some point in the chain they get "locked" to each other. Can you help troubleshoot what would cause this? Wiring issue? EFIS settings? Network setting?

    The panel and harnesses were built by Stein, so I have assumed they are not at fault. But I have traced wires and from what I can tell they all look fine (no obvious splice of CHT1 to CHT2 for example). And have looked through EFIS settings but do not see anyplace where the 4 cylinders are treated separately, i.e. no way for CHT1 and CHT2 to both display temp from Sensor1.

    Confirmed with a heat gun on one EGT sensor - the EFIS shows both EGTs heating up in synch.

    Thanks for the help!
    Matt
    Last edited by LR2001; 08-30-2014 at 07:41 PM.

  2. #2
    Senior Member
    Join Date
    Mar 2010
    Location
    gilford, nh
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    333
    if you disconnect one egt probe do you still get the same problem? If so I think it is in the 3500.
    Alan

  3. #3
    Remove the CHT/EGT connector from the back of the EFIS and verify the following with an Ohm meter:

    EGT 1 Test
    Pin 13 & Pin 25 should have almost zero resistance
    Pin 13 or Pin 25 should have an open circuit when measured to ground on the plane.


    EGT 2 Test
    Pin 11 & Pin 23 should have almost zero resistance
    Pin 11 or Pin 23 should have an open circuit when measured to ground on the plane.

    Verify that Pin 13 and Pin 11 have an open circuit when measured between them

    If this all checks out you could send us the screen to test, although I don't think I have ever seen a problem like this caused by the EFIS.
    Rob Hickman
    Advanced Flight Systems Inc.

  4. #4

    Results

    Hi Rob,

    I did the tests you outlined below, and everything checked out exactly as you indicated. The only exception is:

    For EGT-1 test, you say that pin 13 -OR- pin 25 should have open circuit when measured to ground. In my case pin 13 -AND- pin 25 are open circuit when measured to ground.

    Same for pins 11 and 23, BOTH are open circuit measured to ground.

    Does that mean something??

    Thanks,
    Matt

  5. #5
    Not sure what is causing it, if you send us the screen we can test it.
    Rob Hickman
    Advanced Flight Systems Inc.

  6. #6
    Thanks, Rob. I am going to have to send to you for testing. The fall colours are coming out now in Canada so it is prime flying season - let's revisit this when we are grounded by snow!

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