REFLECTOR: egt and cht problems

[Ron Brown]

My Grand Rapids Technology instructions are similar with the addition that the GRT itself is to be grounded at the engine as well - not at the instrument panel.  

Multiple grounds at different ends of the plane could cause the problems you are describing.

Ronnie

----- Original Message ----- 
  From: dean fitzbag 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Sunday, May 01, 2005 7:53 PM
  Subject: Re: REFLECTOR: egt and cht problems


  The instructions are to be sure to ground the instruments to the engine, not the panel.  The reason is that the probes have a ground to the engine.
    ----- Original Message ----- 
    From: Douglas Holub 
    To: Velocity Aircraft Owners and Builders list 
    Sent: Sunday, May 01, 2005 4:13 PM
    Subject: Re: REFLECTOR: egt and cht problems


    It sounds to me like a voltage drop across the main ground wire between the battery and the panel. It would be easy to check. Just measure the voltage between ground at the panel and battery ground and see how much it changes as the current load changes.  A few tenths of a volt could give you the error you are seeing. You could get that much error even if all of the connections are clean and solid, depending on the gauge of your main ground wire between the battery and panel. I don't know what's inside the JPI temperature probes, but if the voltage across the probe goes up when the temperature goes up, then the error is in the right direction. Ground at the panel will be a little higher than ground at the battery, so the JPI instrument will think there is less voltage across the probe than there actually is.  Running a seperate ground from the temperature probes to the panel ground should fix it.

    Doug
      ----- Original Message ----- 
      From: dean fitzbag 
      To: reflector 
      Sent: Sunday, May 01, 2005 2:11 PM
      Subject: REFLECTOR: egt and cht problems


      I have JPI slim line instruments.  They are a egt, cht, outside air, oil pressure, oil temp, tach, ameter-voltmeter, and fuel pressure.  All are connected to my master panel for the plus side.  All except the egt and cht are grounded to the master panel ground.  The egt and cht are grounded directly to the engine.  All is fine on the ground and in the air provided that no large current is flowing out of the alternator.  If I turn on the gear pump, nav lights, strobe lights, landing lights, electric fuel pump or if the battery needs recharging, then the temperature read outs of the egt and cht will read low.  The more current, the lower the reading.  There is no problem on the ground since my engine isn't running fast enough to drive the alternator output high enough.

      The JPI folks insist that I must have the ground on these two instruments somewhere other than the engine.  I have a direct wire connection.  The voltmeter reads 13 to 14 volts.  The ameter reads 2 to 3 amp on the ground, normal 8-9 in the air without the additional load of the above devices, and up to 15 and above with them.

      I have the factory wiring system.  The engine ground and panel ground meet at the battery.

      The cht will drop about 50 degrees with the nav lights, and another 100 degrees with the landing lights.  The drop seems to be proportional to the extra current.

      Any ideas?


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