REFLECTOR: egt and cht problems

[Joe]

Dean,
If grounding were the problem I would expect variability not repeatability in the error.  I would not expect the nav lights to cause a consistent 50 degree variation; however, EMI (electro magnetic interference) is likely to produce an error proportional to the current amplitude.  I would guess your TC extension wiring is probably in proximity (and running parallel) with power (+ or -) wiring.  A true DC waveform generally will not produce EMI, but the alternator does not produce a true DC waveform and can induce a current into adjacent wiring that is variable with alternator frequency and current.

If this is the problem, moving the TC wiring to a quieter (electrically) path may resolve the problem.  If that is not possible, then using twisted shielded TC wire may be the answer.  If a shielded wire is used, remember to only ground the shield at the instrument end.

Joe
(Industrial Controls, Instrumentation, and Automation for a living)



----- Original Message ----- 
  From: dean fitzbag 
  To: reflector 
  Sent: Sunday, May 01, 2005 3:11 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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