REFLECTOR: egt and cht problems

[Sid Knox]

Dean, 
You have got me to wondering if my EGT/CHT instrumentation ( Vision Micro Systems EPI 800) could be suffering from a similar ailment.  When I bought the Velocity a few years ago, on my first night flight the EPI 800 gave me a low battery alarm (and showed about 11.5 V) which certainly will get one's attention!   This was traced to an improper grounding technique at  installation such that with the position and strobe loads, the analog input to the voltmeter had an unwanted offset added to it.  I  wired the DC return as it should have been and the voltmeter problem was solved.
However, I am trying to remember, but I think my EGT and CHT probes are of the "grounded" style also and if so, my temperature readouts may have a similar error.   Maybe many of us have the same problem and don't know it.  How could I not know it?
Here is my thinking:  
1)  almost all of my flying is in the daytime and thus no landing or position light load.
2)  gear pump runs for a few seconds only.  When the pump is running, I am looking at the gear lights, not the EGT/CHT numbers.
3)  when landing at night, there are too many higher priority tasks than watching EGT/CHT temps.

So, next time I go fly, I will make a point to see if my EGT/CHT readout system suffers as yours.

Sid Knox
Oklahoma

Velocity    N199RS
Starduster  N666SK
KR2         N24TC
W7QJQ






----- Original Message ----- 
From: "dean fitzbag" <dfitzbag at msn.com>
To: "reflector" <reflector at tvbf.org>
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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