EGTs and CHTs disappear when fuel pump is on

[DCBrown198]

All of a sudden my EGT‘s and CHT‘s disappear from my EMS display when I turn the electric fuel pump on. They reappear as soon as I turn the pump off.

Previously this did not happen. I did a system software update before this started.

I have a Lycoming IO-540.

Has anyone else seen this?

David

 

[airguy]

That's almost certainly a power/ground problem that is raising the voltage on the engine block above ship ground potential. Carefully check your grounding between the engine and the airframe, and the between the alternator and the engine.

When you turn on the fuel pump it is drawing more current, causing the alternator to pull harder. The alternator is grounded through the engine, and the engine is grounded to the airframe, which provides the return path to the battery. If any of those connections is weak it will cause the engine block (and the EGT/CHT probes) to be at an elevated voltage above ground potential and cause the readings to go "offscale" for Skyview.

 

[DCBrown198]

Thanks for this info…

So following this logic if I first test the system with the engine off (fuel pump is not grounded to engine) and I still get this problem can I excluded the alternator from the problem?

David

 

[djones]

To add what Greg said, I do have a question. Are you monitoring the fuel pump with the SkyView? If nothing is found on grounds and you are monitoring the fuel pump, try putting a 10K resistor in the wire from the fuel pump to the EMS. Especially if you have multiple 12 volt contact inputs such as gear, pitot heat, etc. You may need to change the voltage ranges in sensor setup once the resistor is installed to compensate for the lower voltage seen by the EMS

 

[DCBrown198]

Thanks again Don.

I show in my installation docs that I intended to show fuel pump voltage to Pin 23 (Type C). I'll need to see if I in fact did install that as I later decided against a "Fuel Pump Powered" indication on Skyview. I do have a Pitot Heat status signal input at pin 9 (Type A). If the fuel pump power is presented to Pin 23 I'll install the resistor as you suggested (versus eliminate it...that way it will function later if I do want Skyview to display).

Appreciated!

David

 

[DCBrown198]

Don...I installed the resistor as you suggested. Success!!!!

Much appreciated!

David

 

[airguy]

Don - is that a total-current sink limitation?

 

[hcccs]

That's almost certainly a power/ground problem that is raising the voltage on the engine block above ship ground potential. Carefully check your grounding between the engine and the airframe, and the between the alternator and the engine.

When you turn on the fuel pump it is drawing more current, causing the alternator to pull harder. The alternator is grounded through the engine, and the engine is grounded to the airframe, which provides the return path to the battery. If any of those connections is weak it will cause the engine block (and the EGT/CHT probes) to be at an elevated voltage above ground potential and cause the readings to go "offscale" for Skyview.

I have a similar problem but the other way around. I have an ICOM A200 and when I transmit my EGTs and CHTs go up to maximum. I asked Dynon about this and they said it was "known issue with certain earlier EMS modules". I could also be that "the wiring running in the same bundle or nearby to a coax cable for the com radio".

 

[airguy]

I have a similar problem but the other way around. I have an ICOM A200 and when I transmit my EGTs and CHTs go up to maximum. I asked Dynon about this and they said it was "known issue with certain earlier EMS modules". I could also be that "the wiring running in the same bundle or nearby to a coax cable for the com radio".

That one is a known RF interference issue that can be resolved with some filtering.

 

[djones]

Don - is that a total-current sink limitation?

Yes, we first started seeing it when certified guys started monitoring multiple 12 volt inputs like gear, pitot heat, fuel pumps etc. It mostly showed up with 3 or more 12V inputs to the EMS, but....