Fuel pressure fluctuations when I key the PTT

[bmcconnaha]

Any idea what could cause the fuel pressure gauge to bounce all  over when I hit the PTT.  Intermittent.  Doesn’t typically do it with the engine off.  Not having issues with the radio outgoing issues either, just the gauge bouncing all over.  Started as a small twitch, now it’s all over the place.  TIA 

Edited August 21, 2021 by bmcconnaha

[Hank]

I'd look for loose connection, cracked wire insulation, or what the PTT wire is rubbing against. 

[PT20J]

I'm assuming you have the Signa-Tek gauges with Kulite transducer. If so, the transducer is a wheatstone bridge that gets an excitation voltage of about 10 volts (exact voltage is set during calibration) from the gauge. Any fluctuation in the excitation voltage could cause needle fluctuation. When you key the transmitter, the com radio draws a lot of current. This might cause a momentary fluctuation in the bus voltage and cause the gauge to act up. I would check all the connections for security and then I would measure the voltage at the connector to the transducer when the mic is keyed. Pin 1 is +10V and pin 2 is the return. If that voltage is steady, I'd next try measuring the output of the transducer between pins 3 and 4.

It would also be good to isolate the engine running effect. Two things are different when the engine is running: 1) the alternator is supplying power to the electrical system, and 2) there is fuel pressure. I'd try running the boost pump with the engine off and see if you can reproduce it that way.

Skip

[Bartman]

My digital OAT goes haywire when I push the PTT button, but it has otherwise been solid performer for me since 2006. I got used to it and ignore it when pushing the PTT.