Hawkeye7A

Hawkeye7A

Well Known Member
Not sure exactly where I should post this so, Doug, feel free to move it if you see fit. I fried my (standard Vans) alternator a couple of weeks ago which also took out three other instruments. I replaced the alternator with the Plane Power unit (a TRUE drop-in I might add!) and the three instruments. All systems are go now but I noticed a strange anomaly (which I'm ashamed to say may or may not have been present with the old alt; I never noticed) but when I'm idling or for what ever reason have low manifold pressure indicated, when I press the PTT the manifold pressure slowly climbs to 25-28 (depending on how long the PTT is pressed) and then when released, falls back down to the former reading. All during normal flight operations the MP seems to be working fine. I suspect some sort of ground loop or something. All of my comm audio is shielded cable including the wiring to the PTT button and everything else is working fine. I have no interference with any of the comm or intercom. It's just this one wierd quirk. I have the standard Vans MP gauge with the sending unit. Any ideas, anyone? Thanx.
 
A couple of items to check...

Hawkeye7A said:
Not sure exactly where I should post this so, Doug, feel free to move it if you see fit. I fried my (standard Vans) alternator a couple of weeks ago which also took out three other instruments. I replaced the alternator with the Plane Power unit (a TRUE drop-in I might add!) and the three instruments. All systems are go now but I noticed a strange anomaly (which I'm ashamed to say may or may not have been present with the old alt; I never noticed) but when I'm idling or for what ever reason have low manifold pressure indicated, when I press the PTT the manifold pressure slowly climbs to 25-28 (depending on how long the PTT is pressed) and then when released, falls back down to the former reading. All during normal flight operations the MP seems to be working fine. I suspect some sort of ground loop or something. All of my comm audio is shielded cable including the wiring to the PTT button and everything else is working fine. I have no interference with any of the comm or intercom. It's just this one wierd quirk. I have the standard Vans MP gauge with the sending unit. Any ideas, anyone? Thanx.
Click to expand...

Assuming it's wired like Vans circuits...

http://www.vansaircraft.com/pdf/Gauge_Install.pdf

1. Is the ground lead short (physically a short length) from the gauge to airframe? Make sure it is not shared with any avionics grounds.
2. Is the shield open at the sensor end like the schematic shows, or is it accidentally shorted at the sensor end?
3. Try adding a 0.01 microFarad ceramic capacitor directly across the power terminals (G and I) at the back of the instrument.

All of the above should be easy to check/do....
 
When I transmit in the -10 the oil pressure shoots up to 120 PSI. I moved the antenna coax away from the oil pressure sending wire, and it helped, but is did not eliminate it.
 
Thanx!

Thanx Gil and Larry for the responses. Gil, IIRC I ganged the grounds from my gauges by threes and fours and clustered them to the ground studs in the sub-panel. I'll try clipping the one from the MP and remounting it to one of the mounting screws. I'll also take a look at how close my antenna cable is to the MP sending unit.
 
I've got a similiar situation.

Such a timely post. I was noticing that my oil temp gauge jumps up when I transmit. I jumps about 20 degrees hotter when I'm transmitting. It it close to my radio. I'm going to get under there and see if I can reroute the gauge wiring.
 
Are these the Van's 2.25" gauges y'all are referring to? If so, then I'm afraid you symptoms of moving needles during transmit is a very common issue, and unfortuneately in many cases very little can be done to rectify it other than live with it. Carefull grounding and installation of the audio systems helps, but rarely will make it 100%immune. It's been an issue with those instruments for many moons!

Cheers,
Stein
 
Gila monster's tips are good, but Stein is right on. These are cheese-ball instruments mounted in an EMF saturated beer can. As long as yer steam needles drop back solidly after transmit, just live with it kinda. These really are JCWhitney quality units. I had a VW Scirocco with a CB radio. When you keyed the mike, the windshield wipers cycled. Solved it by wrecking the car.
 
Stein... is it...

SteinAir said:
Are these the Van's 2.25" gauges y'all are referring to? If so, then I'm afraid you symptoms of moving needles during transmit is a very common issue, and unfortuneately in many cases very little can be done to rectify it other than live with it. Carefull grounding and installation of the audio systems helps, but rarely will make it 100% immune. It's been an issue with those instruments for many moons!

Cheers,
Stein
Click to expand...

...an audio issue, or is it the high current taken by the comms while transmitting, or RF pick-up while transmitting?

Sounds like it should be more ground loop or RF related than audio related.

And yes... dumping the cheap instruments would be a good (the best) fix....:)
 
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