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AFS Manifold Pressure Gremlin

Darren Kerns

Well Known Member
I need some help and advice. My RV7 MP reading has been acting up.

I thought my AFS transducer was going bad but when I shut the engine down it goes back up to 29 in. I turned my avionics master on last night and it indicated 3 in. I started it up and the it was showing 6 -9 in. By the time I got down to the end of the runway, it went red which means I lost it.

As soon as I firewalled it, it came back to life and indicated great the whole time while flying. My mechanic and I examined the line running back from the cylinder to the firewall. It was clean. We examine the transducer aft of the firewall and it was clean and connections were good.

I called AFS and they think it could be the transducer.

What else should I be looking at? All CHT & EGT readings are great. Engine is running strong.

I am calling AFS today to order a new transducer. Any suggestions or advice would be greatly appreciated.

Thanks,
Darren Kerns
RV7 N599DT
 
XDUCER

Sounds like you already checked everything. Sometimes the lines will plug and cause that, but you said you checked that. I assume you did clean them out?

Vic
 
Check ground on the sensor
Check all electrical connections
Check the hose for leaks. A friend had his hose go bad after a few years and leaked from a number of small pinprick sized holes.
 
We checked the lines. They looked clean. We also tested them for leaks and did not see or hear anything. We disconnected the transducer and looked at wired ends. They looked good.

I will keep trouble shooting. Thanks guys for the help. I really appreciate it.
Darren Kerns
RV7 N599DT
 
Disconnect the connector and see what the EFIS reads. This is the number when/if you have a bad wire. It sounds like a wiring issue to me.
 
It is either the wiring or the transducer.

1. Try moving the wires at the transducer and see of it fails on the screen

2. Try moving the wires at the EMS connector on the EFIS

3. Replace the Transducer


Rob Hickman
Advanced Flight Systems
 
Thanks Jesse and Rob
Jesse, thanks for talking to me on the phone.
Rob, I ordered a new transducer. I will try checking the wiring to see it fails.

Thanks for all the help and advice.
Darren Kerns
RV7 N599DT
 
Electrons are weird. I recently did some maintenance to my vacuum system and decided to tie wrap some wires for a slightly neater appearance. After I did that, I noticed my RPM indications were erratic (no change in actual rpm, since the sound didn't change). I rechecked the wiring and the transducer and everything appeared in order. Same erratic behavior. Someone at the hangar suggested that the ignition leads could cause interference. The wiring for the RPM transducer was adjacent to one of the right magneto's ignition leads. I went on a test flight and even with the magneto off (grounded) the same erratic behavior continued at high RPM which lead me to believe that the ignition lead was not the cause of the problem. Later I separated the transducer's wires from the ignition leads and the problem disappeared. I would have thought with no energy running through the ignition leads it couldn't affect other wiring, but it did. Electrons are weird.

Jerry Esquenazi
RV-8 N84JE
 
Update AFS MP Transducer

Flew super early this morning. AWOS reported 30.24. EFIS indicated 28.6 before start. Airport 792 ASL. Plane flew great. On landing rollout, MP indication went redline again. Stayed 0 until shutdown then rose to 28.6.

We checked what Rob said & no glitches. Got new transducer but have not installed it.

Is there anything else I should be looking at?
Takeoff was 27.0 in. RPM 2690. Should be getting more MP.

Surely has to be my transducer. CHT & EGT perfect on 1 hr flight this morn.

Thanks for all the help & advice. I need it!
Darren Kerns
RV7 N599DT
 
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