AI failure or vacuum system failure?

[Adi]

Hey folks,

 

I have a 1983 M20J with two G5s and a KG-258 mechanical AI driving my ancient KAP-150 autopilot. I also have a CGR30P/C engine monitor combo with a vacuum pressure indicator.

During a flight a couple of weeks ago, the KG-258 started misbehaving and showing more and more erratic indications. Seeing that the vacuum gauge was reading green (albeit lower end of green, around 4.7"), I assumed it must be an instrument failure.

I purchased a KG-258 from a fellow Mooneyspacer who had it removed for a full panel revamp and it was working when removed (I fully trust that, and they've also offered to refund me the purchase since it's not working). However when installed in my panel, this KG-258 also behaved incorrectly.

It's possible that it may have been damaged when removed, in transit, or when installed, but barring that I want to poll the audience in case there's something I could be missing.

Can I fully trust the digital vacuum gauge indicator or could it perhaps be installed incorrectly? From what I remember the original panel did NOT have a vacuum gauge at all so the CGR installers may have manufactured a T-splitter and put the gauge on one side of the split, I wonder if a leak on the side that goes to the instrument could cause lack of vacuum pressure but still have that undetected on the gauge side?

Is 4.7" - 4.8" of vacuum pressure normal for other folks? I looked back at my previous engine monitor data and it's always been like this since the CGR was installed.

Thanks,

Adi

[Adi]

The installation instructions for the CGR-30P are here: https://www.aircraftspruce.com/catalog/pdf/CGR30PINSTALL.pdf

Page 22 of the PDF ("13" in the footer) shows that the gyro vacuum pressure line is supposed to be connected to one port of the sensor.

[A64Pilot]

I believe your aircraft has a low vac alarm in the annunciation panel?

If that goes out at taxi RPM then you should have sufficient vacuum to run the system, but if you really want to know I’d plumb in a vacuum gauge (doesn’t have to be an airplane one) directly into the tube that attaches to the AI, that way any leak will show up.

[EricJ]

35 minutes ago, Adi said:

Is 4.7" - 4.8" of vacuum pressure normal for other folks? I looked back at my previous engine monitor data and it's always been like this since the CGR was installed.

That would be reasonably normal suction.   The generic vacuum/suction gauges that AS sells are marked for a green band from 4.5"-5.4"

https://www.aircraftspruce.com/catalog/inpages/uma1suctiongauge.php