Vacuum Pump annunciator, light problem

[Glen Davis]

Last week, a friend of mine was using my M 20 J and the vacuum pump annunciator light started flashing. He was 600 miles away from home. He stopped at an airport and a mechanic visually looked at the vacuum pump and said it was defective based on some Residue visible around it. He replaced it with a new vacuum pump. My friend flew back home with no issue. Today, I took the plane for a 300 mile trip and about 10 minutes into the trip the vacuum pump annunciator light again started to blink. In both cases the vacuum gauge showed good vacuum pressure.  It was right in the center of the green, which I believe is 5 inches. The vacuum pump annunciator light  continue to blink for today’s flight but the vacuum pressure gauge continuously showed good pressure, in the green. I’m beginning to think I had the vacuum pump replaced last week unnecessarily. One other point I’ll make is during today’s flight when the annunciator light was blinking I turned on the standby vacuum switch, and the vacuum gauge went full scale beyond the green area. I am mediately shut it off. While the auxiliary vacuum pump was on for a few seconds the annunciator light stop blinking but when I shut the auxiliary vacuum pump off the annunciator light started blinking again. Has anyone any experience with this and an idea of what might be the problem? 

[EricJ]

If the gauge showed vacuum, then you had vacuum.   It sounds like the switch for the light is faulty.

[Glen Davis]

Thank you. Do you know where that switch is located?

[ArtVandelay]

If the gauge showed vacuum, then you had vacuum.   It sounds like the switch for the light is faulty.

But if it stops annunciator when auxiliary is on, then maybe the vacuum gauge isn’t accurate or annunciator is a bit off too.
Could be just a leak in the tubing, the AP moving the tubing temporarily fixed it when he replaced the pump.

[EricJ]

1 hour ago, ArtVandelay said:

But if it stops annunciator when auxiliary is on, then maybe the vacuum gauge isn’t accurate or annunciator is a bit off too.
Could be just a leak in the tubing, the AP moving the tubing temporarily fixed it when he replaced the pump.

Could be all of that, but there was some level of vacuum, suggesting that the pump wasn't failed.   When the pumps fail, they typically seize and break the shear coupling (by design) and they won't make *any* vacuum after that.   The presence of some vacuum indicates that the pump was operating.  If the gauge was indicating normal vacuum, then it is more likely that the switch for the lamp was faulty.

 

[WilliamR]

I'm guessing the OP has a KI256 AI. Assuming so, the "switch" is really a pressure bot located off the suction line for the 256. On the pot there's an adjustment screw. They can sometimes cause a false annun. If the vacuum gauge showed 5 and the instruments functioned well, it's likely the pressure pot needs adjustment (1/4 turn of the screw is probably more than needed). I've had them vibrate a little and cause a false annun. Make sure when your mechanic adjusts the pot to not adjust it so the annun. never comes on.

The standby vacuum probably needs some adjustment, too. Assuming the standby is electric, when testing in preflight as required by most POHs does the standby go off scale high? If so, that can probably be easily adjusted by a mechanic. 

I won't get into how wrong it is for a mechanic to condemn a vacuum pump by just looking at the outside when the gauge shows full vacuum and instruments are working fine (they were working, right?). 

William

 

[Guest]

4 hours ago, Glen Davis said:

Thank you. Do you know where that switch is located?

In some cases it’s screwed into the back of the attitude indicator or the heading indicator.

[Glen Davis]

I can’t thank you all enough. This is a very fine detail as part of a complex system that most owners (and some mechanics) would not have detailed knowledge of.   Now I do.  THANK YOU! 

[Bolter]

5 hours ago, WilliamR said:

/snip/

The standby vacuum probably  needs some adjustment, too. Assuming the standby is electric, when testing in preflight as required by most POHs does the standby go off scale high? If so, that can probably be easily adjusted by a mechanic. 

/snip/

William

 

On my old J, it had the common engine vacuum based backup.  Not a second pump on an electric motor.  I think it was from Precise Flight.  A valve was manually opened by a pull control in the cabin porting engine manifold vacuum to the instruments instead of the pump vacuum.  Does require you control your power settings per a chart to be sure you maintain sufficient vacuum.  

A pic is in this article: https://www.aviationconsumer.com/safety/which-back-up-is-best/

As an aside, I pulled the backup pump+motor when pulling the vacuum system from my Ovation, if anyone is interested in an electric backup.

-dan

[Guest]

8 hours ago, Glen Davis said:

I can’t thank you all enough. This is a very fine detail as part of a complex system that most owners (and some mechanics) would not have detailed knowledge of.   Now I do.  THANK YOU! 

From the IPC.  Item 30 is the hi/low vacuum switch, it is adjustable.  The main vacuum pump and standby electric both draw through the same regulator.