vacuum pump

[skymaster]

at altitude red button occurrs .73p pump installed less than 50 hours . short of replacing the pump i would like some help troubleshooting.

[Kevin McDonnell]

The most common reason vacuum pumps fail is heat. RAM made some shrouds to force cooling air across the vacuum pump during flight. By the way, Richard Colins of FLying magazine has a sotry abotu how his P210 went through vacuum pumps at an amazing rate until he installed the cooling shroud. This colved his problem.

A test you might run ...

On the ground, when you start only the engine with the "Bad" vacuum, at something above 1200 RPM, does the vacuum gage show the needle well into the green arc?

Repeat that test using only the engine with a good pump - and is there a difference?

The vacuum manifold has come adjustment on it to set the pressure it's trying to regulate - that might be worth a look.

Of course, leaks in the vacuum lines will certainly make the pump work hard - and possibly exceed it's ability to achieve the desired vacuum pressure.

[Francisco]

Question about Vac pumps. How do I test the rear and the Front Vacum pumps? It seems the indicators work the same when one engine in on and the other is not??????

[SkyKing]

Francisco,

Are you familiar with those two small red buttons on the lower section of your vacuum gage? When you start one engine the associated red button for that engine's vacuum system should disappear. Actually, if the vacuum source is working right the red button 'retracts' out of view and the gage should indicate the value listed in the limitations section of your POH. If by chance you start just one engine and both of the buttons disappear from view, that would suggest a defect in the vacuum manifold check valve which is located above the co-pilot rudder pedals. So, the easiest way to check the system is to start up either engine and look at the gage and the associated red button to see that it retracts out of view. Then shut down that engine and start the other, checking that the other red button retracts from view and you're getting the the proper value. Your Cessna Service Manual has a section on this in the utility/instruments section.

SkyKing

[skymaster]

chks on ground. red at altitude. J

[KyleTownsend]

just thinking "out loud"

If it is not the vacuum pump (which is yet to be determined),

then I would suspect a leak in the line somewhere between where it enters the cabin and where it enters the instrument vacuum manifold.

This would explain why you only see the indication at altitude (assuming we are talking about a P-model).

The theory would be that pressurizecd cabin air is actually leaking into the system.

If this is true, you could fly it to altitude without pressurization turned on to see if you still get the indication. If this theory holds water, the problem would probably go away in unpressurized flight, since you don't get it on the ground.

[big al 08]

just a thought, as alitude increases the vac. pump needs to work harrder because of less air . you see this when the filter get plogged, with a plogged filter the cac. guage starts to read lower and the worse it get the lower it get, until the red button pops out. maybe a clogged filter or maybe an obstruction has occured ie a freggen but???????????

[Francisco]

Dear Skyking, thanks for your responce to my question as to how to check for vacunn on the skymaster, How ever my 336 does not have the red buttons you are reffering to here is a pic ture of the right side of my panel the vaccumm gauges are on the right ( one small One Big)

?????????

[skymaster]

behind dash loose plastic line. next week the 73p is going on a 2700 mile trek north, Later this summer returning and leasing back aircraft for alskan aerial tour. This may appeal to others who would like to get some bang from their stateside schooling. now required for all p's. insurance, J