Vacuum pump

[Dale Campbell]

I have a 1978 337H normal asperated with a bad vacuum pump on rear engine. It failed in flight yesterday. Is that one of the items you pull the fire wall from inside the cabin. Also is there a better pump on the market than the original and does it need a 337 form to install. Dale Campbell

[Ernie Martin]

My front one just failed in my 337G. It's a dry pump, model 215cc. After doing a little research I ended up buying a new Rapco pump for around $275 (no core needed) from either Aircraft Spruce or Chief Aircraft.

Ernie

[Jerry De Santis]

The vac. pump is driven through a shear plastic pin type coupling. Before you purchase a new vac pump check the coupling. They cost anywhere from 30 to 75 dollars.

Jerry
N34EC

[WebMaster]

If you have de-ice, you need a wet pump.

[Pat Schmitz]

Correct Larry! (mostly!)

When I read your comments, I knew that my P337 did not have wet pumps - it actually does have Dry pumps with De-ice. Here is why:

From the Cessna Service Manual Page 14-9: The dry vacuum pumps are replaced with oil lubricated pumps and oil separators when the optional de-ice system is installed thru aircraft serials P3370052. Beginning with aircraft serials p3370053 and FP3370001 the vacuum pumps are replaced with larger capacity DRY vacuum pumps when the optional De-Ice System is installed.

So not all 337's have Wet pumps with de-ice - only those prior to serial 0053.

The service manual goes on to say

NO SUCTION INDICATED AT ONE SOURCE:

Probable Cause may be any of the following:
Defective Vacuum Pump - or
Disconnected, broken, or plugged lines - or
Defective relief valve - or
Defective suction guage.
It doen'nt mention the drive coupling - but that is also a great suggestion..

Let us know what you find!

[Jerry De Santis]

Larry, My plane has dry vacuum pumps with deice boots also.

Jerry
N34EC

try http://www.sigmatek.com

2000h or 5 years warranty. its a piston pump. direct replacement to that what is still inside.

[WebMaster]

Sorry, mis information.
I had read in some magazine that wet pumps were preferred for vacuum pump installations.
I have wet pumps in my plane, and I have boots.
Hence, I drew the conclusion that the magazine article was correct, and my plane was an example of that.
So, you can have dry pumps and boots.

[hharney]

MY $0.02

I DON'T HAVE BOOTS, BUT I HAVE DRY PUMPS
I REPLACED SEVERAL WITH REBUILT UNTIL I FINALLY TRIED A NEW AND IT'S STILL PUMPING.

I WOULD DISCOURAGE BUYING REBUILT

[Pat Schmitz]

For what it's worth -

My understanding is that most are not truly 'rebuilt' but rather just recertified. The dry pumps have a carbon graphite based material used for the pump vanes. The graphite is designed to be self lubricating, and is in fact designed to wear away gradually over time.... predictable enough wear rate that they know roughly when they should be about worn out...

Most do not recommend or even support rebuilding the pumps.

Here is a great article on pumps - out of copyright consideration - I'll only paste the 'link'

http://www.avweb.com/news/maint/182905-1.html

Originally posted by Pat Schmitz
For what it's worth -

The dry pumps have a carbon graphite based material used for the pump vanes. The graphite is designed to be self lubricating, and is in fact designed to wear away gradually over time.... predictable enough wear rate that they know roughly when they should be about worn out...

Quite right! If the pumpe suddenly stops because of broken torque link and you have no airfilter between pump and avionics, then the carbon graphite dust will be sucked into your avionics.

From then on, you can count the time for overhaul of suction driven devices in your avionics.

Had this seen in several planes, recommend this cheap filter anyway.

[sunnysky]

my aircraft is a 337g with de ice and the dry pumps. I replaced the front pump some 200 hrs ago with a Rapco. Think the cost was then about $700. Still seems to be running good.
Sunnysky.

[Richard]

Interestingly, I "had" the original pump from 1966 on my airplane. It didn't work, but it pumped just enough to retract the orange button in the suction guage. 2400 hours.

[SteveG]

With regard to Jerry's suggestion to inspect the pump for a failed pump coupling, it seems that the only likely cause for a broken coupling would come from a broken vane causing the pump to seize and in turn causing the coupling to shear, which it is designed to do to prevent engine gear train damage. I presume Jerry must have encountered a broken coupling on an otherwise servicable pump at least once but I'm wondering if others have had similar experience and this condition is more prevalent than I would have assumed. Also, I doubt that if confronted with a sheared coupling on a pump with more than a couple of hundred hours of run time I would elect to repair rather than replace a $300 pump after investing the time and effort to remove it, replace the mounting gasket and reinstall. The difference between the cost of the coupling plus the time to repair and the cost of outright replacement being insufficient to justify reinstalling a pump with half its service life already exhausted. With respect to instrument contamination, this seems highly unlikely in a suction system as opposed to a pressure system. As to a better alternative, Airwolf is now selling an oil lubricated pump. I have no experience with it but I doubt that I could justify the additional cost in a pneumatic system with redundency such as ours although it might make sense in a single pump application rather than installing a backup vacuum solution.

[kevin]

I experienced the broken coupling in my '65 337 a few years ago. In my case, it was as described, the pump was toast too. One reason to mention it is on the rear engine, if you break the coupling, you can end up with part of it in the oil pan. If you can't find all the pieces, it is teardown time for the motor. This happened to me when I was halfway across the country, I didn't sleep well that night, and was very relieved when the mechanic called and said he found it all...

Kevin