How does a tandem pump work?

The vacuum side of the pump is a large oil lubricated vane pump. Air in and air out in yellow, out at the lowest point allowing oil to drain:

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Interestingly the oil supply appears to bypass the pump (purple arrow). I need to work out where that goes to.

There is what looks like a reed valve on the return of air and oil to the cylinder head. Need to check it out.

There may also be a valve in the vac connection.

Also I've found the reason this pump leaked. Diesel has destroyed the O ring where diesel under pressure passes from the outer to inner casing (red arrow). That seal cost me for a set of hoses and a Bosch pump.
 
Vac pump lubrication, the oil enters the pump under the vanes then flows into the vanes via a notch in the central shaft (purple arrows)

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@RAB the seal between the diesel and air sides of the pump does not, as I speculated, prevent diesel passing to the air side. It prevents lubricating oil passing to the diesel side. It looks like the air pump runs at crankcase pressure max (reed valve to be investigated) and oil pressure is higher than crankcase pressure. Conclusion, there is no possible failure within the pump that will allow air or crankcase vapors to enter the fuel supply.
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The air pump is a vacuum pump and will have the lowest pressure of all three. The diesel pump is just a feed pump and will have a low positive pressure; the high pressure pumping is done by the injectors. The oil will just lubricate the vacuum pump; diesel does that for the diesel side.

RAB
 
There is a reed valve on the return of air and oil to the cylinder head. This will prevent crankcase vapors entering the pump. Once the pump has created vacuum it will be held and the pump will not work constantly to keep crankcase vapors out.

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If anybody else has a scrap pump could you check please that the shaft seal is fitted correctly, thank you.

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The air pump is a vacuum pump and will have the lowest pressure of all three. The diesel pump is just a feed pump and will have a low positive pressure; the high pressure pumping is done by the injectors.
Did pick up a 2005 BHC TDI today. I would like to test both the diesel pressure produced by the pump and the vacuum.
Could anyone please advise what acceptable values would be?
Are test requirements for Luk and Bosck pumps the same?
 
Are test requirements for all diesel pumps the same? (AMF, BHC and ATL)
Any idea of what the test values should be ?
 
Brilliant @philward this is a new area to me so I am totally lost.... We (family of five) have now passed 1.200.00km with our A2s and I have until now not had any issue with tandem pumps... as I know about.

I look forward to follow next chapter of your interesting thread 👍
 
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