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Our AANs (and ABYs and ADUs) came equipped with low pressure oil switches (to trigger a warning light) and, for some, e.g. the UrS4s, a pressure sender that sends a voltage signal to the UrS4 oil pressure gauge. These items are located on the intake manifold (IM) side of the engine, below the intake.
The single pin oil pressure idiot light switch below, will at some point decide to fail and "pee" a stream of oil between the plastic and metal parts. Might be good to have one on hand in your trunk spares box. (NOTE: the UrS6 switch is different PN that the UrS4 switch (see below))
The oil pressure gauge sender (Item 19) below is an UrS4 only thing because the UrS6's got voltmeters instead of oil pressure gauges.
Here is a photo of an RS2 ADU with sender and switch located as above (Note: It is likely that the sender in the upper position has M10x1 threads and the switch is sitting in an adapter):
For most UrS4s, the oil pressure switch is located in the upper location and the oil pressure sender is in the lower location, as shown here:
Unlabeled base photo courtesy of Tom S. of RS2 in America fame
Here is a view of under a pre-January 96 built UrS6 AAN showing the UrS6 oil pressure switch but the lack of oil pressure sender:
Unlabeled photo courtesy of speedydtp247
Here is a view of a later build UrS6 AAN (in this case quattro20v's April 97 built AAN but this is also true of bangatek's March 96 built UrS6 AAN) (Note: the pressure switch is in the lower position, not the upper position, implying that it is sitting in an adapter):
I for one had always seen that "Potential Location" for the UrS6 oil pressure sender in the lower port of first UrS6 photo above as being part of the engine casting. At some point, one of our forum members, Mance (aka VAP)suggested that he could see an Allen-head socket hex there. Several UrS6 owner/forum members checked out their UrS6 engines and posted "Yup, mines got a plug" so I had to seriously revise my thinking. Furthermore, Chris M. (fcarspec1) stated that in that position, where his UrS4 has a sender, when you remove the sender, you find a check valve in the cavity behind the sender. This implied that behind the large hex-head socket plug in the lower position, there was also a check valve.
To investigate this, I went out to my spare UrS4 AAN (Oct. 92 build date) and removed the oil pressure sender and switch. Interesting how small the port into the sender is. It wouldn't take much to plug it off.
Here is what the UrS4 block looked like with the pressure switch and pressure gauge sender removed:
The upper hole for the UrS4 pressure switch is very shallow and there is no way that a check valve could be installed there. Peeking further into the lower hole, you can see the triple square socket head of the check valve (PN 077103175B):
The hole size, i.e. to fit the M16x1.5 thread on the pressure sender was most likely dictated by the size of the check valve rather than the other way around.
The check valve (PN PN 077103175B). The check valve is very common across the VAG line (VWs, Audis, Skodas, etc). For our AANs (and ABYs and ADUs), this check valve is used to keep oil in the head so it doesn't drain down, keeping oil available to the hydraulic lifters immediately on start up, for example. Here is what the check valve looks like:
By following the PN of the adapter (PN 034103179), I was able to find a sealing plug (PN N0160241) in the Industrial motors section that implies that the adapter has M10x1 female threads (because the sealing plug has M10x1 male threads). This further implies that both the lower UrS6 hex plug and the adapter have M16x1.5 male threads. The later UrS6s have the pressure switch located in the adapter in the lower position and a hex plug (probably M10x1 thread) in the upper position (where the UrS4 has a pressure switch). Had enough oil pressure sender and switch trivia for one day?