Relais X clicks when driving my 1991 S4 over a bump in the road.It also happens during normal driving sometimes.I checked the schematics - relay X has a control line 85+/-, a permanent 12v bus 30 from the battery, and pin 50 to the starter.

It is unclear how this works. Can anyone shed a light on this?
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Relay X works as a terminator of the connection. When the starter enages the two sides both sides have 12v and the relay releases.
Relay pin 86 is connected to bus 15. Pin85 on bus 50 makes a connection with the starterrelay when the starter is not activated. Maybe bus 50 has a bad contact and breaks the chain to relay X.

Relais X is a normal 53 standard. You can swap with any simple relay.

A simple short term solution is to bridge Relais X 30-87. With a healthy battery this is no problem. Shut off the HVAC when starting. 

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I swapped relay. Nothing changes.

Next I checked the cable loom and whiggled the ignition lock. - no deal..
I checked the height adjustment for the steering wheel, whiggled the assembly, but .... no deal. No difference.

But the #50 brought a solution. I checked the 14 Pin connector in the engine bay. Ity looked crappy and not tight. Some signs of oxidation. I used some WD40 and plugged-unplugged the connectior ten times or so. I attached a tie-wrap to hold it tightly together. 

I made a testdrive over a rough road and all was perfect again. Problem solved.