You need to connect the gauge so that it measures pressure in the large line from the FD to the WUR. You can connect it at either end, but the choice will probably depend on the fittings that come with the gauge. You want the shut-off to be on the WUR side. You close it to measure system pressure, and open it for control pressure.

So connect the gauge. Unplug the electrical connector from the WUR, and remove the fuel pump relay and jumper pin 30 to pin 87. This will cause the fuel pump to run constantly. Let it run for a few seconds and close the shut-off on the gauge to get system pressure, then open it to get your control pressure. Reconnect the the electrical and you should see 3.4-3.8 bar in under 5 minutes.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe