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spuggy spuggy is offline
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Join Date: Jul 2003
Location: Glorious Pac NW
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The factory manuals say that the manifold pressure limiting switch (overboost protection) should read infinity (open) at 1.5 bar, and 0 (short) at between 1.1-1.4 bar. You can check it on the bench with a footpump and a multimeter- a Schrader tire valve will screw straight on. Remove the valve core and be careful not to overpressurise...

The boost pressure sensor is a separate unit; The boost gauge in the tach will peg at maximum as soon as you turn the key if it is disconnected from this sensor, and should return immediately to zero when grounded. When checked against the connecting lug and housing, this sensor should read between 5 and 13 ohms at 20C (68F) at atmospheric pressure (engine not running).

They both bolt onto the pop-off valve housing after 1978.

I'd guess that you have an electrical issue that is isolating both the overboost switch and the boost sensor for some reason, causing both the tach gauge to peg and the overboost protection to kick in, despite the mechanical boost gauge showing normal boost levels.
'77 S with '78 930 power and a few other things.
Old 04-22-2007, 01:19 AM
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