Quote:
Originally Posted by 911T70
That's very interesting information! Don't want to hijack this thread but I'd like to ask a question: the gauge on my 70T has an offset of about 20 psi on the positive side. If I gently tap the sender with screwdriver, the offset disappears but returns once the pressure has increased. I suppose, based on your description, that the potentiometer is sticking. Should I simply replace the sender or is there a way of fixing it?
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Inside of the sender unit is a swivel (for lack of better words) with a dual contact spring pressing against a wire wound resistor. The swivel is pushed up by the pressure building up in the engine, but when the pressure decreases the swivel is returned via a little return spring. You should see the lag when the engine (pressure) is coming down towards idle, but not when the engine is going up from idle. Since it is a mechanical device there is friction in the system and this friction can increase with age. The sender unit (even when new) needs the vibration of the engine to smoothly return from high pressure to low pressure. We had pneumatic cylinders installed on the calibration stations for these senders just to keep knocking the base of the fixture (simulating engine vibration).
The friction in your sender unit may has become too large for the return spring (or your engine is running too smooth). Also, your engine's main vibration is maybe in plane "X" while the swivel of the sender is moving in plane "Y" (perpendicular to the engine vibration). You may get a fix by trying to rotate the sender 90 degrees (if possible without over-tightening and breaking something). On the other hand, it might be time for a new sender (this is what I would do).
Juergen