MPS mystery, huh Options

[r3dplanet]

Last winter I had some fun rebuilding my MPS unit, which is for a '73 1.7 liter engine. The MPS part number ends with 049.

According to the calibration tests on the pbanders website in conjunction with my trusty Meterman LCR55 and MityVac, the unit is perfectly calibrated. It holds vacuum without issue. The readings at 15, 4, and 0 in.Hg all match perfectly. And I mean perfectly. No variation from what the LCR55 shows to the pbanders table.

The mystery is that when I hook it up to the car the engine won't start. It tries to start and I can sort of force to keep going by blipping the gas, but it won't idle on its own. However, when I install another verified trashed out unit, the car starts right up and idles and climbs smoothly.

Um. Huh. I'm hoping that there's some other adjustment elsewhere in the system that my tired brain won't remember. Somehow maybe my car is adjusted to work around the (leaky) old MPS? Or maybe I didn't rebuild the MPS correctly. But it was in and out of the thing for days and I'm convinced the physical operation is correct. But I'm always willing to be wrong.

Confused.

-m.

[stugray]

I have never messed with one of these things, but I am curious.

The MPS has 4 wires, so I assume that one pair of wires is an AC signal that "stimulates" the circuit (called a Primary winding).
Then there appears to be a moveable piece (assumed to be an iron core), and another "sense" coil (the secondary).

If I am understanding how this works, the ECU sends a AC signal into the Primary and senses the amplitude of the signal on the secondary. By moving the core in & out, the amplitude on the secondary varies in amplitude and the ECU can determine the position of the core (and therefore the diaphram).
In my business we call these a LVDT (Linear Variable Differential Transformer).

So my question is: How can you calibrate this with just the inductance meter?
It seems that you would need a known input signal to the primary, then an osciloscope to check the amplitude of the secondary?

What am I missing? If there is any variation of the primary winding between one unit to the next, then just calibrating the secondary will not give consistent results to the ECU

Stu