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.

This is the 2nd time I've seen this happen

I don't know the answer though
I know to those reading along it sounds impossible
but on the other car I witnessed this
the MPS held zero vacuum and ran fine
a REAL NOS brand new, never been used MPS was sourced still in the bosche packaging... it held vacuum and although not tested on a meter it was assumed to be calibrated correctly since it was truly NOS.

with the NOS unit plugged in the car would not start

the NOS unit was plugged into my djet car and ran perfectly as part of the test...

I do not know the solution though
brant

The calibration values need to be indexed for atmospheric pressure based on elevation from sea level. And is that the correct meter, seems to me its not 'Meterman'

I was hoping you would chime in.

The Meterman is a Wavetek. Wavetek is the manufacturer, Meterman is the model.

Also, I live at 1017 feet above sea level so I don't think I should have to reindex the values. The pbanders values were taken at 1280 feet. It should be close to enough to start and run the car to my mind. Good thinking though.

-marcus

What all did you do during the rebuild?

I would run the calc for the altitude elevation just top be sure.

Brads calibration numbers were derived by characterizing known NOS or excellent condition MPS units. My experience has been that different identical PN NOS MPS units have different calibrations right out of the box and the numbers can be different than Brads numbers. My experience is that Brads numbers seem to result in lean conditions and richening them up is beneficial, or maybe its just the specific engine I am running requires a richer setting due to age or wear. I cannot say for sure because there are many factors to consider. When I have the opportunity, I intend to do more research on this with the EFI Associates testers that I recently aquired from the Cap'n with their pulse width monitoring capabilities and hopefully derive better calibration information.

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

I did mine in Forest Grove.

Same meter and same results.
The car did not really run good until I set the AFR down to 12.75.

After the LCR adjustment. It was running an AFR of almost 18.
Once you bring it down under 15 the engine comes to life.

I'm almost a year after these setting. Runs and starts almost like a new car.
There's are those few minutes between were the warmup is over and it gets completely up to temp that it studders a bit.

There is no AC in a 12VDC electrical system Stu. The MPS translates intake manifold vacuum through a mechanical diaphragm into a determinable and adjustable electrical signal (inductance) as one of several sensor inputs to the ECU which then determines injection pulse width for any one combionation of momentary engine characteristics. Brad Anders page has the operational theory and calibration specifics.

Jeff,

I just read the documentation that Paul Anders provides:

http://members.rennlist.com/pbanders/manif...sure_sensor.htm

The device IS a Linear Variable Transformer just as I suspected.
AND you can certainly have AC signals in a DC car.
However Bosch "cheated" and used a pulse as the stimulus signal instead of a sinusoid.

So the device works as I thought. I read the calibration procedure, and while I see how this works most of the time, I can also see how it can fail to work some of the time as well.

So "into a determinable and adjustable electrical signal (inductance)" is not 100% correct as the characteristic of the MPS that changes with pressure is the "transformer ratio" between the primary & secondary.

The "proper" way to calibrate such a device is as I stated above: Inject a known signal into the primary winding while measuring the amplitude of the secondary.

Paul's method obviously works for most of the units, but the variables in the primary (which is not checked other than resistance) could be causing some units to work while others do not.

Stu

Hey Stu,

Sounds like you are deeper into the theory and electronics than me, I am mostly a wiring guy with a casual interest in the electronics. No worries.

I assume the 'known signal into the primary' means setting the vacuum at known levels, then adjust the secondary inductance. I think there is more to it than that though. Brad has suggested that each MPS was custom calibrated to each engine with its specific sensors and ECU, which could affect the actaul calibration of each MPS, and make each one unique.

I intend to research this more carefully when I get the chance to spend some time with the new-to-me EFI Associates analyzer I picked up from the Cap'n that displays actual injector pulse width. Then I should be able to generate improved calibration data with a more precise degree of precision fo each engine.

Geoff..? Yo Geoff you out there?

The diaphragm is probably fine, you probably have a problem with your board on the mps. Oh wait, how is your CHT? hooked up, right reading at cold???

sound strange I know, but have you looked at your TPS (Throttle Position Switch) and verified that the contacts are clean. Also if you mess with the idle screw or the brain box adjustment will it idle at 1200-1400 RPM?

No, what I mean is that you would inject a sinusoid drive signal into the primary side inputs and look at the amplitude of the signal that comes out of the secondary, then adjust the vacuum like you did before to the three vacuum levels.

Of course the experiment would need to be done with a NOS MPS just like Paul Anders did.


Sorry to sound like a nerd, but I helped test the board that measures the LVDTs that are used to focus the Kepler spacecraft telescope. They can meause linear changes at the micron level.



I just measured the CHT sensor and determined that it is a type 2252 Thermistor if anyone is interested. That means it should read 2252 Ohms at 25 degrees C.

Stu

there are 3 different models of CHT's each with different calibration

Guess why they use a primary and secondary winding is that the signal is then "amplified" and easier to accurately measure (?)

/Lars S