Testing and Adjusting Manifold Pressure Sensor

As you can probably guess from my last post, I am pretty frustrated that there is not a way to properly test the manifold pressure sensor. I want to be able to tell in the future if the one that’s working now has changed its operating characteristics, so I came up with a method to test.

Up to this point the only information I had for checking the operation of the MPS was:

The Jaguar service manual has these resistance tests.

Many websites suggest checking if the MPS holds vacuum. Mine held 0.5 Bar with no leakage

Jaguar’s dealer training documentation has this description for symptoms of a sick MPS.

But nothing I found led me to definitively know the MPS was the issue. It was time to learn more about these little devices.

Manifold vacuum changes (as engine load changes) result in the iron core armature sliding through a primary and secondary winding. This changes the inductance, which in turn signals the ECU to change injector open time that leans out or makes the air/fuel mixture richer.

I didn’t have any way to measure inductance, so I bought this simple LCR meter and started testing.

Measured inductance across terminals 7 – 15 and 8 – 10 while changing vacuum at the manifold pressure port.

There was a definite difference between the good and bad sensors…….

….so I wondered if the bad sensor could be adjusted to work again. To get to the adjust screw on these type 3 sensors, you first need to heat up the plastic cap in some hot water.

It then expands enough to pop off the end and expose the adjustment screw.

I then adjusted the screw until none of the readings would cause any leaner operation than the good sensor.

Adjusting so the coil across the 7 & 15 terminals never crossed the good sensor line into leaner territory…..

…..made very little difference to the readings on the other coil (8 & 10 terminals).

I was curious if this was an improvement over how the car was previously running with the bad sensor, so I put the adjusted sensor back in the car to test. The car still misfired on low RPM acceleration with this sensor, so the 8-10 coil needs to run closer to the green line.

This testing is interesting, but probably not useful for the rest of you unless you live at 6,069 ft elevation, or can compare your readings to a known good sensor.

If any of you suspect your sensor of not working, and would like to send to me in Denver, I would be happy to compare your readings to the working unit. It would need to be the same unit, Bosch part number 0280100123.

Now I feel like I have control again…..and have a way of testing this unit if the car starts running rough.

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