MPC (MAP) Sensor readings.

Hey, I am working on a 1972 Mercedes 280 SL V8. I have a sensor 0 280 100 012 that does not hold vacuum but the resistances are correct. I replaced it with a sensor that holds vacuum and shows proper resistance. I then checked voltages on the car with a vacuum pump. I cannot however get any voltages to change on any wire at all no matter what the vacuum is. How does the ECU read this sensor? I have almost 9 volts on the 2 outer wires, and almost 2 volts on the 2 inner wires. I took the old one apart and manually pressed the armature in while connected to the car. Again, no change in reading on any of the wires. Is there something wrong? I also checked for current on both coil circuits, none at found in any position. I just don't understand if my "good" sensor is actually doing anything.

Thanks in Advance for helping out.

Hi Burton,

our MAP sensors are variable transformers. Transformers do not work with DC voltage. They need a varying signal to work. In our case that is the injection pulse as generated by trigger contacts and air temperature sensor. That pulse is transformed in MAP sensor to different voltage levels depending on armature position. Its ouput is used together with an rpm correction and forms the so called basic injection pulse which is finally extended by a pulse width modulator.

Or just short: You cannot measure it via DC voltage on a multimeter.

Ok, so it there any waveform I can see, or pattern that will change when the car is running that I am able to measure? It sounds as if it directly affects a circuit that is a trigger circuit. Is this correct? So the Trigger signal to the computer is modified by the sensor, to change pulse width of the injectors? If so, can It be measured within the MAP circuit, or is it measured in the trigger circuit? Or can it only be measured in the injection Circuit? It would make sense to me that it would be measurable within its own circuit during injection operation, but will I see it with a Lab scope? I may have repeated myself many times hear, but I am thinking out loud as I write. Thank you for your information thus far. I am a 88+ injection guy up to this point and understand modern systems very well and am well versed in the use of a Lab scope, although I am self taught so sometimes terminology does not make sense to me as I don't have a schooled background. I find this early injection fascinating. It is highly interesting how they worked around these problems to come up with solutions that made these cars run!!

Hi Burton,

you can measure with a lab scope while running the car on secondary coil. You will see that the signal varies with different loads on engine.