testing a MAP sensor with a meter and vacuum pump?

[lilredwagn]

Searched but didn't find

I would have sworn I saw on here that there was a way to test the MAP sensor with a mityvac and an ohmmeter. This may be electrically impossible, but I never took electrical engineering, and from what little I know, in function, the sensor seems to be acting as a variable resistor so I may be asking a really stupid question. I may even be way off base as to knowing what a resistor even is :D Wouldn't be the first.

I know the FSM procedure, but thought someone had a way of checking the response "slope", as it were, over different pressures, rather than just the "on" and "idle" states.

[5-90]

I haven't done this yet myself (I should...) I could see checking the response of the MAP using an analogue ohmmeter and a vacuum pump of some sort (a Mityvac should serve neatly.)

I prefer to use an analogue to check variable resistance response because digital ohmmeters have "buffer" circuits in them that slow response. You could end up cruising right past a "flat spot" and never catch it - same if you're going to check a TPS. I use an analogue ohmmeter on those as well.

5-90

[lilredwagn]

Update: Tested resistance across all combinations of terminals. Resistance was measured, but applying vacuum had no effect on the reading.

[old_man]

IIRC you have to energize it with 5volts and then read the output. It isn't a simple pot, it has some electronics in it.

[Runnin'OnEmpty]

Old Man is correct, the MAP has to be tested with the connector in place and the key on, by backprobing the wiring. Also, it's output is a frequency modulated signal, so I doubt if a voltmeter would tell you much because the meter's response rate would be too slow.

You could probably tell if it was working, though. Once you identify the 5v power wire and the ground wire, the remaining wire would be the signal wire. The signal will average out somewhere between 0-5v. If it's 0v, then you can assume it's bad.

[xjbubba]

I don't believe the MAP is "frequency modulated". It's basically a variable resistor, sensitive to pressure changes related to barometric pressure and engine manifold pressure. My Chevy FSM gives the following test procedure using an external vacuum source, such as the "mityvac": " Disconnect and plug vacuum source to MAP sensor; connect a hand vacuum pump to the MAP sensor; start engine; note MAP sensor voltage; apply 34 KPa (10" Hg) of vacuum and note voltage change. Subtract second reading from the first. Voltage value should be greater than 1.5 volts." Additionally, for clarification, the FSM also includes the following note: "Applying 34 KPa (10" Hg) vacuum to the MAP sensor should cause the voltage to change. Subtract second reading from the first. Voltage value should be greater than 1.5 volts. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor". The Chevy's MAP reference voltage, 5volts DC, is the same as the Renix, and probably later Jeep MAPs. For the Chevy, the MAP voltage should read 1-2 volts at warm idle, and 4-4.8 volts a WOT.

[Timber]

xjbubba has it right about what a MAP does. It measures ambient barometric pressure when the ignition switch is first turned on and during engine cranking. It doesn't take barometric measurements at any other times, but it does continue to measure that of the manifold.

[Runnin'OnEmpty]

I don't believe the MAP is "frequency modulated".

http://www.picotech.com/auto/waveforms/map_sensor_digital.html

[Timber]

Any electrical signal can be "read" in terms of frequency; that does not make the circuit a frequency modulated one, though.

[xjbubba]

Point taken, however, I beleive the Jeep and the Chevy MAP sensors have this wave form: http://www.picotech.com/auto/waveforms/map_sensor_analog.html

[Runnin'OnEmpty]

So the real question is whether the MAP is digital or analog? Digital MAPs do have a frequency modulated output, but the analogs appear not to.

Are we talking about Renix systems here, or the later model Chrysler electronics?