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testing a MAP sensor with a meter and vacuum pump?

lilredwagn

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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.
 
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
 
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.
 
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.
 
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.
 
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.
 
Any electrical signal can be "read" in terms of frequency; that does not make the circuit a frequency modulated one, though.
 
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?
 
See post 6. You just back probe the signal output wire from the MAP and the battery or Map sensor ground wire to the DC scale on multi meter. The FSM lists the DC voltage versus Vacuum reading of the vacuum tool. Those values are posted in many old threads here. I think the old Lundford (sp?) site has the data listed as well, and no doubt Cruiser54s new site lists it too as well as the wire colors. It does need the 5 volt ECU input voltage.

I may be wrong, but I thing it is nothing more than a variable resistor. Or at least it acts like one when tested with multi meter per the FSM. It is a likely a simple cheap pressure-force gauge.

I think this is the GM one used on all XJs and the data should be the same too.

http://www.fastfieros.com/tech/map_sensors_and_how_they_work.htm
 
Reviving this thread for an answer to the question above.

http://www.allpar.com/fix/codes/sensors/MAP.html

"The MAP sensor is a strain gauge which changes its output voltage based on the amount of vacuum or boost it is connected to. This vacuum/boost signal moves a silicon diaphragm inside the MAP sensor. This diaphragm is thinner in the center than it is around the edges and is mounted with a vacuum below the chip. A steady 5v supply is provided by the ECU, so the voltage from the MAP shows the relative pressure of the manifold.

When vacuum or boost moves the diaphragm the resistance inside the strain gauge changes, altering the voltage it produces (within a range of .02 volts to 4.94 volts). The ECU uses this voltage to regulate fuel delivery. This chart describes the pressure-to-voltage relationship of the stock Mopar Turbo II MAP two-bar sensor:"
 
I think this is the GM one used on all XJs and the data should be the same too.

http://www.fastfieros.com/tech/map_sensors_and_how_they_work.htm

I'll second what EcoMike says here...at least the early Mopar MPI setups (the one I'm familiar with) appears to use the GM-style "one bar" sensor. This is pretty common on a wide range of non-boosted engines. (There is also a 2 and 3 bar variation of the same sensor for engines with boosted induction).

(I've been researching this in another emissions-related thread, so I "have my head in it" so to speak).

The output of the sensor is remarkably linear with changes in pressure, and it's pretty easy to check with a handheld pump per the data shown on the site he links.

The caveat to this is altitude.

If you test with a vacuum pump, the vacuum pump is showing you the difference between vacuum and the local air pressure, which will change with altitude (and, to an extent, weather). Therefore, the vacuum pump will show you relative pressure, not absolute pressure. This will show up as an consistent error across the entire range of outputs. In order to correct the data for the local air pressure, you'll need to know the local barometric pressure, and test elevation. This can be used against the published data to provide a correction factor for your vacuum test.

I'm going to post some data on this when I can...as soon as I can get my own running right!
 
While I am not sure of this, but think I may be, the published test data should be based on what is called in the chemical engineering world "Gauge Pressure" (You referred to it as relative pressure, maybe a meteorological weather term?) as we all use "gauge" pressure measurements to test our sensors. I have never seen, had or heard of a bad Jeep MAP sensor. Especially one that has a bias error? Maybe one that has a hole in the diaphragm is possible on some rare bizarre accident, or a wire inside breaks and it dies completely.

Therefore correction for altitude should no be needed. But then again, if the MAP sensor is actually reporting true Absolute pressure, then I am all wet LOL. But I suspect it too is only reporting gauge pressure.

I know the Renix system used a modified CPS for high altitude corrections. And in closed loop the O2 sensor keeps the mix near stoich, not the MAP. The MAP is there to reprt sudden violent changes in air density during accleration and deceleration and the Delta Pressure the ECU want s to know, not the absolute pressure.

At least this is my current thinking. But do not take my word for it.

I'll second what EcoMike says here...at least the early Mopar MPI setups (the one I'm familiar with) appears to use the GM-style "one bar" sensor. This is pretty common on a wide range of non-boosted engines. (There is also a 2 and 3 bar variation of the same sensor for engines with boosted induction).

(I've been researching this in another emissions-related thread, so I "have my head in it" so to speak).

The output of the sensor is remarkably linear with changes in pressure, and it's pretty easy to check with a handheld pump per the data shown on the site he links.

The caveat to this is altitude.

If you test with a vacuum pump, the vacuum pump is showing you the difference between vacuum and the local air pressure, which will change with altitude (and, to an extent, weather). Therefore, the vacuum pump will show you relative pressure, not absolute pressure. This will show up as an consistent error across the entire range of outputs. In order to correct the data for the local air pressure, you'll need to know the local barometric pressure, and test elevation. This can be used against the published data to provide a correction factor for your vacuum test.

I'm going to post some data on this when I can...as soon as I can get my own running right!
 
So was the Renix MAP a one bar or two Bar MAP? I need to dig out my FSM to see. Had not thought of the turbos issue. I think the link I posted, or one of them was 2 BAR?

I'll second what EcoMike says here...at least the early Mopar MPI setups (the one I'm familiar with) appears to use the GM-style "one bar" sensor. This is pretty common on a wide range of non-boosted engines. (There is also a 2 and 3 bar variation of the same sensor for engines with boosted induction).

(I've been researching this in another emissions-related thread, so I "have my head in it" so to speak).

The output of the sensor is remarkably linear with changes in pressure, and it's pretty easy to check with a handheld pump per the data shown on the site he links.

The caveat to this is altitude.

If you test with a vacuum pump, the vacuum pump is showing you the difference between vacuum and the local air pressure, which will change with altitude (and, to an extent, weather). Therefore, the vacuum pump will show you relative pressure, not absolute pressure. This will show up as an consistent error across the entire range of outputs. In order to correct the data for the local air pressure, you'll need to know the local barometric pressure, and test elevation. This can be used against the published data to provide a correction factor for your vacuum test.

I'm going to post some data on this when I can...as soon as I can get my own running right!
 
Thank you guys so much for such detailed and in depth explanations.

Naxja is a wealth of knowledge and i appreciate everyone that takes the time to answer questions.

Merry chritsmas to all and to all a good night!:cheers:
 
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