Here's a good article from GM's techlink newsletter:
http://www.gmtechlink.com/images/iss.../TLMar03e.html

The MAP sensor is an important part of the modern engine control system. When asked, “What does MAP mean?” most technicians could correctly answer, “Manifold Absolute Pressure.”

The next question, though, would stump most.

What is absolute pressure?
In absolute measurement, the zero point (where the measuring device indicates zero) is an absolute zero pressure. That means no pressure, or in other words, a 100% vacuum.

The pressure gauges I have indicate zero when no pressure is being measured. Isn't this absolute zero?
No. Most pressure or vacuum gauges indicate zero pressure when not connected, or when there is no pressure or vacuum being measured. However, there actually is pressure -- the atmospheric pressure that surrounds the earth.

You mean barometric pressure?
Yes, even though your pressure or vacuum gauge may indicate zero, the atmospheric or barometric pressure is always present. Conventional gauges always measure gauge pressure.

What is gauge pressure?
Gauge pressure has its zero point at the current barometric pressure (fig. 17). Everything above barometric pressure is called pressure and everything below barometric pressure is called vacuum.



A - Gauge Pressure Zero indicated here
B - Absolute Pressure Zero indicated here
C - Current barometric pressure
D - Atmospheric Pressure
E - Vacuum
F - Perfect Vacuum
G - Operating Range of Standard Pressure Gauge
H - Operating Range of Standard Vacuum Gauge

Conventional pressure or vacuum gauges are constructed to measure gauge pressure to keep the cost affordable.

An absolute pressure gauge is bulky and expensive. Laboratory-grade devices that measure absolute pressure cost over $1000.

Tell me about atmospheric, or barometric, pressure.
The two terms are interchangeable. Atmospheric pressure at sea level on a standard day is approximately 14.7 pounds per square inch (psi), or 29.9 inches of mercury (HG), or 101 kilopascals (kPa), or 1 Bar.

These various standards differ only in the units of measure used to express them.

Does atmospheric pressure always stay the same?
No. Two factors can make the atmospheric pressure vary. First, at an altitude above sea level the atmospheric pressure goes down, because the density of the air goes down.

Second, weather or climate can change the atmospheric pressure -- high pressure or low pressure days. This is why the standard sea level atmospheric pressure is listed as being on a standard day.

How do my conventional pressure or vacuum gauges act at various altitudes?
They react the same at high altitude as at sea level, which is exactly the point we are getting to.

Conventional pressure gauges have no way to compensate for different altitudes or weather changes. They will indicate zero either at sea level or at the top of a mountain. However, the atmospheric pressure is certainly different at these two extremes.

Why is this atmospheric pressure measurement so important?
The air in the atmosphere contains oxygen. An engine burns a mixture of oxygen and fuel. For an engine to burn efficiently, it has to have just the right mixture of fuel and oxygen.

To determine the correct air/fuel mixture and the correct ignition timing, the PCM must know the atmospheric (BARO) pressure. If the PCM is to compensate for changes in altitude or weather, it must have an input signal that reflects these changes in atmospheric pressure.

The Manifold Absolute Pressure sensor does this?
Yes. And, on engines that do not have a Mass Air Flow (MAF) sensor, the MAP sensor signal is also used by the PCM to calculate engine load -- how hard the engine is working. This is called the speed-density method of calculating engine load for engines without MAF sensors. It is because of this engine load calculation for speed-density engines that the accuracy of the MAP sensor signal is so critical.

On OBD-II engines, the MAP sensor signal is also used for EGR diagnosis.
What are the normal ranges of the sensor's output voltage?

The most common MAP sensor generates an output voltage between 0 and 5 volts, depending on the pressure being measured. It must be able to measure atmospheric pressure at the lowest elevations, which in some areas is slightly below sea level. The standard atmospheric pressure at sea level is about 101 kPa. In the Death Valley, Utah, which is below sea level, the atmospheric pressure can be higher than 101 kPa. At the top of Pikes Peak mountain in Colorado, which is more than 14,000 (4,267 m) feet above sea level, the baro pressure is less than 65 kPa. So, the MAP sensor must have a measurement range of 105 kPa to about 15 kPa.

How does the MAP sensor measure pressure UP from absolute zero?
Imagine two glass jars glued together at the open ends, with a flexible membrane sealed between them. Drill a hole into the bottom of each jar, and glue a tube into each hole. Now, connect a powerful vacuum pump to one of the tubes.

When the vacuum pump removes ALL the atmospheric pressure from the jar, seal the tube, trapping the vacuum in the jar. The flexible membrane will be pushed in towards the vacuum chamber jar by the atmospheric pressure in the open jar.

The vacuum jar has absolutely no pressure in it, so it becomes the absolute zero reference point.

Any pressure on the atmospheric side will push the flexible membrane in, but higher pressure will push it in further.

Remember, high pressure in this case equals atmospheric pressure, about 101 kPa at sea level.

Now, attach a hose from the intake manifold of your engine to the open jar. Devise an electrical circuit to measure how far the membrane flexes, and you have the basic idea of how a MAP sensor works (fig. 18).



A - Hose fitting to manifold
B - Thin silicon diaphragm
C - Reference pressure chamber (Absolute vacuum, zero pressure)
D - Pyrex glass
E - Sensing resistors on silicon diaphragm

When would I ever measure a reading as low as 15 kPa?
The sensor is called a manifold absolute pressure sensor because its sensing element is connected to the intake manifold, either through a hose or a direct mount. When the engine is not running, the pressure inside the intake manifold is equal to atmospheric pressure, and the PCM will use this "engine not running" MAP signal as the BARO reading.

A running engine acts like a large vacuum pump. When the throttle is nearly shut, the pressure in the intake manifold is very low -- as low as 15 kPa at a high-speed, closed-throttle deceleration. As the throttle is opened, the pressure inside the intake manifold increases because the atmospheric pressure outside the intake manifold is rushing in, limited only by the engine's throttle blade opening.

The accompanying chart shows that low manifold pressure (engine idling) equals low MAP output voltage, and high pressure (engine at WOT or not running at all) equals high MAP output voltage.



What is the function of the three wires leading to the MAP sensor?
One of the wires provides a precise 5 volt power supply from the PCM. Another wire provides the ground circuit, grounded only through the PCM. The third is the signal wire, carrying the signal voltage generated by the MAP sensor to the PCM.

 

Wow Mangus, that is definitly some good reading material.

 

Do I smell a sticky

 

Great article on the function of the MAP sensor,I dont mean to hijack this post but I have a question reguarding the mountiong position of the sensor. I posted in the TPI section before I found this post
Reading the Painless Wiring, Wire harness Installation Instructions, on line at www.SummitRacing.com on page 16 in the instruction book, page 20 on the Adobe viewer. para B. at the bottom the warning, Important: The MAP sensor needs to be mounted with the hose nipple pointed down.
Anyone know why?
How important is the position?

 

I'm going to guess it has to do with the position of the diaphram. The effect of gravity on the mass of the film is taken into account in the (MAP sensor's) calibration. Mounting it in any other orientation changes the readings.

Just a guess.

 

Thanks for the quick reply Mark,
I was thinking maybe it is to prevent moisture collecting and causing damage from frezing. Anyway thanks for the response. I will follow the recomendation.

 

I wonder if the same applies to a 2 bar sensor?

 

Well, you guessed right.
It's all least effected by lateral G's, and straight line accleration, decclerations, mounted *flat/ nipple down*.

Look at some data logs on a car that has really good 60' times, with the sensor mounted incorrectly, and you can see rich or lean spikes, depending on which way the diaphram is oriented.

 

Orientation?, yes.

 

I didn't do this, for looks.

ie, mount the MAP to the manifold.

 

GM's been doing that since what, 1992 with the Lt1 vette?

 

Mangus, good post on that article.

actually, everyone is right about the mounting position of the MAP. in the early days of computer controlled carbs, its mounting position didn't make much difference as far as it being as accurate as possible, those systems were just not that precise. but, if it was not mounted pointed down, condensation would damage the sensor, & depending on where the MAP sensor's vacuum line was hooked to, it could get filled with liquid fuel if the float got stuck in the carb. when EFI came along the need for it to be more accurate became greater, & even more so with the later systems.

one other note, where the MAP is located at in relation to the height of the intake is also important.
it needs to be no lower than the intake, the higher the better. Mopar found that out back when they mounted their MAP sensors on the ECM located in the passenger side kick panel. they had alot of problems with them from the fuel vapors condensing in the hose & killing the sensor.
where Grumpy has his mounted at is just about as close to prefect as you can get.

 

Turns out DENN_SHAH is correct. This is from the Delphi text on the subject:

Recommended mounting procedure: port pointed down within 30 degrees of vertical. This will ensure that any condensate or liquid in the line will drain away from the sensor cell. The pressure sensor should be mounted higher than the vaccuum source to assist in draining the line

Turns out there are some smart people here! Who knew?! Guess I'm just a crack smoker with my gravity and diaphram weight theory. =)

 

oops, double. sorry.

 

What welding up intakes, and remounting the MAPs to be direct reading?. Which is why I bothered to show folks what could be done, if they're car wasn't that way originally.

BTW, have you relocated one, like this?.

Ya, things can be blantantly obvisious once someone splains it.

 

Should I mention the higher the better is extremely important when running an Alky system. As a *rule*, thou, any line connecting the MAP to the Plenum, is too long, and allows for condensation.

You can have a drop of moisture form (in the line), and then just have it *stick* there, and it'll do an excellent job of dampening the MAP signal.

Magine that.. <G>

 

Ah, that was your point. I thought you were trying to say, "hey look at what I came up with" when infact GM beat you to it . But I was wrong.
No I haven't relocated a MAP to the top of a welded up GN intake but I have relocated enough MAP sensors. The 2 on our FSAE car the years I was engine team leader we had the MAP sensor right in the middle of the plenum but had some trouble sealing it since it was carbon fiber and only about a mil thick... so we used a piece of rubber about twice the length of the nipple on the sensor itself and used zip ties to ensure it's sealing... worked great. On my car it's TBI, it's a bit harder to do what you and GM did so I just relocated the sensor above the fuel rails on the back of the TBI. The hose connecting it to my adaptor plate vacuum bung is about 4" long and curves upward.
Now riddle me this, the Honda F4i engine's use a throttle for each cylinder and still runs speed density! Their MAP sensor is about the size of a jolly rancher and it branchs off to 4 rather soft rubber tubes that go to each intake port. Why do you think the did it that way? I'm curious.

Blantantly obvious? Yeah, I credit GM for that one. Just looking at the Lt1 engine bay there are more reasons they mounted it on the intake than are explained in this thread. It's definatly an eye sore but I'm guessing, they probably saved over $100,000 or more by not having to make a bracket and tube to relocate the MAP sensor. That's a big reason why when you're talking about why the big 3 do anything.

 

Assuming, does that. It's often better to just ask a question(s), rather then to guess/ assume. Not to mention that I'd already given some of reasons for doing that. [*some*]

While budget is a concern, when talking emissions, and CAFE numbers, they can alter what looks like an economic discision.

 

not really, on most factory cars it doesn't matter, but as Grumpy pointed out, you get a car that launches hard & it can affect fueling.


haha, i have seen that. it really causes problems when it gets cold enough over night to freeze the water in the hose but the temp is above freezing during the day, on the first mourning start up it runs bad, kicks the MIL & sets the code but once it warms up enough to melt the ice, the MIL goes away & it runs fine the rest of the day. the first time that happened to me was on a carbed motor, i saw that car almost every day for 2 weeks before they let me drive it home over night, next mourning i found it right off with no problem.

 

In my opinion, it's strictly the draining issue for pointing it down. The fuel can rot the hose and sensor and screw up the signal. Te diaphram has practically 0 mass.
The manifold mounted sensors are getting even more trick with "TMAP" sensors that have a temp sensor in the same hole as the MAP inlet and the Temp sensor is built into the MAP sensor (giving you 4 wires on the TMAP sensor). Saves money and machining.
We've gone a step further by running sequential by installing the TMAP in a single runner of a 4 cylinder with no cam signal, and a 60-2. It takes some software signal conditioning, and it just takes a guess at the beginning, and checks it later, and switches if it's wrong. Works bitchin. Saves another buck on the cam wheel, hall sensor, and wiring, and only potential drawback is 50% of the time, the fuel will sit on the valve a little longer until it sync's up (assuming low RPM).

 

This conclusion was already arrived at. Actually, it was proved via the Delphi sensor engineering documents. The diaphram is not practically 0 mass relative to its size, and any instrument sensitive enough to measure a pressure differential so accurately could certainly be affected by even a nearly massless diaphram.

But alas, even the educated guesses can be incorrect. When all else fails, consult the drawings...

 

another problem i could see with using the MAP sensor as the cam sensor is, what happens when the cylinder its on gets just a little weak? i don't mean so weak you can feel it has a weak cylinder, but just before its enough to trigger a misfire code. seems it is possible that it could see a stronger signal from another cylinder. would that be enough to throw it off?

i do agree with it working, i don't think there would be a problem with it getting in sync. but i don't think i would want to use it to fire the plugs.

 

Hook up your MAP and measure the voltage with the sensor is normal to earths surface, then measure with it parallel. If you do this, you're effectively putting 1G of acceleration on the diaphragm... so why isn't my meter showing a change in voltage? Is that enough proof that the MAP isn't oriented to eliminate the error caused by the vehicle's dynamics?

Grumpy, I find it hard to believe that GM didn't use the reasoning of econimics for their placement of the MAP. It's just like the LS1 returnless fuel system... sure it's a performance thing (slightly) but it's largly economics. On all GM vehicles they had the MAP's pointing down for the reasons stated (here and elswhere), moving it to directly over the intake was cost. My stock vac MAP line isn't a typical line, it's hvac stiffness. I couldn't tell any difference between stock location and relocated, the tune wasn't off or anything. With more accurate electronics it might make a difference but I doubt it with the c3/p4 n/a 3rd gen setups. 2 bar, 3 bar, much larger pressure differential so I can see and believe there would be a difference if the lines weren't stiff enough for their length.

 

The suction pulse of a cylinder will only get weak if the cam or valvetrain has crapped out, in which case it may not sync properly, but I'm not about to test that. This was done on a 4 cylinder with waste spark, so spark isn't sequential. Also on a 4 cylinder, having the injections phased 360 degrees out doesn't put the injection in danger of spraying on an open valve at normal RPMs. There's no significant drawback if it screws up.

The map sensors are subjected to lots of vibration, that doesn't seem to make measurable difference. I'm not going to argue about it, it's pointless. I just wouldn't go warning all about the dangers due to g forces, or next thing you know, we'll have a dozen threads of people trying to solve their mystery problems by re-orienting their MAP sensors, and putting them on shock proof mounts.

Mounting it to the manifold provides a c hair faster response, and is cheaper, and has one less failure mode, but won't be noticable unless the line is too long (which sadly, I've seen). Also seen collapsing vacuum lines that are soft from fuel, heat, and age. Rotted vacuum lines. Small vacuum lines stretched over the nipple and torn. Internally cut vacuum lines (from metal fittings) that act like a check valve.

 

You might try actually reading what I post before trying to argue, here's what I said.
What part of *While budget is a concern*, don't you get?.


******
While budget is a concern, when talking emissions, and CAFE numbers, they can alter what looks like an economic discision.
******

 

Might try looking at some cars that run 1.4, 60' times before saying that. Personally, if I see some data to support a point, ie before and after of reorienting a MAP, I'm going to mimic the one that worked.

BTW, has anyone else around here, cracked a MAP open and actually looked inside?.

 

".. they can alter what looks like an economic discision." was what I was refuting. NOT that you said budget is a concern. They moved the MAP on a stock f,y, and b-body's that do NOT cut 1.4 sec 60's. 92 was the 1st year correct? Wasn't that still a p4? Something makes me think it wasn't CAFE or an emissions decission to move it . Call me on this one but I'm just stating the obvious, no need to get huffy puffy with your "might try actually reading" because nobody want's to hear it.

As for 1.4 60' times. I don't have data on a car THAT quick but I do have on for a car that cuts 1.6 times and is a 10 sec runner. He doesn't get above 2g even on the launch but it's close, around 1.8g, and then quickly comes down to below 1g for the rest of 1st gear. When I say quickly comes down below 1g I mean half a second and it's gone from 1.8 to 1. But again, I don't have data on a car that does 1.4 60' times so I won't speculate, just reporting what data I do have that makes me not worry about the orientation.

 

I looked at one for Illinois last night......

Piezo crystal, diaphragm, electronics, never looked, but did I miss anything? I have big $1500 ones at work to play with.

60'... ah never mind

 

You guys are funny. I'll say it one last time - the positioning has nothing to do with vehicle dynamics. Per Delphi's technical data sheets, it is for protection against moisture and particulates from the vacuum line.

Clear?

For Pete's sake, man.

 

I have to agree with you on that. They are funny.

Hey, it's just a MAP sensor. Maybe you should run two of them and have the code decide which one is reading better. OK, time to move on, Next topic MAF sensors. Where is the best place to mount that? perpendicular or parallel to the intake?? (J/K-don't answer that)

 

I've found that mounting it perpendicular to the airflow works out best.

:O

RBob.

 

The best place to mount a MAF... hmm.. I'd have to say the best place for me has been 6" infront of either rear wheel. Only takes a few feet and it's all better

Why are you quoting me? I was providing more proof that acceleration wasn't a factor in it's placement. Although, when given the chance to orient it in a certain way that required no other compromises... might as well do it that way, doesn't hurt!

 

Everyone is making this too confusing. Reality is, proper Map sensor and hose positioning is important only because water collection in the supply line (hose). This is a commonly found TSB in manufactures from G.M., Asian and others. Not only the Map sensor but also the hose should be above the height of the intake vacume connection with no upward climb. Such as upside down map or hoses that drop down into the intake and then climb up to the top of like a Stealth Ram. Water blockage will give wrong voltage readings in the map sensor circuit and cause drivability problems without turning on a check engine light . That is only if it falls between what would be known good perameters. usually anything between .2 to 4.8 volts.

 

so im getting a code 33 and bought a new map sensor for an 88 camaro. does the 88 even have a map sensor being a map set up? if it does, where is it?

 

88 would be on the firewall. Rt center

 

Only if you want to ignore what some people have seen.

Reality is, there's more to llife then condensation, but, I'm not going to argue with folks.

If anyone can prove the MAP innerds are MASSfree, lemme know, other then that, it will have some inertia, and be subject to G forces.

 

I have read this thread a couple times now, and think I should point something out:

GM, and Delphi electronics specified the mounting position based on the condensation issue, etc....

Grumpy noticed that if it's mounted incorrectly, it CAN cause an issue, even if it is for mere milliseconds. Damage can occur in milliseconds!

Therefore, I'm going to make sure that mine is properly mounted for BOTH reasons! (even though I doubt I'll be pulling enough G's to make that diaphram flex, I don't want it to be an issue!)

IMHO..... Point is moot, due to the fact that all the reasoning discussed, points to the vacuum connection pointing downward.