MAP sensor value chart?

Is there a chart developed giving an output for the MAP sensor for any specific vacuum level?

4.0 renix 89...

I know at atmosphere it 'should' be 5 volt output, but what about 15in vacuum, 18in vacuum, etc?

Output voltage "B" should drop to 0.5 to 1.5 Volts with a hot, neutral, normal, idle speed condition and no vacuum leaks is all the FSM says.

I agree with what the manual says...have read it in quite a few places.

I have seen posted where some say 2 volts at idle is 'ok'..

But high mileage engine may not have the same vacuum reading as one with lower mileage...

What would one consider 'normal' vacuum at idle?

What vacuum level was the FSM info based on?

Hook up a hand vacuum pump to a new MAP sensor and read the output volyage at discrete intervals of say "1 of vacuum and make your own table.

Normal idle vacuum with a stock camshaft is 17-20inHg. That leaves the manifold absolute pressure between 10-13inHg (atmospheric is near enough 30inHg) so the MAP sensor output voltage should be around 1.7-2.2v at idle.
There's a direct linear relationship between manifold absolute pressure (MAP) and MAP sensor output voltage.

Dr. Dyno said:

Normal idle vacuum with a stock camshaft is 17-20inHg. That leaves the manifold absolute pressure between 10-13inHg (atmospheric is near enough 30inHg) so the MAP sensor output voltage should be around 1.7-2.2v at idle.
There's a direct linear relationship between manifold absolute pressure (MAP) and MAP sensor output voltage.

Click to expand...

Dr. Dyno,

You sound like you know what you are talking about, but how do you know the MAP output is linear?

Also the 1987 Renix FSM I have here says 0.5 to 1.5 V at hot idle. Thoughts?

These sites give an explanation:

http://www.autoshop101.com/forms/h35.pdf
http://www.autoclassroom.com/2007/01/use_the_map_sensor_as_a_vacuum.html

Keep in mind that the MAP sensor in our Jeeps is a GM unit and is interchangeable with those found in GM vehicles.

Interesting. So it is a GM part on a Jeep.
What a novel idea:shiver:.

But that web site shows the idle voltage to be between 1.62 and 0.88 volt the same (or nearly the same ) as the FSM I have from 1987, and not the "1.7-2.2v" range you listed on your first post below. I think you may have been mistaken about the need to calculate the absolute pressure first as the web site seems to have listed the voltage as a function of the gauge vacuum of the intake manifold? Yes, No?

The MAP sensor output voltage is proportional to the manifold absolute pressure (it's not called a manifold absolute pressure sensor for nothing).
Manifold absolute pressure = Atmospheric pressure - Intake manifold vacuum

Dr. Dyno said:

The MAP sensor output voltage is proportional to the manifold absolute pressure (it's not called a manifold absolute pressure sensor for nothing).
Manifold absolute pressure = Atmospheric pressure - Intake manifold vacuum

Click to expand...

I seem to vaguely (LOL) remember the definition of absolute pressure from my first chemical engineering class at the University of Houston,
!!!1LOL, but you did not answer my question! Are you saying the web site you are sighting as a reference for the volatges, http://www.autoclassroom.com/2007/01/use_the_map_sensor_as_a_vacuum.html and the Renix OEM manual are both wrong?

You said in your first post the Idle voltage should be "around 1.7-2.2v at idle." but the site you referenced says "voltage reading output of the MAP sensor indicates that the reading should be between 1.62 and 0.88 volt" which agrees with my Renix OEM FSM.

I think the problem is that the table you sited lists the voltage as a function of intake manifold vacuum (the actual gauge vacuum) and not absolute pressure. Therefore the correct voltage of a working sensor would be 1.62 to 0.88 volts at 600 ft above sea level (where the data in the table was taken according to the author) like the web site and Renix FSM says, and not 1.7 to 2.2 volts.

Now that I look at the table again the 1.7 to 2.2 Volts you suggested for idle does not even match the 10 to 13" hg of absolute pressure either??? 2.2 volts in the table is 14". 1.7 volts is between 16" and 17".

Each sensor is slightly different and the manifold vacuum readings will vary slightly from engine to engine (even from the same manufacturer), but the normal 17-20inHg vacuum at idle would indeed suggest that the MAP sensor output voltage should be 1.1-1.6v as per the FSM, and not 1.7-2.2v as I originally stated.

Glad you guys had your talk......

was interesting and I did take it all in.
for my example...

checked engine vacuum, (150k mile engine), 15 to 16 in vacuum on gauge at warm idle.

and (if memory serves) i had posted sowwhere my MAP sensor was showing ~2 volts, maybe 2.2

so..... my MAP is most likely OK even with mileage.

I did not want to just replace it to have it show the same voltage as my old one.... Like I did with a CPS:gonnablow that was showing output on the 'low' side, but the same as a new CPS..


.

The part I learned and found most interesting in this exchange is that the MAP sensor was linear (Voltage changes linearly with respect to the change in vacuum), thanks to Dr. Dyno. And my thanks to Dr. Dyno for his patience with my naging question.

I had been looking at the temperature sensors on the Renix jeeps recently and they are all definately non-linear, the intake manifold air temperature sensor, the ECU's coolant temperature sensor and the Guage (coolant) temperature sensor, and the coolant temperature gauge itself are all non-linear.

Glad to hear you avoided a needless expense. By the way, have you checked to make sure you don't have a tiny vacuum leak hiding somewhere like the intake manifold gasket, buried vac. tubing up under the the flange of the firewall, brake (vacuum) booster....etc. 150,000 miles does not sound like a lot for these engines. Mine has 245,000 miles and is still pulling something like 17-20 as I recall.

You never said why you were testing the MAP sensor and CPS sensor.

i have checked it pretty darn closely... with my recent experiences with it and a no-start/flooding situation.

I have not confirmed the reading with a different vacuum gauge... the one I have is an OLD snap on BIG rectangular fuel pump/vacuum tester

I know these will run 200k+ as there are two others in 'family' that have exceeded that mark.

I am not certain this one will. I am second owner, purchased at 72k and I drove it quite a bit until a couple of years ago. PO drove it short trips, but did service the vehicle, as did I. But I think this recent situation and one before washed the cylinder walls and I hope did no damage... both times there "had" to be gas in the Oil...... the viscosity was reduced and SMELLED...

I had posted on this and other forums for advice on my particular situation..

http://www.naxja.org/forum/showthread.php?t=902557

and on the JU forum... my user name is the same there if ya wanna search.

I did not actually "find" an absolute problem, though now it is in running state, and I did put some $$ in it, but sometimes ya just have to try something ...that was the perplexing situation, I could not 'find' a problem in the sensors and there is no apparent test for the ECU..... and it did not start immediately after replacement of the ECU.

I am still testing it by 'short' NO RESTART trips until I get 90% plus crank rate.... ( i thought i had it before until it stranded my dad 60 miles from home... :flamemad: