Topic: Map sensor values in hhg?

[Rio_Grande]

I don't have my volt. Meter with me but I have my scan tool. I found what the voltages should be but can't find the vacume specs. Does anyone know what my map. Should read pressure wise at idle? 

[sharpxmen]

it depends what altitude you're at but from memory (should be verified) it's between .6 and .8 at idle, something like .65v at -20inhg. I can see if i can find the actual values for the 1 bar map sensor tomorrow (should be somewhere in the FSM but you can try a google for 1 bar map sensor voltages or something similar if you don't have the FSM).

[sharpxmen]

I probably gave you the value for TPS at idle, looked up the test procedure in the FSM (this is from '95YJ/XJ manual):

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
TEST
To perform a complete test of the MAP sensor and
its circuitry, refer to DRB scan tool and appropriate
Powertrain Diagnostics Procedures manual. To test
the sensor only, refer to the following:
Inspect the MAP sensor vacuum hose connections
at the throttle body and sensor. Repair as necessary.
CAUTION: When testing, do not remove the electrical
connector from MAP sensor (Fig. 36). Be sure
that the MAP sensor harness wires are not damaged
by the test meter probes.
Test the MAP sensor output voltage at the MAP
sensor connector between terminals A and B (as
marked on the sensor body) (Fig. 37). With the ignition
switch ON and the engine OFF, output voltage
should be 4-to-5 volts. The voltage should drop to 1.5-
to-2.1 volts with a neutral-hot idle speed condition.
Test the powertrain control module (PCM) (terminal-
5) for the same voltage described above to verify
the wire harness condition. Repair as necessary.
Test MAP sensor supply voltage at sensor connector
between terminals A and C (Fig. 37) with the ignition
ON and engine OFF. The voltage should be
approximately 5 volts (+/-0.5V). Five volts (+/-0.5V)
should also be at terminal-6 of the PCM wire harness
connector. Repair or replace the wire harness as necessary.
Test the MAP sensor ground circuit at sensor connector
terminal-A (Fig. 37) and PCM connector terminal-
4. Repair the wire harness if necessary.
Test the MAP sensor ground circuit at the PCM
connector between terminal-4 and terminal-11 with
an ohmmeter. If the ohmmeter indicates an open circuit,
inspect for a defective sensor ground connection.
Refer to Group 8W, Wiring for location of engine
grounds. If the ground connection is good, replace the
PCM. If terminal-4 has a short circuit to 12 volts,
correct this condition before replacing the PCM.

[sharpxmen]

here's a table for map voltages vs absolute pressure,
http://injector-rehab.com/shop/mapsensor.html
the one on the Jeep is a 1 bar map sensor. At idle you usually have around 18inHg vacuum which is about 8.8psi, since the atmospheric absolute pressure is about 14.7 at sea level you need to look at the table entry for ( 14.7-8.8 )=5.9 and it comes to 1.5v to 1.75v according to that table in the link. Of course less or more vacuum would vary the value but it is somewhat consistent with the FSM which is listing it at between 1.5 to 2.1v for idle.

1 psi = 2.04 inHg
1 inHg = 0.49 psi
for vacuum for example -20 inHg = -9.8 psi => 14.7-9.8 psi = 4.9 psi absolute pressure (where 0 is absolute vacuum and 14.7 psi is absolute pressure at sealevel)

[Rio_Grande]

Thanks for the info. Mine is running 28.6 at idle, as soon as I touch he gas it is all over the place.

[sharpxmen]

Thanks for the info. Mine is running 28.6 at idle, as soon as I touch he gas it is all over the place.

you mean -28.6inHg? that's wrong i would think, i'm fairly sure mine was showing around -20 or little over (between -18 and -20). do you have a mechanical vac gauge?

[Rio_Grande]

It is showing 28.6 on my obd2 meter. I have a spare I may try and swap it. I do have an old style vac gauge we used back when things were simple.

[sharpxmen]

what does it show with the ignition on and engine stopped?

[Rio_Grande]

 0.0 at ignition on engine stopped. I ran a live data recording so I can go back and look at everything. Started, running at an idle 10.5hhg, as soon as I push on the gas it jumps up quickly to 18-29, if I let off it settles back down.  Unfortunately I was going to put all the values on her, but apparently when I turn my meter off it looses the recorded memory.