Short to voltage testing

autojoe

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#1
Make
Jeep
Model
Grand cherokee
Year
1999
Miles
90000
Engine
4.7
Have a p0108 5 volts on map signal wire. Map sensor connector disconnected have 5 volts on ref circuit 5 volts on signal wire and 0 volts on ground wire. I know some potentiometers the pcm uses a 5 volt bias voltage on the signal wire. What's the best way to tell if the 5 volt ref circuit is shorted to the map sensor signal wire? Thanks.
 

grcauto

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#2
Using your scan tool look at the MAP value with KOEO and wiggle the wiring and see if it changes. Also look at other sensors and see if they are reading right. Does the engine start?
 

billr

wrench
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#3
With the MAP sensor disconnected, connect a 1000-10K resistor between the signal and ground pins in the harness, then measure voltage between those two pins again (across the resistor). If you still have 5V like that, there is a short in the wiring or bad PCM. MAP sensors are not "pots", they don't have a bias resistor inline with the signal. However, they may have a very high value (1M) pull-up to the 5V so that a high-impedance DMM reads 5V when there is no load on the signal wire.
 
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#4
did you check - never mind to early to think -
I happened to be in WJ manual at this moment....
4.7s its always something

does this help?
 

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