Question about Jeep Grand Cherokee

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HI. PUT A NEW MAP SENSOR IN AND EVERYTHING WAS OK. TWO DAYS LATER THE MANAGEMENT LIGHT, ESP LIGHT AND THE RED LIGHTNING BOLT LIGHT CAME ON. THE DIAGNOSTIC SAYS FAULT CODE P0108 AND P2638. NOW WHAT. THANKS

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  • vmcmotors May 25, 2011

    THANKS. COULD THIS ALL BE BECAUSE I INSTALLED A MOPAR COLD AIR INDUCTION KIT? WHAT WOULD YOU DO?

  • vmcmotors May 26, 2011

    ONCE AGAIN THANKS. MY ONLY PROBLEM IS THAT THE COLD AIR INDUCTION KIT WAS FITTED A MONTH AND A HALF AGO AND I HAVE DONE ABOUT 3000KMS WITH IT IN. I AM JUST WONDERING IF FAULT CODE P2638 COULD EFFECT CODE P0108 OR VISA VERSA. THANKS

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Welcome to Fixya

P0108 - The MAP (Manifold Absolute Pressure) sensor measures engine manifold negative air pressure. It's usually a three wire sensor: a ground wire, a 5 volt reference wire from the PCM (Powertrain Control Module) to the MAP sensor, and a signal wire that informs the PCM of the MAP sensor voltage reading as it changes. The higher the engine vacuum, the lower the voltage reading. The voltage should range between about 1 volt (at idle) to about 5 volts (WOT wide open throttle).

Causes


Potential causes of a P0108 code:

  • Bad MAP sensor
  • Leak in vacuum supply line to MAP sensor
  • Engine vacuum leak
  • Short on signal wire to PCM
  • Short on reference voltage wire from PCM
  • Open in ground circuit to MAP
  • Worn engine causing low vacuum

Possible Solutions


A good way to diagnose if the MAP sensor is to blame would be to compare the KOEO (key on engine off) MAP sensor reading on a scan tool with the Barometric pressure reading. They should be the same because they both measure atmospheric pressure.

If the MAP reading is greater than 0.5 volt off of the BARO reading, then replacing the MAP sensor would likely fix the problem. Otherwise, start the engine and observe the MAP reading at idle. It should normally be about 1.5 volts (varies according to altitude).

a. If it is, the problem is likely intermittent. Check all the vacuum hoses for damage and replace as necessary. You can also try wiggle testing the harness and connector to reproduce the problem.
b. If the scan tool MAP reading is more than 4.5 volts, check the actual engine vacuum reading with the engine running. If it is less than 15 or 16 in. Hg, then the PCM is probably not seeing enough vacuum (due to a worn engine, perhaps) for a given operating condition (which causes a higher than normal voltage signal to the PCM) and setting the code. Repair the engine vacuum problem and retest.
c. But, if the actual engine vacuum reading is 16 in. Hg or more, then unplug the MAP sensor. The scan tool MAP reading should indicate that there is no voltage present. Check that the ground from the PCM is intact and also that the MAP sensor connector and terminals are tight. If the connection is good, then replace the map sensor.
d. However if, with KOEO, & the MAP sensor unplugged, the scan tool shows a voltage reading, then there may be a short in the harness to the MAP sensor. Turn the ignition off. At the PCM unplug connector and remove the MAP signal wire from the connector. Re-attach the PCM connector and see if with KOEO, the scan tool MAP reading shows voltage. If it still does, replace the PCM. If not, check for voltage on the signal wire you just removed from the PCM. If there is voltage on the signal wire, find the short in the harness and repair.


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Posted on May 26, 2011

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P2638 The Transmission Control Module (TCM) requests torque reductions over the CAN bus during a transmission shift to improve shift quality. The TCM verifies that the PCM / ECM has performed the requested torque reduction by monitoring the response to the request. If the response is not within a specified tolerance, the TCM increments a counter, and if this counter reaches a threshold, the DTC sets.

  • When Monitored: Engine intervention active for at least 20 ms, no engine torque errors, engine torque demand is greater than 0.
  • Set Condition: Torque Reduction acknowledge bit - not set, no shift aborts, the error flag Torque Reduction Acknowledge is not set, Powertrain controller not supporting torque requests.
Possible causes: CAN BUS CIRCUIT DTC PRESENT, TRANSMISSION CONTROL MODULE , POWERTRAIN / ENGINE CONTROL MODULE Very difficult to narrow down this code, I would not spend to much time as it may be a by-product or glitch from map sensor code.

P0108 Map sensor circuit high=Possible defective map sensor(which you already replaced) Possible short at harness(loose) connection or Defective PCM/ECM(Very common) also problems with harness connections at PCM. You need to check for vacuum leaks all around intake manifold which would be most noticable when engine is cold. Use carb cleaner around intake and listen for engine surges with engine running at idle and note locations of leaks for repair. This code is saying you have a vacuum leak or no signal from map to pcm intermitent or all the time.

Posted on May 25, 2011

  • MechxD
    MechxD May 25, 2011

    I was going to ask you if you had any aftermarket intake componets, to answer your question, I have seen simple things like K&N replacement filters give false readings to MAF and MAP sensors it all depends on the sensitvity of the paramaters in which the software/PCM is set to. As with any modification to the vehicle you change its relative state in which it came from. Some vehicles take well to mods and require little manipulation to hardware/modules however some do not. With a cold air induction your performance may increase however other areas can be effected. Most people who mod vehicles are not concerend about warnings and DTC's they just look at it like a byproduct and it is what it is. Remember with any new problem sometimes tracing your steps will lead you to the answer, hence you may want to convert back to stock if you wish to see your lights gone for now. I would set back to stock clear codes and watch the timetable at which it take to set the fault(s), take note of engine temp, how long vehicle was setting (not running), and things like the climate right before it sets lights and faults. These can be helpful to narrow down problems. I like to use freeze frame data via scanner to see exactly when faults occured then its just logic that solves the problem from there on.

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