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1999 Jeep Grand Cherokee Limited 4.7L - P0108 MAP Sensor Code: Possible Short or PCM Issue?

Model: 1999 Grand Cherokee Limited Fault Code: P0136 Posted: 2019-03-15 17:34

I'm troubleshooting a persistent P0108 diagnostic trouble code on my 1999 Jeep Grand Cherokee Limited with the 4.7L engine. The engine light comes on, and scanning shows only P0108 — indicating a MAP (Manifold Absolute Pressure) sensor signal too high. I've verified the wiring harness connections: Pin 1 is 5V from ECM, Pin 2 is grounded, and Pin 3 is the signal wire to the PCM. When the key is on but engine off, I see 5V on both Pin 1 and Pin 3. After starting the engine, same readings persist. Even when unplugging the MAP sensor plug (without reconnecting), I still observe 5V on Pins 1 and 3. I replaced the MAP sensor first — no change in behavior. The code returns after clearing with HPTuner. I've checked for other codes, airbag light status, cruise control, horn — all normal. No modifications have been made to the vehicle. My concern: Is it possible that the signal return (Pin 3) is shorted to power? Would this explain why 5V remains present even when unplugged? Could there be a wiring issue near the connector or within the harness? I’ve tested continuity between the PCM and MAP sensor connectors with the engine off. I found low resistance (<2 ohms) between the signal wire at the PCM and the map sensor plug, but infinite resistance between the 5V reference and signal wire when both connectors are disconnected. Using HPTuner, I've monitored the MAP reading — it consistently shows around 15.1 psi (104 kPa), with no variation from idle to near 2000 RPM. This suggests the sensor is not responding dynamically. I’ve also tested the PCM connector C1 and found no voltage on K1 or K7 when disconnected. I’m now considering whether a short exists between the signal wire and power, or if the issue lies with the PCM itself due to internal circuit failure. Any guidance on next steps would be greatly appreciated.

Related fault codes
P0136
Comments (18)
Anonymous 2019-03-16 08:53

Are there any other diagnostic codes? Always verify using a scan tool. Is the airbag light illuminated? Do cruise control and horn function? The ground connection appears fine. The issue may be a short in the signal return wire to the PCM. Pin 1 (orange) is the 5V supply, Pin 2 (cavity 2) is ground, and Pin 3 (DG/RD) is the signal return. Check for frayed or pinched wires within inches of the connector. Any recent modifications or repairs? Try unplugging PCM C1 and probe pin 27 — should show 5V. If not, a PCM issue is likely.

Anonymous 2019-03-16 09:47

Only P0108 was present; confirmed with HPTuner. Horn and all systems work normally. The vehicle has never been modified. I'll test the harness at the PCM as suggested. Thanks for the help!

Anonymous 2019-03-16 10:23

If the signal return (Pin 3) is shorted to any power source, the 5V supply could be lost — but in this case, it's still present.

Anonymous 2019-03-16 15:53

Yes, you would lose the 5V feed if there’s a short. However, if voltage is detected only when the sensor and PCM are disconnected, that suggests a wiring fault — likely due to a short from the signal wire to power.

Anonymous 2019-03-17 19:12

With the PCM unplugged, I found no voltage on K1 or K7. There is continuity between K1 (MAP sensor plug to C1) and between K7 (C1 to MAP plug), but infinite resistance between K1 and K7. If there’s no short, then the issue may be with the PCM itself.

Anonymous 2019-03-18 06:56

With both PCM and MAP sensor disconnected, do you have 5V on the signal wire at the MAP sensor? Some systems use a 5V bias voltage on the signal line. Did you replace the MAP sensor?

Anonymous 2019-03-18 07:03

Do you have access to a scan tool for real-time data monitoring?

Anonymous 2019-03-18 13:00

With both PCM and MAP sensor disconnected, I see no voltage on any of the MAP wires. The new MAP sensor was my first step — I replaced it immediately. After clearing the code, the issue persists. I use HPTuner for diagnostics, which allows me to read codes, clear them, and monitor engine parameters.

Anonymous 2019-03-18 16:56

When performing voltage checks, are you using the battery negative terminal as your meter’s ground? Did you check resistance between the PCM's MAP signal wire and the MAP sensor signal wire with both connectors disconnected? It should be under 2 ohms.

Anonymous 2019-03-18 17:02

Can you read the MAP data PID on your scan tool?

Anonymous 2019-03-18 18:42

Before unplugging C1 at PCM, I had 5V on K1 and K7. When I switched to battery negative as ground, readings remained consistent. With both connectors disconnected, resistance between the wires was barely over 1 ohm. However, when testing with both ends of the wire disconnected (using a multimeter), I found infinite resistance between all three wires — confirming no short. The MAP reading stays steady at 15.1 psi from idle to ~2000 RPM. Thanks for your patience and support.

Anonymous 2019-03-18 19:06

Confirm: When PCM and MAP sensor are disconnected, you measured resistance between the signal wire at PCM and the opposite end at the map connector — should be under 5 ohms (ideally less than 2 ohms). Now, reconnect the PCM plug but keep the MAP sensor unplugged. Use a multimeter to probe the signal wire at the PCM and the 5V reference terminal of the disconnected MAP connector with key off. Please share your resistance reading.

Anonymous 2019-03-18 19:45

Correct — I measured 1 ohm on both the 5V supply and return wires when both connectors were unplugged, using a small pick set. When PCM was connected but sensor disconnected, resistance readings ranged from high megohms to infinite.

Anonymous 2019-03-18 20:06

If you have low resistance between the signal wire and the 5V reference wire as described above, there is a short to voltage. I recall some pressure sensors use a 5V bias — but this should only appear when the connector is disconnected due to an internal resistor in the PCM.

Anonymous 2019-03-19 07:07

If you have extra wire, try cutting the 5V reference wire near the PCM and see if the signal wire loses voltage. If the PCM outputs 5V on the signal wire when it shouldn’t, that points to a faulty PCM. Some systems do use a 5V bias — but this is not always documented in service manuals.

Anonymous 2019-03-19 07:13

The 5V bias voltage should only be visible with the connector disconnected due to an internal resistor in the PCM between the 5V reference and signal circuit. In your case, you see 5V both when connected and disconnected — this suggests a short or a faulty PCM. Check resistance between the 5V reference and signal wire for continuity. Inspect the harness visually for damage from heat or vibration. If no continuity exists, the PCM is likely defective.

Anonymous 2019-03-19 07:28

Monitor MAP data on your scan tool while moving the harness to help identify any intermittent wiring faults. Also, disconnect the PCM and inspect its connector for corrosion, water damage, or pin degradation.

Anonymous 2019-03-19 07:41

Also monitor map on scan tool while moving harness to try to pinpoint any wiring issues. Also disconnect the pcm and inspect pcm connector for corrosion or water intrusion or other damage pins.