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I'm currently working on a 2012 Jeep Grand Cherokee with an intermittent no crank issue. If you turn the ignition on and short the starter relay it starts up straight away.

I have a good live feeding the relay and the relay itself is new.

The brake pedal switch is ok and live data confirms this.

When the start button is held down or the key is held to start I have a momentary live signal to the relay coil but the other side of the coil is not getting its ground.

The ground side of the relay is fed from the Engine ECU (PCM as Jeep call it) If I ground the ground side of the relay it starts straight up but then leaves a fault code saying 'starter relay short to low' so I can't leave it like that.

I have good dealer level diagnostics and live data states the PCM is authorising the starter motor, but it's not switching the relay.

I'm having trouble accessing the wiring diagram for a 2012 3.0 CRD RHD

The problem I have now is the intermittent nature of the problem every time I think of something to test the car starts up.

Does anyone have a complete wiring diagram for the starting and electronic ignition / push button start circuits?

Or know of common reasons for a no crank with no fault codes?

I have tested or eliminated the following: - The wire from the ECU to the BCM/Fuse board that feeds the Ground of the starter relay. - The relay itself - The brake pedal switch is new - Live data suggests the ECU is authorising a crank signal

For the bounty I would like or as much as possible: - Full starting system wiring diagram - BCM and ECU pinout - Electronic ignition switch wiring diagram - Any workflows or troubleshooters Jeep use to diagnose no crank. Nothing generic must be specific to a Grand Cherokee 3.0 CRD automatic with push button keyless entry.

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  • Both + and - are controlled by the ECU (PCM), not the BCM. I cant find a diagram for it unfortunately. BCM may play a roll in anti-theft which would keep it from starting. Have you checked all your fuses?.
    – Moab
    Sep 12, 2019 at 16:31
  • I'd start with the fuel pumps relay
    – user38183
    Sep 16, 2019 at 3:14
  • Fuel pump relay wouldn’t stop it cranking
    – Newbie Noob
    Sep 16, 2019 at 5:01
  • Have you looked at the cam sensor
    – user38183
    Sep 16, 2019 at 5:48
  • @hellomoto - Cam sensor wouldn't stop it from cranking, either. Yes, stop it from running, but not keep it from cranking.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 17, 2019 at 1:14

2 Answers 2

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The computer looks at many sensors to determine whether to ground the starter relay. First it looks at the passive remote keyless entry system to see if it detected your key fob coming near the vehicle to open the door. Then it looks at door switches and hood switches. Then it looks at the key transponder. So to troubleshoot this, you'd need a scan tool capable of giving you all the live data that the computer is seeing to determine which data point is out of whack.

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  • I know all of that, thanks. I have multiple dealer level scan tools and the one I’m using is very good with Jeep. All live data is fine and the live data states that the start signal is being sent to the relay, therefore it seems there is a wiring issue or module fault
    – Newbie Noob
    Sep 12, 2019 at 16:37
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Well, PCM (powertrian control module) is usually only when the engine and transmission controls are in one unit. If it has a separate trans module then ecu/ecm (engine control module) is generally used.

Unfortunately, I do not have any diagrams for rhd models and we do not have diesel Cherokee's here so I can not truly answer your question but I still wanted to offer a little information to you. Considering live data is authorizing the start and when you ground the relay it does start (and run I assume) then I would strongly consider the fault to be internal to the ecu or a wire break in the harness. I do a lot of ecu repair and its not all that uncommon the driving component(s) for the starter circuit goes faulty. You may consider finding someone in your region you can send out the ecu to and have it tested/repaired.

Typically when the internal components of ecu fail for the starter their will be no DTC's .

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