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No Bus - No start (Not usual reason)

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Old Aug 2, 2016 | 02:40 PM
  #1  
indeepwithajeep's Avatar
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From: Portland, OR
Year: 1998
Model: Cherokee
Engine: 4.0L
Default No Bus - No start (Not usual reason)

I was helping someone get some readings for the relay fan switch lines. I stupidly checked the ASD relay by accident. I shorted out pin 86 with a 1 amp diode in route to it grounding (so not too much current drawn).

Ended up with a no bus on the cluster.

So far I've:
Checked all fuses under hood (Passed)
Attempted a PCM restart (Failed, cluster wouldn't respond)
Attempted a PCM scan for faults (Failed, error couldn't read ECU)
Tested the OBD2 bus ports (Passed, 2.5v @ each (pin 3 & 11)
Tolerance tested CPS just because (Pass)
Tested ASD relay (passed)
Tolerance tested ASD to PCM C3 pin (passed)

Fairly safe to say its my PCM? anything I missed? I was thinking about pulling my fuse box to check wiring.

I'm helping a friend build a fan controller is the reason I got into this mess. Dumb on my part mistaking that relay.

Last edited by indeepwithajeep; Aug 3, 2016 at 08:35 AM.
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Old Aug 4, 2016 | 08:46 AM
  #2  
indeepwithajeep's Avatar
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bump
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Old Aug 4, 2016 | 08:53 AM
  #3  
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From: Syracuse, NY
Year: 1996
Model: Cherokee (XJ)
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Usually people jump to the computer as a cause way to soon (without testing).

In your case, you might very well be right. Can you get a hold of a computer to swap and test?
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Old Aug 4, 2016 | 10:52 AM
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indeepwithajeep's Avatar
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From: Portland, OR
Year: 1998
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I found someone local with one.. He's not going for my price. I'm seeing online they'll program the PCM to your vin and millage. Is this something I'll need? Will I have things not work even though the PCM number matches?

I know they store miles both on your cluster and ECU. So if you swap a cluster that reads differently your PCM will retain the miles in the background.

Between that and a warranty is the reason I'm not going much higher for the local guy. I know the dealer wants ~$140 to reprogram your cluster. So if I'm paying $60 for a PCM that needs to be programmed I'm paying more and not getting a warranty.

Last edited by indeepwithajeep; Aug 4, 2016 at 10:55 AM.
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Old Aug 4, 2016 | 11:02 AM
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indeepwithajeep's Avatar
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From: Portland, OR
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I thought about jumping the ASD to make sure it's not sensor related. Although if I'm not able to connect to the ECU I'd think that wouldn't matter much.
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Old Aug 5, 2016 | 05:58 PM
  #6  
indeepwithajeep's Avatar
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Update:
I got ahold of the FSM which had much better schematics. I traced it a fuse to the junction box (Chiltons only showed it tied to the power distribution center). Anyways the 20amp on fuse 11 was popped. She lives again.
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