Diagnosing a 2001 Jeep Grand Cherokee Laredo 4.0L that wont stay running.
p1686

Time Line:
00:00 - 00:50 demonstrate the problem
00:50 - 01:45 With SKIM unplugged
0:145 - 03:05 PCI bus works demo door locks & console
03:05 - 03:50 Door locks
03:50 - 05:15 5v sensors
05:15 - 06:10 Load testing SKIM power and ground wires
06:10 - 07:40 PCI bus diagnostic connector
07:40 - 08:10 oscilloscope on the PCI bus wire (not shown)
08:10 - 09:25 inside fuse box, BCM and data port location
09:25 - 10:40 Disabling SKIM VS replacing computer
10:40 - 11:15 What the dealer tech discovered
11:15 - 12:57 where the computer is located & one last thing I checked

PCM = Powertrain Control Module
BCM = Body Control Module
TCM = Transmission Control Module
SKIM = Sentry Key Immobilizer Module

Things to check

Door wires:
These Jeeps tend to break wires going to the doors, the door locks talk on the pci bus so issues with these modules or the wires going to them could potentially cause this issue

Bulk Head connectors:
next to the BCM (drivers side behind fuse box) there are several bulk head connectors, there are several more on the passenger side. These connectors have a bolt though them. to check for corrosion of the connectors.

Fuses:
there is a fuse box under the dashboard on the drivers side as well as one by the battery under the hood, check the fuses.

PCM / BCM connector corrosion:
unplug the connectors to the BCM and PCM and check for corrosion.

5V sensors:
Some of the 5v sensors on the engine use the same 5v rail as the PCI bus, if any of these sensors go bad and short the 5v to ground it could cause this problem. unplug sensors (cam throttle and something else?) also check voltage between 5v and ground on one of the sensors plugs. I was told not to load test this line. note that the 5v on the diagnostic port is a diff power rail per the forms.

SKIM Module:
Some people report corrosion in the skim module, check for corrosion on the SKIM module connectors. You can also open up the SKIM module and check for corrosion / broken antenna joints, bad capacitors etc.. note that you have to desolder the antenna wires to get the board out.

SKIM Module power and ground:
using a 12v bulb across the 12v constant and ground wire if it lights up and stays lit its good. do the same for he 12v key power.
Pink = 12v constant
Black = ground
Blue = 12v key on
Yellow + purple stripe = PCI bus (all the pci bus wires are yellow & purple in this Jeep)

PCI bus continuity test:
use a volt meeter and some wire to test the continuity between the PCI bus pin on the computer plug to the SKIM module plug. Also check if the PCI bus line has either continuity to 12v or ground.

PCI bus diagnostic cap:
remove the PCI bus diagnostic cap if the Jeep works then one of the auxiliary PCI bus modules may bad / shorted/ bad wiring.

Scope PCI bus:
If you have an oscilloscope put it on the PCI bus and see if it looks normal. you should see 0V then discreet jumps to between 5-12v then back to zero.

PCM Repair:
There are several youtube video's showing how to open up the PCM and make repairs. the simplest seems to be to repair / re solder the connections for the 5v power supply and the 5v power supply is relatively easy to get to compared to the computer itself. Repairing the solder joints on the pins of the PCM itself appears to be much more involved and require cooking the board out of the potting material that seals both the board from weather and to the case. I've not tried either of these.

Ground Wires:
Ground wire corrosion can cause just about any electrical gremlin out there in a car, several forms have people reporting that methodically cleaning all the grounds in the vehicle fixed issues similar to this one... I did not go down this route.

Useful Links:
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note that according to their site a SKIM delete does NOT include any diagnostics, so if you send them a bad computer they will delete the SKIM function and send you your same bad computer back. This is why I chose to just buy a new computer and delete the skim. I figured its most likely the PCM but could still be the SKIM and a new / refurbished computer with the SKIM delete would take care of both. ~ 300. for my computer they did not require a core, but they do on some. I put my order in over the phone (required VIN )

#Jeepstalls
#keylight
#SKIM

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