Jeep Battery Drain
This applies to late 1990's to early 2000's Jeep Grand Cherokee vehicles. When these show up with dead battery complaints, parasitic drain testing techniques may lead us to believe that the BCM has failed and is continuing to draw power from the battery when it shouldn't. Before condemning the BCM, there are some things that need to be considered.
Here is a partial wiring diagram showing some of the anti theft switches diag&#8203;.&#8203;net/file/f5nlalfzd…
we try to look for anything that would keep the BCM awake. Any of the door ajar or cylinder lock switches can cause this. As shown in the wiring diagram, the door ajar switches are drawn in the at rest position with the doors closed. The cylinder switches at rest position is with no key inserted and no movement to the lock or unlock position.
Here is how the switch data PIDS appear on the DRB 3 factory scan tool. Most aftermarket scan tools are similar (fig 1).
The PID status is either open or closed. The somewhat confusing part is that the PID status will show the opposite of the door position . For example; the DR DOOR AJAR PID (highlighted) reads CLOSED, meaning that the drivers door should be open. The BCM apples a reference voltage to the switches. The switches close when the corresponding door is opened, pulling the reference voltage to ground.
This particular circuit (arrow) is interesting :
The lock cylinder switches are in parallel. Any one of switches that is struck closed will cause the data PID to read CLOSED; causing the BCM to remain awake and drain the battery.
Here is how the data PID (highlighted in fig 2) should appear when the cylinder lock switches are in the open position (no key is locking or unlocking the cylinder):
In short the PID state should be opposite of the door state. Put another way; the data PID applies to the switch position and not the actual door position. We hope that this wasn't too boring for anyone, and hopefully it helps someone. Thanks for taking the time to read. Be safe and be well.
Thanks for the info. I always appreciate some info on the finer points of reading PID values. The fact that the PID refers to the switch state and not the door state could confuse a lot of people because that is not the way you would read the PID intuitively. Door open, Door closed, simple right. Ummmayby not :) Like when you read the PID for a MAP and your looking at 10inHg and someone thinks…
Also it is common for the wires to break in the door jambs.
Hi Dennis. You are right. We see broken wires in the door harness quite often. Hope you are well.
and then there is Jeep, that actually DOES show engine vacuum, instead of absolute pressure in the manifol. :-)
Why not. Make if totally confusing for people :)
HI Bill. I hope this helps. Take care and be safe.
These era Jeeps are prevalent out here. Haven't seen this yet, so great tip for me, thanks!
Hi Geoff. Thanks. I kind of thought this might be a boring tip. My brain power is quite limmited compared to evryone on this platform. My best to you and your family. Se safe and be well.
John- It might have seemed boring to you,but these things are always intriguing to the diagnosticans among us.(probably most if not all of us here) Good info!!
Thank you very much Rudy. I hope you and your family are safe and well. Take care.
I have read a few Key-Off Draw issue cases on newer stuff, caused by door latches, but never would have even considered these old (pile-o-crap) Jeeps had a BCM that "smart". Heck, I'm not sure I knew they had a BCM! :-) Best to you and yours also.