Rear Defroster/Immobilizer - Not a Problem, It's a "Feature"

[dyn blin]

Soliciting the tribal wisdom-

A few months ago we added another P80 to the stable- a '96 850 Wagon NA, intended for my multiple kids' use. It was a neglected example, but has turned out pretty well as a successful project as we've worked it back to Stage 0.

Last week, I noted the rear defroster not working, and quickly traced it to chaffed wiring in the transition to the tailgate. I made some quick work of that with a home-made harness and swapped the blown fuse- this is where it gets interesting:

When the car is running & the defroster timer turns off, the doors lock. If the car is turned off while the defroster is still active, the doors lock and immobilizer arms. If I turn the ignition off while the defroster is on with an open driver-side door, the doors lock and when I close the door, the immobilizer arms and give the single hazard flash as it would if I use the key fob.

It's not causing any problems beyond the head scratching it's inflicting. I'm curious if anyone recognizes the behavior as a modification the PO made?

[scot850]

That is weird! I can't think of anything that may cause that. Have you reset the system using the driver's door lock? I had problems with the 97 850 that went down to my brother in Santa Rosa any time I removed the battery and then re-connected.

I'm sure your soldering was excellent, but I'm wondering if the issue could be in the replacement wiring harness maybe having a short still in there, like the wiring to the tailgate lock mechanism?

I don't think any 850 came with an option to have the doors lock on start up to prevent car jacking like many newer vehicles have. You may have an immobiliser system issue, fob issue (sticking buttons?) or ignition key electrical portion.

I wish you luck!

Neil.

[dyn blin]

That is weird!
Neil.

I know! Right?

Yes, the behavior is consistent even after a door lock pull-push cycle and "outside" diver's side key unlock and power (battery) disconnect.

I clipped the wires to the tailgate and used Deutsch connectors after shrink-sleeving the wires to give them some flex resistance , but it's definitely worth tracking the wiring further in both directions to see if there is a short. However, intuitively, I would think the behavior more likely to happen when the defroster is switched on if it was a short.

After thinking about it, I suppose the other possibility is a 5-pin relay set-up customized by a PO, but I would have thought I would have seen it by now. 'been through the car end to end more than a few times without noticing anything wonky like that.

Thanks for weighing in on the puzzle.

(PM me on how your brother made out with the x-fer)

[abscate]

Yeah, you still have a short in your tailgate loom.

I found this feature in NY a couple years ago...if I used the lock button the short energized the immobilizer circuit, overloaded it, then popped the fuse and left me with a no-crank no start.

Luckily I found the remote jump terminal and made it home and figured it out the next day

Root cause....rear tailgate loom.

[dyn blin]

Looks like this weekend's do-list is set...

[abscate]

My weekend will be re-doing the BMW PCV system..the CCV valve broke in two and gushes smoke, so I get to do the job again!

It makes the Volvo PCV look easy....

[dyn blin]

Unsatisfying update: I pulled the tailgate trim and followed the wiring down to the lamps, brake lights, wiper, defroster and lock- it all looked good and no voltage changes to the door locks when the rear defroster switched "on". I'm going to trace it the other way in the future and post back. This weekend is dedicated to front struts and finding a SAS vacuum leak so we can get the NA through emissions next month without having to keep the tank full (2 cars with that 442 code to trace now).

[abscate]

Dyn - you are cutting open the sheath to inspect that wiring, right? The faults arent visible unless you open up that nice wiring loom, unfortunately.

Dont run with the cutting tool

[dyn blin]

Heh, good general advice for any sharp objects and Moose-projects, especially when going back into the house to refresh the coping beverage..

Yes ,I previously exposed the wire run from the sheathing loom in both directions and re-sheathed it with split-mesh sleeving like I use on my motorcycle. I do this whenever I have to go in to make future projects easier to access and troubleshoot. I re-traced the wiring for splits and cracks. as far as I could "forward", and "back" to the terminations and connectors to the component in the liftgate. It's interesting there is a full 12volts hitting the door locks when the defroster is switched "off", so I presumed it was a going to be a pretty obvious short. The fuse hasn't blown again, either, despite retesting the "feature" a dozen times over the week. I *want* it to be an obvious short, but no joy yet.

I'll be under the dash this weekend placing a intermittent wiper relay and checking to see if the flood light relay is there.

While I'm there, I'll look to see if there is anything wonky on the central locking and the anti-theft relays. IIRC, they are the 2 "long" relays in the driver's side under-dash interior bus, correct?

[abscate]

How about grounds? Are they all in place? Voltage hunting for new grounds will cause lots of interesting stuff.

Can anyone find the 1997 tai.gate wiring diagram and post here?