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'96 Deville, '00 Seville
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Discussion Starter #1
My 2000 Seville has a capacitive short somewhere in the Class 2 Data Link Communications wiring that I cannot locate. The symptoms are an occasional dead battery due to stuff being turned on when the car is parked, numerous random error codes stored in the DTC, and once it wouldn't run because the computer thought the car was being stolen (Theft module error)!

The wiring 'short' can be readily observed by connecting an oscilloscope to the data line and observing a triangular waveform (rather than squarewave) which wanders in between high & low states (but rests at ground level in the absence of data). The voltage levels for the high/low states are so far out of spec (near 1.5v), that it amazes me that the vehicle even works/runs at all. The waveform looks like there is a capacitor between the data line and ground.

This problem has existed ever since I have owned the vehicle - approx 4 years. A couple years ago, I grew frustrated by a frequently dead battery, and tried to find the problem. I disconnected each module one-by-one (there are well over a dozen), saving the instrument cluster for last since it is so hard to get to. (I noticed two modules had been replaced by the previous owner - they are not OEM.) When I pulled on the wires to the instrument cluster, the short mysteriously disappeared! Not able to recreate the short, I put everything back together.

But now the short is back, and no amount on tugging on wiring harnesses will make it go away. I have a service manual, but am lost trying to trace the wiring. If somebody could direct me to the LOCATION OF SPLICES in this data line, I would appreciate it. In particular, S421 and S200. If I could disconnect various runs of wire to find out which one is shorted, it would help immensely. Unfortunately, I can find no WIRING HARNESS LAYOUT DIAGRAMS in the service manual. And even the schematics are ambigous in places.

In reading the service manual, it appears that if I simultaneously unplug the Theft Deterrent Module, Dash Integration Module, Bose Amp, and Inflatable Restraint Module that I will isolate most of the data bus wiring. However, the short is still present with these modules unplugged. So that leaves me wondering how I can isolate the wiring to the 'end of loop' modules I have unplugged, and modules such as the Memory Seat & Remote Keyless Entry which appear to not be in the loop(s).

Basically, I WOULD LIKE TO KNOW where the CLass 2 Data Link homerun cables are routed throughout the vehicle. Armed with this information, I think I could tract down the short and isolate it. Or, if someone could direct me to a manual or wiring diagram more informative than my service manual, I would appreciate that also.

BTW, there are no aftermarket items connected that I am aware of (remote start, etc.). Any accesories that I have personally added have been wired to the cigarette lighter circuits, which is why the battery goes dead faster when the RIM wrongly energizes that circuit!

Any suggestions are appreciated - I am really frustrated...
 

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1997 Cadillac STS (BOSE w/AUX In, Massage Seats, URPAS, "Z")
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6,405 Posts
The wires are running along the the rocker panel front to back on either side of the car, under the carpets inside a wire way. This is where the wires between dashboard and electronics bay are running. A FSM will have the data bus line diagram in the Electrical Diagnose section, there are just 10-15 modules connected after all (in loops of 2-3 modules). The data wires are purple (but that's not very helpful). The FSM has ALL the information you need.
 

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I must be having a very similar problem. I have been having increasing problems with intermitent stability engagement and multiple random codes that come and go such as wheel sensors, then VSS, now steering sensors, radio and memory seats voltage out of range, etc but I always have U1300 Class 2 shrt to grnd.

I havent had battery die and no apparent voltage problems even though I have B1327 low current and B0856, B1983 but I my stability control is constantly randomly engaging. It used to only be every once in awhile and now all of the time. Ive had to pull the ABS fuse to disengage it completly and of course the set off some more codes and took away 1st gear.

I just have no clue where to start looking. I thought at first it must have been one of these sensors but most of the codes will go to history and then go away just to get completly new codes like wheel speed sensor codes that are gone now and I didnt change the sensors. I did change the VSS but the code came back but is gone again. I did have C1287 steering but it is gone now and now I have current C1780, C1783, C1785.

I just dont even know what to look at. Does this sound like a short somewhere instead of bad sensor?

Sorry Slowtyper to not help you but instead have a question, but is sounds like we have similar problems.
 

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1997 Cadillac STS (BOSE w/AUX In, Massage Seats, URPAS, "Z")
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6,405 Posts
Solution: disconnect one by one module off the data bus and observe when the issue disappears (hopefully is not the IPC, as is the module you need to see the codes).
If is a short to GND is even easier, connect an ohmmeter between data line and GMD (you should see a very low value if the line is shorted to the ground), and start-disconnecting modules until the reading is not low (I am expecting KOhms as good reading).
 
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