Need some help. "unkown driver" Nothing in the interior works,

[Brando5641]

2004 Suburban Z71

 

Truck was dropped off for displaying unkown driver and wouldn't start. Truck has 2 keys and they both do this. Had the vehicle for 3 days and started the truck 30+ times and couldn't recreate the symptoms. Gave the truck back and got a call the next day it had done it again but now the truck will run but nothing inside the truck works. Hooked my scanner up to it and it wouldn't connect. Had 7v on pin 2, pins 4 and 5 have good ground, and 16 has 12v+ at the DLC. Started doing some searching and found a TSB for this issue and it stated to check the wiring under the drivers seat and the ECM harness next to the battery box. Both appear to be fine.

Not being able to communicate with the vehicle I'm assuming its a serial data issue.

I found the SP205 splice connector and jumped pins B and M and was still not able to connect to the truck. The writeup i found wanted you to take jumper wires and one at a time bring each module online and then when you lost connection the last one to bring online was the issue.

 

I'm at a loss of how to proceed. Gut tells me its the BCM but I need some assistance on how to diagnose this issue better to avoid throwing money at it.

[Doug_Scott]

Your post sounds like you are a repair facility. One thing I have always done when I was still on the bench was to admit when something stumped me, and tell the customer that they are best off going back to the dealer for something like this. I hate working for free almost as much as I hated charging customers for my learning something. Once you charge them for fixing it, and two days later the same issue returns, I could not ask them to pay me again. Did not take many instances of that happening before I started just being straight with them. If I was not confident in my recommendation, I would simply tell them it was beyond my skill set. About the only thing I hated more than being wrong was getting caught at being wrong. Customers will trust the mechanic that is straight with them first.

[Brando5641]

I'm not a shop but working on it for a friend. I agree I am past my capability's with the equipment I have. I am going to take your advise and recommend he takes it to someone more well equipped. I hate to do it but I guess you cant win them all.

[Jsdirt]

Here is a prime example of why I HATE modern vehicles.

 

I'll take the poor fuel economy and increased maintenance of points and a carburetor any day of the week over problems like this.

[Doug_Scott]

Here is a prime example of why I HATE modern vehicles.

 

I'll take the poor fuel economy and increased maintenance of points and a carburetor any day of the week over problems like this.

 

We had similar issues back in those days as well, bad connections in the bulkhead disconnect for example. Or bad grounds on one of those 14 wire daisy wheel connectors that was the very first thing bolted down, then the dash was built up on top of it (1973 Imperial for example). Idiots used a sheet metal screw to hold it down, no form of any locking washer. They changed to a 1/4 inch stud the following year, still hidden though.

[Jsdirt]

At least in those cars, if I was faced with that at my house, I'd just rewire the circuit myself, bypassing the connector altogether. Do that on a modern vehicle, and it won't even run.