NEED HELP GMT800 PDM and DDM Down?

[marigoldquadrasteer]

All right, GM electrical nerds, I've been having class two data issues.

Said issues are: keyless entry not working, dash cluster turning on and off sometimes, and a bunch of U1000 codes.

So I bought a cheap oscilloscope and probed the infamous splice pack 205.

I am getting what I understand to be proper 7 V pulses from all the pins that should be giving me those EXCEPT for the passenger door module and the driver door module, which are both stuck at 0.8 V!

Do we think both of those went bad? As far as I can tell, they don't share a ground (G305 and G306).

Unfortunately I can't test whether or not keyless entry will be fixed by unplugging them because... well, they are what does that job lol.

 

[TJBaker57]

getting what I understand to be proper 7 V pulses from all the pins that should be giving me those EXCEPT for the passenger door module and the driver door module, which are both stuck at 0.8 V!

How exactly did you do these tests?? Were any wires disconnected during your tests?

If you did not disturb or otherwise disconnect anything while testing then probing all the various 'pins' is essentially probing various points on the exact same wire.... as every serial data wire is connected to every other serial data wire by the spice pack.

 

[marigoldquadrasteer]

I was testing at the splice pack with the comb out.

 

[TJBaker57]

I was testing at the splice pack with the comb out.

OK then... When testing for voltages there will be none unless the module under test is actually broadcasting.

Most modules have a fair quantity of responsibilities that may keep them active for a time in the absence of other modules.

The door modules really have nothing to say when they are isolated from the rest of the system. Except in your particular vehicle where I would expect the FPDM to perhaps be the module responsible to the keyless entry system. In that particular case it might transmit in response to a key fob button press.

Otherwise they remain powered at all times normally and it is a "power mode message" from the "Power Mode Master" that causes them to wake up and talk. Excepting the keyless entry system at the FPDM if so equipped

So I would not expect to see frequent serial data activity on a door module which is isolated.

 

[TJBaker57]

Here is a short video showing the voltages as displayed by a DMM. Naturally you cannot get accurate voltage readings of microsecond pulses using a DMM but you can see the activity.

In addition to the DMM display here you can see on the phone screen the actual messages from the serial data bus.