CTM is a major part of the communications bus. Clean it up good, make sure you are getting good contact, and see if it doesn't solve the issue.

 

HeyYou, I can’t find mine, got the 4.7 2000.

 

Should be in the same place. Drivers side kick panel. I know over the years they put them in different places....... If you have the manual for your truck, it should show you where it is.

 

Gotcha I’ll take a look at it, Wednesday.

 

HeyYou
Thanks!
No - it didnt solve the problem.

Orion - get the Service manual from the link I gave above. You are going to have some good use out of it!!

So far - I realized, that in my first video, in the beggining - was from the wrong pin =/
Thats from the PCI BUS pin on the cluster connector!!
So I have taken measurements with Oscilloscope.... on that same pin.
There is some difference between the measurements from the connector behind the cluster and from the same PCI Bus pin on the OBD2 connector.
They look like this:
Behind cluster:

OBD2:

Im talking Swedish - so never mind =) Im getting help in interpretation of it all =) My friend sais that it looks like the pattern is not oscillating, moving quite enough, when I showed him the one from the OBD2 connector. But as you can see, from the cluster, the pattern is only half that strong.
Waiting him to get back to me on some more videos.
At first I was searching for the "Diagnostic Junction Port" - I found it.... it looks like a OBD1 connector. And full of that same wire =) Violet/Yellow.

 

So yesterday, I went thru the dash and cleaned up the grounds and checked the CTM. All things where good. Some grounds needed cleaning. Didnt fix my problem.

I than went into the fuse box inside the the hood and cleaned out where a wiring harness was plugged in at on the left hand side. There was corrosion there cleaned it up really well and both po1687 and po1688 went away. Not sure why but seemed to fix my problem. I’ll update you in a week if it happens again but I think I had a poor connection somewhere.

 

So - in my case I cannot find anyting dodgy anywhere.
I have followed the violet/yellow wire, with a tone generator, to see if there was a brake/short somewhere. And I can follow the tone to the ECU.
Starting to think its the ECU/PCM, thats the problem. How common are those on these cars, do you think?

 

it is common, did you ever check under the hood fuse box and the connectors in there? Mine was the wiring harness plugging into the fuse box right by all the fuses had to clean up it a little and it solved my issue.

 

Yes I did check the connectors on the top of the fuse box. All clean and fine.
I also checked at the ECU.
5V references in and out OK.
Grounds OK.
12V OK.
This is what the oscilloscope shows on PCI buss on ECU:

For comparison:

 

So, the fault was the instrument cluster. I got an other used one from USA today.
I also had faulty scaner module on my SnapOn Verus. It couldnt read the other modules besides engine ECU. After buying a new module - there was no problem reading all the other modules as well =)
So not easy to trace a problem with foulty equipment......