FIXED! "SERIAL DATA " need info... 1996 Buick Skylark

New to the forum.. Recent cluster change. Repair guy says I have Classic symptoms of  SERIAL DATA loss. 
Can someone direct me to it's source, and the components that are serviced by SERIAL DATA. He says something is pulling the link down and I need to figure out what... then the cluster will work fine as many things on the cluster also use the serial data link.. ANY help would be GRRRRREAT . Thanks
Dave G. 

Serial data is a broad term used to describe how information is shared between the modules on the car. Every module on the network is the source and recipient of serial data.

Someone may be able to help you if you would describe the issue you are experiencing and what tests you have performed.

thanks for answering,
if I may, , the problem is centering around speedometer failure. once back from complete reconditioning, Anti theft is not on , and many warning lights are on.. oil, antilock brakes etc. Repair guy says this is characteristic of serial data loss... that something is pulling it down... : ABS module for example. I have replaced the computer as I thought it had failed. Sent the cluster off for reconditioning as I thought the problem might be there. The vehicle has run flawlessly, up until speedometer failed, then engine temp, the, gas, then lights began coming on. even still occasionally it will start and run perfectly, just that many indicators are on. Speedometer does work now. Is there danger to the system if I just start pulling connectors off modules? not sure the voltage, can I read it with a digital meter or better with an analog. Do you know where I could get a wiring diagram, ? I have 3 Chilton books, none address serial data that I could see. Thanks
Dave G.

I took a guess and figured you have a v6 4dr? It probably doesn't matter, but the diagram I saw of the cluster shows a tan wire labeled UART.
That's your data com line that transmits data (ie oil pressure, temperature, rpm) from the engine computer to the cluster. Vehicle speed is sent on it's own dedicated circuit.
It looks like there's only 5 modules hooked into the UART circuit: 
Yout can disconnect then one at a time to see if the problem goes away.
Which brings me to my question, what is the problem?
I'm confused reading your post, the speedometer didn't work, now it does or the car runs like crap when it doesn't work sometimes?

I'm slow, you've got to spell it out clearly for me.

yes it's a Skylark limited 4 dr. v6 3.1L
sorry for the confusion. hopefully to clarify,
Every 20 attempts to start the engine, it will start and run great. otherwise, engine will start but ..when you release the key engine will run for a second or 2 then quit. I have the car on jack stands so when I do get it to start, I have put it in Drive to see speedometer and odometer work.
So It looks like repair guy fixed the non-working speedometer when I sent it for reconditioning.
History:
Son had driven the car for a few months without a speedometer at first no big deal.. Then the other failures occurred which drove me to replace the computer, which did not correct the problem....and that is why I sent the cluster off to be reconditioned.
Reconditioned cluster is now connected in the dash. still fully accessible.
Several lights are on and stay on..oil, anti lock, etc. even when the engine is running.
So called repair guy and he said serial data link is not talking.
I have never traced the serial data link before..
Hope this helps.
Thank you for you help !

Do you have a scanner, or access to one than can be connected to the DLC and see if there are communication codes in any of the modules? This would give you some direction as to the problem

Yes. I have 2, a Bosch OBD 1300 and a PT (Performance Tool) OBDll / EOBD. It did not occur to me to try that.   I figured if the link is down it would not work at all.... But if it might give some direction, I will give it a try. Thanks... This site is difficult for me to navigate.. ok, yes the reader found 2 codes. P1573 (serial data loss on ABS module) and P1629 ( Anti Theft ) I unplugged the connector to the module. no change to the cluster. However: after about 5 attempts, coincidentally it started.   I began a response to you listing some voltage readings at the ABS connector with engine running and not running,   key on and key off. , but when I went to EDITOR to create a chart, I lost it and ended up here at my earlier post. oh well,  If there are certain readings that might be helpful, let me know, I took a bunch and recorded them on a pad.  
Thanks Dave G.

Happy 4th, !!
Now that I know that the DTC connector to the scanner is Serial Data, and that I can read codes, at least tells me that the link is not totally down. We did not have trouble with ABS prior to the cluster removal. Now I'm needing to know the actual pathway for the Serial Data. Is it possible that a malfunctioning cluster being in the mix, could be what is causing the problem of the ABS module not reporting " not talking" . ?? Mr. Noah provided a drawing which shows a common tie point for a tan wire for all modules. I'm not following as my car does not have a module under passenger seat. ( seats currently removed for working space. ) and do not see how all can communicate on one wire..my problem.. I need more info
You tube videos show how LEDs can be used to watch different signal frequencies across different pins on the DTC connector. Is this something I can reasonably expect to use as a diagnosis? or am I going the wrong way ?
One other question ..? This car has ANTI-THEFT. We bought it with NON RESISTOR keys. We can assume that the anti-theft was somehow bypassed from the day we bought it. (no evidence of resistors added into any wires under dash or on any key related wires) The anti-theft light would flicker but never be on and stay on. .. We never NEVER had any trouble starting the vehicle. If anything it seemed over-responsive where it would start within 1 second of engaging the starter. Very reliable and no codes ever ...

The vehicle has two networks. UART which appears to be your problem on the tan wire. Class 2 which is the purple wire at the bottom of the diagram Noah posted. That is probably the one your code readers are using to communicate with the PCM. So the fact you can pull P codes doesn't really tell you anything about the state of the UART. And both of the codes you did pull point toward a problem with the UART......

Regards no module under the passenger seat does the vehicle have airbags? They might've been optional on a '96??

I've attached an article that goes into detail about how these older GM networks function.

First, Many thanks for the responses I got in the search for answers concerning our Buick..
Second, glad for the forum as there is so much to learn..
Third.. Learning to NOT be a parts changer.. !!

So...
The plot to the electronic project thickened when the lower intake manifold gasket went from a weepy leak to a full on blown gasket..
This was now a make or break moment.. Dash completely apart, electronic problems and now a blown lower intake manifold gasket..
I thought fix the gasket,,, I might get a better price if it at least ran if I could actually get it started to sell it..
That repair turned out to be a very good distraction from the electronics problem as "THAT" I can easily fix.. plus I now had a good reason to uncover my Safety Kleen basin/tank..( I love the smell of that stuff).. Anyway,
I cleaned all the parts and reassembled that GM 3.0.. All went wonderful and after a dozen tries , the car started and ran as it did before..
Now back to the electronic problem,,
I visited a local shop here in Fayetteville NC. (Bills Auto Electric). They instructed me on EXACTLY where to find the AIR BAG module. It is under the passenger seat, under the carpet.. I removed the seat and cut a 3 sided door in the carpet to reveal the module. With battery disconnected, I unplugged the very corroded module. Remade all the miscellaneous wires that hung randomly from the column. Reconnected battery,, and gave it a try. Started immediately. The dash lights that were ALL on prior... went out.. except for the Air Bag light which made sense.
Repeated starts were perfect. Gauges all working properly, check engine light out.. ..
Was able to source an air bag module from a junk yard in Kentucky, and it was a perfect match...
So, the Buick that was 1/2 a step from the "FOR SALE" sign is back on the road!
THE BOTTOM LINE IS,
The serial data line was corrupted by the corroded air bag module. With it out of the circuit, the remaining modules communicated properly.
Seems if you can remove each module to identify the offending module without hurting anything.. Lesson learned, also if someone says the module is under the seat, and you don't see it, look under the carpet!!
Thank you all again.. and
Thank you Scanner Danner!

Blast from the past! Nice work.
I merged the two topics for you to keep everything in the same place.
Thanks for updating us with the fix!