GM Low Speed Data Wires - Power Steering Issue with Speedo Loss

Robinjo

Robinjo

Goblin Guru
What all does the GM Low Speed data wires control?

Perhaps the speedometer, Ambient Air reading on the dash and some aspects of the Power Steering?

Edit: adding context.
I keep randomly loosing the speedometer, outside temperature reading, mileage and get 'Power Steering' on the dash display.

Final Fix: Vehicle Speed Sensor (VSS) harness connector had a break in the wires. The location of the break required a new connector.
 
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G

Gtstorey

Goblin Guru
From another source (more generic, not model specific);
"Low-Speed – The low speed GMLAN protocol conforms to the ISO 11898 standard, and is used for transmitting and receiving within simple systems that only require 100-200 ms response times (i.e. power door locks). This is a single wire network that typically communicates at 33 kbps using a 0-5 Volt toggling signal. This circuit connects to the DLC at Pin 1 "
 
E

ennonne

Member
The low speed network in a 2006 Cobalt* goes to:

Data Link Connector (DLC)
Body Control Module (BCM)
Radio
Vehicle Communications Interface Module (VICM)
Theft Deterrent Control Module (TDCM)
Instrument Panel Cluster (IPC)
Inflatable Restraint Sensing and Diagnostic Module (SDM)

Most of it is gone in the typical Goblin. Big thing it does/can do is supply data to the cluster that it receives from the BCM. Most of the data that the BCM relays on to the IPC is gathered on the high speed network. (RPM is a good example. ECM broadcasts RPM on to the high speed network. The BCM reads that and sends it to the cluster on the low speed network. And, voila, a working tachometer!)

It's worth noting that the low speed network is not a loop. Modules can simply be removed. Doesn't really matter how it's hooked up, unlike the high speed network.

*Others probably similar, I looked at a 2006 and 2008 Cobalt/G5 wiring diagrams.
 
Robinjo

Robinjo

Goblin Guru
Is this the cluster of green wires that were connected together? If I remember correctly, there was a small pack that was cut out too?
 
Robinjo

Robinjo

Goblin Guru
Ok, I’m still having an issue that I think is the Low Speed Data issue. The problem is intermittent.

I randomly lose my speedometer, power steering (with message on the dash) and ambient temperature reading. Sometimes it all of them at the same time and sometimes it’s just one or two. When I lose the speedometer it also doesn’t add miles as I drive. Any one have ideas to check that I’m not seeing? Is it something other than LS data to consider?

I have checked the harness to the gauge cluster all the way to the splice. Those solders are good. I need to check the wire from the splice to the BCM and from the splice to the PS unit.
 
Desert Sasqwatch

Desert Sasqwatch

Bigfoot Goblin
Dare I say it....it's a ground - it's always a ground. :p This sounds like a connector pin making intermittent contact or a broken wire inside the insulation - all fodder for the electrical gremlins! Does wiggling wire harnesses make it come and go, if you can reach them when it's happening?
 
Robinjo

Robinjo

Goblin Guru
I don’t know a lot about the LS data setup, hence the chain of questioning earlier. If one wire on in the system has a short would it impact the others? The connector pin is a possibility. Is it only the BCM and units or is the PCM connectors in the LS data?

I’m decently sure it’s not a ground but will look into it.
 
Desert Sasqwatch

Desert Sasqwatch

Bigfoot Goblin
All of the high side wires are connected together and all of the low side wires are connected together, so a short will affect all the electronics connected to it. An open in the loop could also, but will depend upon the connection - serial connections yes, parallel connections probability not.
 
Ross

Ross

Goblin Guru
Robinjo said:
Ok, I’m still having an issue that I think is the Low Speed Data issue. The problem is intermittent.

I randomly lose my speedometer, power steering (with message on the dash) and ambient temperature reading. Sometimes it all of them at the same time and sometimes it’s just one or two. When I lose the speedometer it also doesn’t add miles as I drive. Any one have ideas to check that I’m not seeing? Is it something other than LS data to consider?

I have checked the harness to the gauge cluster all the way to the splice. Those solders are good. I need to check the wire from the splice to the BCM and from the splice to the PS unit.
Click to expand...
Those symptoms sound like the high speed LAN issue I was having. Especially the power steering message. The BCM needs the high speed LAN data to pass onto the instrument panel via the low speed LAN.
 
Robinjo

Robinjo

Goblin Guru
Ross said:
Those symptoms sound like the high speed LAN issue I was having. Especially the power steering message. The BCM needs the high speed LAN data to pass onto the instrument panel via the low speed LAN.
Click to expand...
Can you expand on this a bit?
 
Ross

Ross

Goblin Guru
The power steering is only on the GM high speed LAN... and you are getting an error on it.
I had a power steering unit that was intermittent, sometimes the gauges would go dead, sometimes the power steering would die.
 
Robinjo

Robinjo

Goblin Guru
Ross said:
The power steering is only on the GM high speed LAN... and you are getting an error on it.
I had a power steering unit that was intermittent, sometimes the gauges would go dead, sometimes the power steering would die.
Click to expand...
Mine is similar except it’s only the speedometer. Tach and fuel gauge keep working. What was the fix?
 
Ross

Ross

Goblin Guru
For my issue, I replaced the high speed LAN connector at the power steering, and a new power steering unit.
Interesting that the speedo stops working, but the RPM tach works. Both of those are data from the PCM, since we don't have a TCM.
 
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