Throwing all kinds of codes after passenger airbag replaced on recall

[brods]

You are correct, there is a speed sensor (3 wire, 5v, ground, sensor signal) in the fan that reports actual fan speed back to the engine so it can compare that to desired fan speed. Most 5v circuits on FCA vehicles feed more than one sensor, so when the 5v gets shorted to ground or battery it takes down any other associated sensors.

I'll answer my question.
The clutch fan uses 5V which is Vcc from the computer. (it may even use 12V and knock it down, I don't know) The clutch fan, like all discrete components, uses that 5v to power its electronics and create a simple signal the computer can read as speed. In one of the CF's failure modes, it shorts or more accurately, pulls down that 5V - certainly this is not a failure mode Dodge intended to happen but it happens. Since many other discrete components/sensors are also powered by Vcc, anything on that buss that the computer derives DATA from are effected.
All the sensors around the truck are simple digital components of a computer circuit (some may be or analog components that convert to digital through D to A converters).
The 5V itself is not a bus but the signals created by items powered by it are effected. The output of these components are on various busses. With digital circuits, a low Vcc means they may go to a hi (least likely), low or tri-state (unstable area between hi and low) output which can wreak havoc on a data bus.
This is why a failed item on the truck effects something else that is seemingly unconnected.

Scott

Sounds like the same answer to me? Correct me if this is wrong: isn't the buss just two wires going from controller to controller? So sensors may be giving inaccurate data because they are not receiving the proper input voltage, but the buss is physically unaffected, yes?

 

[cerberusiam]

The 12V power is fused so a dead short will just blow the fuse there and the fan fails to engage. A worse issue is a partial short on the 12V circuit internal to the fan that spikes voltage on the 5V circuit. Too much and ECU just shuts down the bus and all comms. Random spikes and noise raise havoc with all the sensors as the data bus is just a common circuit to communicate on. There is one bus with multiple signaling protocols running on it, some are affected more than others with the noise generated from bad components.

 

[brods]

Really confused now. So what you guys are saying is that one device (in this case the fan clutch) can take down the communications buss which the modules need to talk to each other???? One would think the modules would have been designed with circuit isolation so that any device/sensor failure wouldn't take the whole system down.

 

[cerberusiam]

No network is immune or designed to handle externally generated voltage spikes or noise. It can handle sensor going out of range, that is the rationality codes, but what happens in the case of the fan or even the data port is not under ECM control.

 

[sag2]

OK, this has got a little off track. When the 5v gets shorted it takes down other sensors, not the BUS. I don't recall I have seen a shorted 5v take down the BUS, but i suppose it could be possible. The 5v is made inside the ECM and it is a clean voltage source that is very steady as far as voltage. They use a lower voltage than battery so they can make sure the voltage stays in a very narrow range, usually within a couple hundredths of a volt.