Horn Continuously Sounding
This is a basic case study that may be fairly elementary for the DN but it shows how complex even simple devices have become.
Vehicle is a 2014 Jeep Wrangler. The vehicle owner stated that when he was in the house the horn on his Jeep began sounding so he pulled the fuses to silence it.
First step was to verify the concern. So the fuses were reinstalled and the horn was silent. Pressing the horn button made no difference. So now it does not work at all.
Scan intelligent power distribution module (IPDM) that controls the horn relay and no codes. The IPDM data indicates that the horn relay is not requested even when the switch is pressed
Ok time to pull a wiring diagram
So it looks like the horn switch is hardwired to the steering column control module through the clock spring. Then the steering column module reports the horn request through a LIN bus to the cabin compartment node ( instrument cluster) then not shown in diagram the CCN ( cabin compartment node) sends the request by CAN bus C to the IPDM to ground the horn relay and sound the horn.
Ok so now scanning the CCN it has a fault code for horn request stuck. Now let’s look at some data in the CCN. The data states that the horn button is pressed but the horn is not requested. The CCN has disabled the horn because it has realized that the horn request is stuck on.
So the air bag was removed the horn switch unplugged the data rechecked and it still indicates that the horn button is pressed even when it is unplugged.
Why does the CCN see a horn button request when the horn button is not even connected ?
Next the steering column control module was unplugged and the CCN still indicated that the switch was pressed
Time to be sure the LIN bus between the column control module and the CCN is electrically good
so the LIN bus is functional which means the wiring for the LIN bus is not shorted or open. So that indicates data is being transferred between the column module and the CCN.
The fault must be in the CCN if everything else is disconnected and the CCN still sees the horn button pressed right? Well if the steering column module loses communication with the CCN when it was disconnected experience tells me for safety reasons the CCN may resort to the last known state of the data before the column module was disconnected. That data before the module was disconnected indicated that the horn button was pressed so it is probably default data. Try clearing codes to see if the status of the horn button changes. No it does not. Data still indicates horn button is pressed. Ok disconnect battery and perform a battery reset. Ok now the data changed to indicate the horn button is not pressed.
Now the column control module was reconnected. And the horn button data pid switches to on and the horn sounds. At this time the horn button is still disconnected and the column module is connected and the horn is sounding.
Remove the horn fuse so more diagnosis can be performed without the horn sound. Scope the LIN bus with the column module connected and the data on the bus is noticeably busier.
The increase in LIN data indicates that there is data being transferred from the column to the CCN when the modules are reconnected.
At this poInt I’m going to call the column control module faulty. New column control module installed and all is well.
Good one Robert. Nice use of the scope. I don't believe this is really that elementary. This is a great demonstration of how the LIN bus operates, Thanks for sharing, Dean
I agree with Dean wholeheartedly. Also a great time/place to use the uScope if you have one. Then the scan tool remains live.
I believe he is using two separate tools here, a solus edge for the scanner and a vantage pro for the scope. The uScope would have been just as handy here though, I enjoy using mine when I want a dedicated trace but small footprint.
Thought it was all on a Verus, anyway, uScope was on my mind from elsewhere.
I agree the uscope is a great tool. I am hoping to pick one up at the big event in a couple weeks.
Robert, Thank you for the detailed case study. Great explanation of LIN bus and how the modules are interlinked. Good job diagnosing.