Horn activation stalls the vehicle

Michael Technician California Posted   Latest  
Unsolved
Electrical
2011 Lexus IS250 2.5L (4GR-FSE) 6-spd (A760H)
No Dtc's
Vehicle Stalls When Horn Is Activated

Customer complaint, vehicle came in for no horn operation. Horn works with key off but not when the key is in ACC or igntion. When in ACC, the horn will not work at all. With the vehicle running the horn does not work and when you push it, the vehicle will stall right away. Checked fuses and there are no blown fuses in the entire vehicle. The horn fuse has 12 v on both terminals when removed, does not make the vehicle stall. The vehicle runs perfect as well. Monitors have run and there are no DTC's in any modules. I also checked the network topography and made sure all the correct modules are installed and communicating. Plugged into the communication networks with scope and linespi and there is no interruption in the communication networks when activating the horn and the vehicle stalling. Vehicle was recently in a small collision but all that was needed was a rear bumper, taillights and an exhaust tip. No airbags were deployed. Checked initialization within the modules in case there was a disconnected battery at one point during the body shop repair and all the calibrations are done. Problem still exists. Removed the airbag module and checked wiring through the steering column with continuity testing and found no shorts up to the fuse junction box near bottom of steering column. Checked under hood near fuse junction box but visually did not see any faults. Hooked up scope to horn pad wire and signal ground (pin 5) and with the horn fuse installed, the circuit has 12 v and goes to 0 v when grounded causing the vehicle to stall. With the horn fuse removed, the circuit still has 12 v and goes to 0 v but does not cause a stall. Suspected an SWS issue so i checked the SWS/ clock spring and graphed SWS data while causing a vehicle stall and the SWS unit data is good in the ABS module live data without fault. Checked my live data against Scan Tool Known Good Live Data in Service Information finding no issues. Any ideas why this stall happens with the fuse installed but doesn't pop the fuse, and still has 12 v with fuse removed but doesn't cause a stall?

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Marlin Technician
Oregon
Marlin Default
 

Pursue the source of voltage on the load side of the fuse, which is apparently shorted to battery power. Consider what circuit could be supplying battery voltage in the condition in which it is present. Based on the provided info, I do not know when the power is present/not present, but pressing the horn button must pull the voltage low, possibly because a wire is nearly worn through or because…

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Bob Owner/Technician
Massachusetts
Bob Default
 

I will bet a weeks pay that you will find this fault caused by either a bad ground for the horn circuit or high resistance in a power feed circuit.

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John Instructor
Pennsylvania
John Default
   

Agree with Bob. Think about this for a minute, the horn works when the engine isn't running and it stalls the running engine and does not work. You have two circuits that are being affected by the same failure, the first is the horns of course the second is spark, fuel or both. You wrote "Hooked up scope to horn pad wire and signal ground (pin 5) and with the horn fuse installed, the circuit has…

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Robert Owner
Pennsylvania
Robert Default
 

I agree with John and Bob. Also, assuming the issue began when the collision repairs were performed, perform a careful visual inspection of the collision repairs and the area in which the repairs were done. Use a wiring diagram and envision what might have been done by the body shop that would affect the horn circuits . . .

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Jim Curriculum Developer
Maryland
Jim Default
 

The horn pad switch is an input to the wiper/washer switch assembly. That input is conveyed across the network MPX system to the front controller. The power for wiper switch and horn button are different circuits from the horn relay power. The horn relay is activated by the front controller when the request is received. Note that the network in question includes the certification ECU, Body…

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Bob Owner/Technician
Massachusetts
Bob Default
 

Michael, I dug up some videos I did from a similar situation. This was a 2013 Rav4 from the body shop and it wanted to stall if the headlights were turned on or the right blinker. Here is one of the symptoms. Here is some initial testing that confirmed a bad ground. And this video proves that the new body panel is at fault. The test light was still hooked to battery positive and the ground…

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John Instructor
Pennsylvania
John Default
 

Hi Michael. So where does this stand, did you get it figured out?

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