ADAS and verifying all the systems in a vehicle operate as intended by any OEM go hand and hand like peanut butter and jelly. Since my departure from the dealership world as a diagnostician and my migration to the mobile adas (advanced driver-assistance systems) and diagnostics industry this principle is of paramount importance. Recently I performed a anything but routine adas service and used voltage drop in diagnostics to find a solution to multiple electrical problems.
To recall a recent calibration service where a standard front radar static calibration was performed successfully but the test drive was anything but successful. No more than 2 minutes into the verification drive the instrument panel lite up flashing abs and skid control warnings. Along with front radar system and front emergency braking disabled.
Consequently a quick rescan of the Toyota Rav 4 ecu’s sent me to the abs(skid control) ecu as the culprit causing so much havoc. This ecu itself issued a current dtc C146D ABS motor relay circuit short. While the Pre Collision system contain a current dtc (diagnostic trouble code) for C1A50 Brake system fault.
Just to clarify the the Pre Collision system on this vehicle consists of 12 components.
Millimeter Wave Radar Sensor
Driving Support ECU Assembly
Brake Actuator Assembly (Skid Control ECU)
Forward Recognition Camera
Airbag Sensor Assembly
Spiral with Sensor Cable Sub-assembly
Stop Light Switch Assembly
Steering Pad Switch Assembly
VSC OFF Switch
Combination Meter Assembly
Skid Control Buzzer
Main Body ECU (Multiplex Network Body ECU)
Each one operating in harmony with the other to perform distinct functions of the Pre-Collision system. Arriving back at the shop a visual inspection revealed no obvious faults with the abs/skid control ecu or wire harness. My quick search for technical service bulletins turns up nothing either that pertains to this concern.
Diagnosis
Given the green light to perform diagnostic’s I pulled up a schematic, dtc definition and skid control ecu description of proper operation. Checking for power at pin 13 in relation to pin 1 chassis ground battery voltage is present. This led to a voltage drop diagnostic test where the ground side failed with over 500mv and the resistance check of the chassis ground is 2.76ohms. Both results way out of specification for any OEM concerning a ecu ground. Since my ground on pin 1 is out of spec I chose to install a temporary overlay ground and delete the current dtc for C146D ABS Motor Relay circuit short.
Then key off to let all ecu’s go to sleep and the key turn back on to run mode and new dtc that is the opposite of the short was now present became C146C ABS Motor relay circuit open. These short but quick tests point to a damage relay that was in series on the circuit board with the skid control ecu pump motor.
RECOMMENDATION
The pump motor circuit short on the ground side pin 1 and the overlay temporary ground I install cause this circuit to become open. Which places the original cause for adas pre-collision system shutting down as the surface mount in series motor relay on the pump motor circuit. So along with a new ground for pin 1 an abs/skid control unit needs replaced too. In this case parts are ordered and the customer will have them installed at a dealership. Where afterwards a zero point and yaw rate reset will need to be perform to complete new abs module initialization.
Part 2
After about a week I received a request to come back out on this very same vehicle due to the new abs module that still needs zero point and yaw rate reset. This vehicle now has a whole new set of electrical problems that were not there before. Along wit the instrument cluster has nearly every warning for anti-lock brake and several adas disabled system messages flashing.
A quick pre scan reveals a grocery list of diagnostic trouble codes
While the new physical symptoms are a left headlamp low/high beam assembly that flickers rapidly and then flashes. When applying the left turn signal. It is at this very moment when all the warning lights on the Instrument cluster come on.
Simultaneously on the scan tool various modules show no communication with the abs module then become history.
Moving on to check power and ground at abs module 0 volts is seen. Which led to checking both fuses for the abs module and they are fine. Removing the engine air filter assembly exposes a odd wire hanging out. That had repairs in two different areas on the abs harness connector which were not there weeks ago. Upon a closer inspection the harness itself is soaked in brake fluid that has seep into both solder sleeves. First step I disconnect the battery and clean the harness. Second step properly repair the ground circuit wire to abs unit connector. Reconnecting the battery and performing another scan. Now reveal all abs no communication dtcs become history and clear out upon scan tool refresh.
Only one dtc remains and it is for the new abs module needing to have zero point and yaw rate reset to initialize the new module. Both resets are performed and abs module initialization is successful.
Voltage drop and it’s practical function in Diagnostics
Moving on to the new concern, operation of the left headlamp assembly was still flickering rapidly. In the process of applying left turn signal all Instrument warning lights came on again. Scan tool showed many modules reported no communication to the abs module!
Looking over a schematic it became apparent that abs module and all ecus stating no communication have two grounds in common located in the driver side wheel well. Observing a dimly lit driver front turn signal and low beam lights lead to voltage drop testing.
Where 3 more ground circuits are discovered to have 0- 8v pulsing on each circuit using a graphing meter to verify. New ground circuits are created to each connector on Left headlamp assembly.
Succeeding in the correct operation and a clean post scan systems functioning as designed by the OEM.
Tools Used
N2 Neuron https://www.curienllc.com/shop
Phil’s Short Electrical Piercing Probe http://AES WAVE
Autel MaxiSys 909 http://Scan Tool