A customer rang us recently and asked us to investigate a fault on his 2008 Jeep Grand Cherokee, he reported a warning message directly after starting the engine indicating a 4 wheel drive malfunction. I began by carrying out a diagnostic check of the 4WD module, I retrieved one static error code namely C1417 - Front Differential Control Circuit Low. This error code relates to the front differential lock control solenoid circuit.
An hypoid bevel differential will by design transmit torque through the path of least resistance therefore if traction is lost on one wheel all the traction on that axle will be lost, which of course is not desirable on an off road vehicle. To prevent this on this vehicle the manufacturer employs a device to lock both front and rear differentials when 4 wheel drive is selected.
The first thing I always do is a visual check so I removed the front undertray and examined the visible wiring around the front differential but there were no obvious defects. When diagnosing faults like this it's important to formulate a procedure to arrive at your diagnosis as quickly and efficiently as possible for obvious reasons. In this case an examination of the 4wd system wiring diagram revealed the front solenoid circuit consisted of 4wd module, pair wires, and solenoid.
I located the 4wd module (under rear seat) and disconnected the multiplug, I began with a resistance check of the circuit, this was done by probing Pin 14 (Supply) and Pin 22 (Control). This immediately revealed an open circuit, next step was to locate where the circuit had been compromised, I disconnected the 2 pin plug from the differential and bridged the connectors, back to the ohmeter where continuity was now present.
It was now evident a problem existed in the circuit within the front differential, the customer was informed and he gave me permission to remove the differential front cover and investigate further.
Once dismantled the cause soon became evident, a broken wire in the harness connecting the solenoid to the differential casing. A new harness and solenoid was installed, the error code erased, and following a road test the vehicle returned to a grateful customer.