21 Jun 2024
The voice of the independent garage sector

Building in a fault

The subject this month is something I am sure we have all come across; Parts that cause more problems than they solve. The vehicle in question was a 2006 Chrysler 300C with EDC 16CP31.
    
This car drove in to us under its own steam with a constellation of lights shining in the dash, including the battery warning light. As usual, we started with a global scan of the vehicle while on battery support. Upon completion we found several low-voltage fault codes, but the one I was most interested in was U1132 Lost Communication with Generator – Active.
    
Armed with this information I formulated a test plan:
1) Test the vehicle battery
2) Check voltage at the battery to see if it was charging at all with the engine running; The answer was yes, and we could use an amp clamp as well but I saw no need
3) Find a wiring diagram for the system so we know what should be where and connected to what, I.E Comms line
4) Find the alternator on the vehicle physically to do testing
5) Do volt drop testing on ground and B+ side as we need both of these for the alternator and comms to work correctly
6) Connect the scope so we can see what is happening on the LIN bus control wire
7) Make a decision on the fault according to outcomes.

Upon testing the vehicle battery, the result was; ‘good – needs charging’. This was only to be expected, so a substitute battery was put in and the original put on charge. It is always best to start with a known good and we already had a copy of the DTCs that were present. With the multi meter installed across the battery and 12.6v shown, the car was started up and the lights turned on to load the system. The voltage was going down even when picking the revs up. This proved why the battery light was on in the dash and why we had the alternator LIN bus malfunction DTC.
    
Next, I found the alternator, which was on the driver’s side under the engine. To get at it, I had to go through the suspension and subframe. As I had the scope at the ready, I first checked between battery ground and the alternator casing. This showed less than 100mV, so good. The next check was between battery positive and the B+ terminal at the rear of the alternator. Again, the same result here, less than a 100mV. This was good, both passed. For the next test, I connected the scope using a back probe in to the Lin bus connector. I found a good signal, 12 volts to about 0.5 of a volt, so it passed that test. Please refer to Fig.1.

Wiring and response integrity
At this point we knew the LIN bus signal came from the engine control unit ECU, so we didn’t need to test there as the signal was good. It was looking like the alternator is was fault, but how do we prove it, as well as the ECU-to-connector integrity? What I did at this point was to ground the signal down, pulling it to ground and reread the DTC. As expected, we now had two DTCs for the LIN bus, two malfunctions, but two different DTC codes. This proved wiring and response integrity of the circuit, so I deduced that a new alternator was required.
    
What arrived was an aftermarket example, due to availability problems with the OEM part. With the new one fitted by my colleague, which is not the easiest to do, the shout came out “Kev it’s still the same – not working!”
    
As always, you get that sinking feeling and question yourself. What did I miss? Back to the job then. Rescanning the original DTC returned ‘U1132 lost communication with Generator –Active’.  
    
Believing it was the alternator at fault, I ran through the tests again just in case I had missed something, but the results were conclusive; Definitely the new alternator at fault. So, another one was ordered, this time from a different manufacturer. The part duly arrived, only this time I wanted to try before we fitted it to the vehicle. With the second new one in front of me, I extended the LIN bus communication line to it outside the vehicle. I thought, I know, I will put the jump box on to the alternator to give it live and ground, this should allow it to talk. By now, some of you will be ahead of me doing the test this way. Without the B+ and the vehicle ground connected to the alternator, how can the circuit be complete for feedback logic to work? When the CTC was checked to see if it cleared, it did not.

Steady charging voltage
We extended the B+ wire and a ground connection along with the LIN bus wire to the second new alternator on the tool box. The DTC was checked again and erased and did not return. Next, I cycled the ignition a couple of times to make sure it didn’t return and rechecked; No DTC returned for loss of comms with the alternator. With this product seeming to be okay, it was installed and tested. Charging was occurring and control of the alternator was taking place. With the scope recoupled to the Lin bus signal and the headlamps turned on and off we could plainly see the LIN  bus control signal altering on the scope screen. We could also see the charging voltage staying steady, at around 14.2 volts, thereby proving the repair.  Please refer to Fig.2
    
With this saved to the scope for future reference we could now hand the vehicle back to the customer with confidence in our repair. A full post repair global scan was also taken once we finished the job. This allowed us to be aware of DTCs in other system which bear no relationship to the repair we carried out. This enabled us to advise the customer of up-and-coming likely future repairs, should they wish to do anything about them.