Mercedes Benz C Class Hybrid Isolation Fault – P1CA100

One of our most interesting case studies yet. The final findings are a real eye-opener!

The car in question arrived on the back of a low loader – it being unable to start or achieve ‘Ready’ mode. The dash would light up and display a red battery malfunction light.

We initially scanned for DTCs (fault codes) and the obvious one jumping out at us was: P1CA100 – Isolation Fault, Contactors Open. Without delving into any depth at all here, the high-voltage system on a Hybrid Vehicle must be kept separate from the low-voltage (12V) system. The high-voltage system needs to be “Isolated” from the 12V system. What our fault code indicated was a breaking down of this Isolation. High voltage was getting to where it shouldn’t be getting….

(This article is NOT going to go into detail on the methods of Isolation Testing. When working on any HV System, appropriate PPE should be utilized. When using an Isolation tester, appropriate PPE should be utilized.)

So, we have to admit: we initially went down a rabbit hole. We pursued an avenue of testing that ultimately proved fruitless. Whilst this avenue didn’t lead us to our answers, we picked up valuable pieces of information that we could pass on to our customer regarding his HV Battery condition and Health….

We took the “Contactors Open” part of the fault code and interpreted it as: ‘fault is present with the contactors open’. Contactors open implied the HV Battery was in a state of NOT supplying voltage to the car. We therefore assumed the Isolation Fault lay WITHIN the High Voltage Battery. We dropped the battery and split it. There was evidence of water ingress and corrosion.

Evidence of corrosion and water ingress inside the Mercedes Benz C Class Hybrid Battery

Upon examination and testing, isolation was found to be reduced but not to such a level to set the Fault Code we were experiencing. By cleaning, airing and drying the battery, we increased its isolation properties.

Inside the Mercedes Benz C Class Hybrid Battery

We had further testing to do. An obvious test point was at the High Voltage Distribution Module. Amazingly we found the 3 phase pins to the Electric Machine (Motor / Generator) were shorted directly to the chassis ground. BOOM! We were onto something.

Mercedes Benz High-Voltage Distributor

Our fault could only be either: damaged High Voltage line(s) to the motor or the motor itself. We had to drop the motor enough to disconnect the high-voltage lines from the motor end. That we did and the fault was found to be within the motor itself. At this point, Mercedes Benz would’ve ceased testing and would just quote the customer for a new Electric Machine (motor). Our inquisitive minds (and a customer willing for us to carry on testing) removed the motor from the vehicle. We were gobsmacked with what we found…

Mercedes Benz C Class Diesel Hybrid Electric Machine (Motor / Generator)

We found a bolt that had come loose and dropped into the windings causing a direct short!! Other loose bolts were about to join the party as well. Unreal. Said bolt was secured back into its rightful place and all other bolts torqued up. Vehicle built up and ‘Hey, presto!’ vehicle working again – as it should. A very, very satisfying diagnosis.

Mercedes Benz C Class Hybrid P1CA100 – Isolation Fault with Contactors Open. A loose bolt is the cause of the lack of isolation.

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