Why my VW Polo TSI failed to start after the BCM Max upgrade

When I was done with all the work and it was time to test the coding and functionality, the car just wouldn’t crank. The voltage was 12.3V and yet the starter just wouldn’t crank.

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After a lot of acrobatics in my temporary workspace for August 2021, the wiring work was complete and the BCM was installed, it was time to connect the battery and complete the VCDS coding and adaptation work.

Factory Mode and Key Coding

Most BCMs are shipped in the Factory mode and this needs to be deactivated before we can code it completely. You can read more about it here. The instructions I found online on multiple platforms did not help me code the remote keys. So, I was without a working remote key fob for quite a long time. Then my friend Shardul came to my rescue and remotely helped me complete the pairing of the key with the newer BCM. It was Shardul, who told me that most instructions were not best suited for the latest BCM and it was thanks to his experience that I could move ahead and adapt my keys.

Batteries Don’t Fail To Amuse

Before working with wires, I had taken the negative terminal out to prevent any drains. Then when I was done with all the work and it was time to test the coding and functionality, the car just wouldn’t crank. The voltage was 12.3V and yet the starter just wouldn’t crank. VCDS threw up a fault regarding the Ignition Signal being intermittent, and after checking everything, I realised that one of the BCM connectors was loosely fit. I reconnected the loose connector and the TSI motor roared back to life, making me give out a huge sigh of relief. Before the BCM connectors were properly fit, I had some intermittent issues with BCM-related functions like the Door Locks, Windows etc.

Then, I got to test everything and it was all good. I ticked all installed modules in the Installation list and the Auto-Scan had no errors after the coding was complete. After that, the BCM was suitably parametrised to support the Headlights that I intend to use, without which it will not enable headlight control. This is a vehicle-specific procedure, and hence BCMs cannot just be swapped unless you have a matching parameter. I decided to wrap up the work the following evening, but then I noticed on one occasion, that the car was cranking slowly as if there was a drain. The battery dropped from 12.4V to 7.5V in a matter of five minutes so I was sure there was a drain somewhere.

There were no blown fuses and parasitic drain tests came negative. Since it was dark, I decided to call it a night. The next morning, when the battery was reconnected, the voltage was back to 12.3V. Now I was confused. In most cases, when the voltage is 12+ volts, the car at the least tries to crank. In this case, even at 12+ volts on the voltmeter, the car was just refusing to crank. It wasn’t even trying to crank, so I was wondering what the issue could be. Again, I thought it could be an issue of loose BCM connectors, but this time, that did not seem to be the issue, as the BCM connectors were clamped as tight as they could be. 12.78V and no cranking, made me believe this was not a battery issue and so the witch-hunt was on. In most cases when it’s been a bad battery, has been when the car tries to crank, but doesn’t manage to start, but this was clearly different.

I called up all the experts I knew, and sadly they were all busy. I was convinced there was some mistake somewhere. So I rechecked all fusible links, fuses, and wiring changes I had made and could not find a mistake anywhere. There was no short in the car that the multimeter could detect and so I was genuinely at a loss. My mindset and morale were low and I momentarily gave up trying to figure out what was wrong.

Some friends thought it was a faulty BCM which was either malfunctioning or perhaps not connecting properly, but nobody I knew who had installed BCM Max had ever reported an instance like this. My only hope was to try another BCM, but BCM Max is not something which is commonly found lying around to try. The conspiracy theories roared in, like the BCM must have been factory rejected for having a battery drain and hence available on sale, but none of these theories highlighted a possible solution that I could implement.

The next few days were not the best days of my week, as I watched my car sit idle in the parking lot. It was frankly a demoralising site. A perfectly drivable, enjoyable TSI was sitting idle in the garage because I was too obsessed with making it better. And to make things worse, I was moving abroad for my Masters the next week. I wanted to do everything right before I left, but I started doubting my choices. I thought I wanted to do too much in too little time. After some retrospection, I made up my mind to not fret and be upset before I was to leave home and was mentally prepared to face any consequences and conflicts with dad because of a job that seemed to not have gone so right.

I quickly realised that for someone like me who was shortly about to leave home, a dead car should be the least of my concerns and so I began to get over it, and my dad’s words, “Love living things, not non-living things” helped me feel comfortable in a tough time like this.

Calling the VW Doctor

A few days later, when Nikhil Patil was finally free, Nikhil, Sheethal and Joe, all passionate car lovers and my friends, came home to check on the problem. Nikhil had a quick look, inspected some basic electrical components and frowned and asked me for a pair of jumper cables. His diagnostics did not take longer than a minute, as he checked different components and already had a confirmed diagnosis for me.

I was stumped. I thought of every possible problem, double-checked on it, and never even thought of jumping the battery. This is perhaps because the electronics were working fine and the voltmeter gave it a clean chit for voltage. Never in my right mind did I suspect I should jump the car first. I was so obsessed with finding a fault or a short circuit in the car, that I forgot the most basic step. The Polo’s elder sibling was then roped in for a jump-start.

Well, some people are called the doctor for a reason, and Nikhil Patil is undoubtedly Dr. Nikhil Patil, VW Skoda Audi and beyond, for his passion, experience and expertise in German cars better than the manufacturer made them. Confidence-inspiring issues like these, have made it a no-brainer for me to recommend Nikhil and his team for all retrofit tasks.

I dug around a little after everything was up and running and realised that, unlike the BCM High, BCM Max had an inbuilt battery management mode which was why most electronics like locks and windows were dead when the BCM detected a weak battery. That being said, the 1.0 TSI has a really small EXIDE battery which probably drained really quickly when I was busy working on the car. This minor hiccup took 2 days of my time but finally, my worst nightmares were behind me.

Wrapping Up, Total Work Time

Wrapping up the wires and packing trims was a slightly boring task, but I completed it the next day, and the car was ready to roll out after being idle for nearly a week. I realised how much I missed this engine when I was finally revving on the expressway, on my way to one of my favourite places, The Mechanix Automotive, Baner, with a new set of features on offer.


The BCM Max is a fairly rare part, but I have recently seen a few BCM Max units available on eBay, both new and used. Unfortunately, most of them do not have the appropriate firmware for working on our headlight systems and radio frequencies, thus making it difficult to predict whether or not it will work, plus since this is not a plug-and-play upgrade, this is something a majority of owners will not resort to, because of limited resources available to diagnose and fix electrical issues related to such retrofits.

I would have loved to discuss further about the parts and pricing of the wiring kits, but that would end up making this an unpleasant thread because of the disappointing experiences I’ve had with a few people on this planet, and I like to believe it is better to share good things and avoid experiences which could propagate hate.

The Polo 6R community around the world is beautiful, and they have performed some of the most exciting retrofits on the PQ25 platform, and this energy and sense of collaboration is one of the reasons, I continued to improve my car, and it is the reason why I continue to share updates about my car, because there could be a little something that I share, which could inspire someone else. I am thankful to all of the enthusiasts and friends I’ve worked with, and without their help at the right times, I wouldn’t have been able to grow and improve my cars. It is quite sad that this retrofit space is lately being highly commercialised by individuals, but I’ll reserve my thoughts on this for a darker day.

Net Experience

The BCM Max upgrade ended up feeling like a full-body workout, with a lot of sets of abs crunches. I’d say it is incredibly painful to do this upgrade on a Polo because of the limited working room. The entire task would have been a lot smoother with the dashboard off. If I ever have to take the dashboard off, I’ll definitely consider redoing some of the wire routings behind the dashboard. It seemed like a nightmare while I was doing it. It feels like the biggest mistake when there’s a big obstacle but in the end, the achievement is a car which is a lot better than it was, a year ago.

Completing this retrofit was about the satisfaction of performing a task of high difficulty, and the learnings I obtained as we faced and solved problems. Thanks to that hustle, my car is ready to pack in more features and it’s finally a car which is a proud asset to have in 2021 and the many years to come because it feels technologically on par with the other offerings at present, if not, better in some places.

I’m sure this thought resonates in the minds of many original PQ25 VAG users who aren’t proud of the cost-cutting Volkswagen has been doing on the MQB-A0-IN cars, and in the next few posts I’ll try and share a few of the improvements I’ve made after this BCM upgrade, and for the benefit of potential enthusiasts, I’ll perhaps post about Direct TPMS first. That’s it for now folks, enjoy the backwaters of the Khadakwasla dam, in Pune.

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