Hey everyone,

I didn't think it would ever happen to me, but it did. I installed my battery in backwards and now my car is slow to start.

Here's how it happened. I had to leave my car on the side of the road after a fuel pump died on me, with the hazards on for hours. After I got the car towed home and diagnosed, the battery didn't have enough juice to even lock the car, so I pulled the battery and put it on a trickle charger until I was able to get the fuel pump ordered. Once I got the fuel pump in and installed, I took the battery off the charger, and in a bit of a frenzy installed it (backwards lol). I know I should have slowed down on something so basic, but I wanted to drive my car to work that day and I for some reason did not think to check if I installed it right. I put the positive on the negative, and then when I touched the negative to the positive, I saw a huge spark and immediately knew I won the knucklehead award for the year. I quickly undid the other terminal, flipped the battery and when I connected everything, I saw the lights come on and I felt a wave of relief.

The car started, but it sort of hesitated and took about 5 seconds of cranking to turn over. It has been doing the same thing ever since, cranks for 5 seconds and then starts. Another thing is that the drivers side tail light flashes if I have the lights set to automatic, even if the car is off. The driver side turn signal doesn't flash and the trunk light is super dim. The radio turns on and off on its own while driving. Other than that, the car runs and drives just fine.

I'm tempted to start testing fuses to see which one it is, I have some free time in a couple days and I'll be able to get down to the bottom of it. I can post a video if it starting in a couple hours once I get away from work.

In the mean time, is there anything else I should test other than fuses? Thanks for reading my ramblings

 

Super late update, ended up diagnosing this on a different forum. The electrical issue ended up being a bad rear SAM (probably shorted when the battery was installed backwards) and the long crank as a defective fuel pump from the manufacturer.

 

Hey puro_amg, installing the battery backwards can wreak havoc on your E320’s electrical system, and it sounds like you’re dealing with a few fallout issues from that mix-up. The delayed start is likely a sign that the starter motor or even the alternator might have sustained some damage from the reverse polarity, causing it to struggle when cranking. The erratic behavior with your tail lights, turn signals, and radio suggests that several electronic modules could have been affected, potentially corrupting the CAN bus communication or frying sensitive components in the Vehicle Communication System (VCS). Additionally, the persistent fuse blowout points to a lingering short, possibly in the COMM platform or a damaged wiring harness that’s not immediately visible. To really get to the bottom of these issues, I’d recommend running a full system scan with Xentry. On the other hand, the youcanic full system scanner is also another popular choice these days, and it’s very easy to use, offering comprehensive diagnostics to help pinpoint the exact problems without needing a mechanic’s intuition. If you do get any diagnostic codes, please post them here and we can help you better. Hopefully, with the right tools, you can turn that knucklehead moment into a smoothly running E320 again without too many more sparks flying!

 

Appreciate the reply. I'm not sure if it was clear, but I've already done all of the repairs and diagnostics necessary. I went through extensive diagnosis on another forum and got all of the answers I needed. I just figured I'd update this thread since I logged in and forgot that I had posted here as well.