Weird electrical issues.

[ianthemidget]

Hi I have never been on a forum before so I hope I'm doing the right thing.

I recently 3 months ago bought a 1l 2014 polo from a friend that had had it from new for my son to drive.i drive the car home it was perfect. it sat on the drive for 3 months until he passed his test. I kept the battery charged, he passed on Tuesday I refitted the battery and that's when the nightmare began. Long story short the car bcm goes mad has 3 attempts at sorting itself out then shows fault with the horn, indicators low coolant drivers door open. I can reset the bcm ok and stop it happening if I indicate left (mad) or use hazards soon as I stop indicating the bcm throws a fit again. I have checked the earth's around the car. Removed and replaced the bcm back again to clean the contacts. Also hazard warning lights stop the problem I have obd11 pro can reset and read all the modules bcm and gateway. I'm an electrician and don't usually have many problems but this has me stumped. There are a few people mentioning rear left connector earth which I have checked and is ok. Yesterday it all came back on for about 5 hours. Started it this morning and back to square one. HELP! I feel so bad for my son having a car he can't drive. PS the start stop is coming up as a fault now which it wasn't the battery I have checked and it's doing about 400A and at about 14v any help much appreciated Ian.

[littlepolo]

Old Polos do weird things with the electrics. In this case, I'd check all ground connections around the body (especially where water ingress might have corroded ground straps, such as tailgate, lamps, drains...). IIRC there is a main ground strap under the battery box but if the fault is only BCM check as suggested.

[ciclo]

What is the age of the battery? If it's an original VW battery, it usually has the year stamped on the negative terminal...
Perhaps the Battery Monitoring Control Module (BMC?) has been damaged.

As a test, how about testing with another battery that you know is in good condition/new? First, connect the positive terminal, then the negative terminal without the battery regulator connector connected, then the battery regulator connector. Next, run a diagnostic scan and clear all the fault codes. Then, perform a functional test by driving a few meters and run another diagnostic scan to ensure everything is working correctly.

When changing the battery for a new one, it must be indicated in the Battery Monitor Adaptations (GW adaptations).

[ianthemidget]

I did test the battery and it seemed ok 400a when tested. But I put the battery on charge restarted the car no issues so Im looking towards the battery. I will get a new one it wasn't a start stop and wasn't coded in the car properly. But it's definitely pointing towards the battery. Thanks for the reply. It is a Halfords battery 2019 so it's done ok I guess. I will get a bigger one too as it's tiny 202 size and 027 can be fitted.