[shuotong]

I have encountered engine ground cables corrosion problem and while there are extensive DIYs of how to replace these cables, I would like to provide a diagnostic to other owners to save yourself from a headache. I highly recommend you to use a heat shrink tube to protect the new cable from future failure again.
All E90s and M3 have two ground wire, one for the engine block and another is for cylinder head, which BMW calls "interference suppression cable". Main grounding cable is located under driver side of your engine, and the interference suppression cable is located at the passenger side, top of the engine and it's attached to the cylinder head. Your engine will be running with either one of them still present, but to this point both of my ground cable is fully corroded and need replacement. My main ground cable which presented under the car looks far worse than the interference suppression cable, I believe this applies to everyone as the interference suppression cable is better protected by the hood, so even you have no starting and charging problems right now, you may want to check your main grounding wire is still working before both wires broke.

Quick test for main grounding cable without raise your car & remove engine shield:
Remove the nut holding the interference suppression cable to chassis (P1), then lay aside the cable from the mounting point. Don't forget to temporarily reinstall the nut because there's another cable attached to it as well. Then try to start the car. Your car should crank and start, otherwise the engine grounding cable is bad.

Symptoms:
car won't start during a track event, at first I thought battery is discharged because I left the car on without the engine running, so I asked my friend to push it for me. However after I hook up my diagnostic scanner, the battery shows 12.2V with engine running, drive home for about an hour and voltage dropped to 10.8V (alternator is not charging). I have multiple screenshot during the track event to monitor my coolant temperature, and before my car had this starting problem, the voltage while engine was running was at 14.0V.
Although I can drive with push start, after I have inspected the ground cable and

Cleaned error memory and the following pops :
CAS
A0B4-starter operation
A0C1-output, terminal 50
A0C2-terminal 50, signal too low

Diagnostic:
My alternator and starter were replaced within the last 40000km and it's very unlikely for both of them to fail at the same time. To make sure it's your ground cable causing the no start and no charging problem, use a booster cable from the negative jumping point of the chassis to the engine block or cylinder head, thus creating a ground temporary for the engine block. If you're able to start your car, then it's your ground cable.


Heat shrink tube:
Replacing the cable alone is not good enough, as your new cable will corrode overtime as well. If you live in northern climate that's a bigger issue. When you get your replacement cables, bring them to some places that sells heat shrink tube (Home Depot etc.) and test what size of tube you'll need to fit them.
Cut enough length of the heat shrink tube so it fits over the terminals and cover the entire length of copper. You can reference to P2-P4. I did not remove the OEM label so it looks a bit bulked in the middle where the label attached, but it doesn't affect the performance, as the key is to seal the terminals so the air/water can not get into the copper portion. Heat and shrink two terminals first, then heat the middle of the cable to let tubes firmly attach to the wires.

[Wolf 335]

There was a thread where someone gave a very nice explanation as to why you should not heat shrink the ground cable.

Your cable, even though heat shrunk, is still not sealing against moisture.

[shuotong]

Originally Posted by Wolf 335

There was a thread where someone gave a very nice explanation as to why you should not heat shrink the ground cable.

Your cable, even though heat shrunk, is still not sealing against moisture.

This 9th gen Accord transmission ground cable has a soft tube outside the transmission ground wire, and the portion it protects does not change colour at all compares to exposed portion when I slightly pull the cable out of the tube, the other side is protected by black electrical tape and I removed it just to see how well it works, and I saw very promising result. Older BMWs have insulation on the engine ground cables as well, I don't see why not to use heat shrink at least to drastically reduce oxidation of the exposed portion. Only post I found against use of heat shrink tube is from M3 post, and I don't find it's credible because the guy did not even know how to use it on a flat cable.
Most of the heat shrink tube I have worked with including this one has a transparent inner layer(possibly some sort of glue), which become very sticky under heat and seal the rounded terminals.
I have also found a revised the interference suppression cable 12427555222 for N52 equipped 1 and 3 series, doesn't see it's insulated from BMW Canada's diagram, but the actual product picture from FCP or ECS is indeed insulated. I'm waiting for it to arrive and see if I can further improve my engine grounding. As of now, both old cables were snapped.

[David9001]

Originally Posted by shuotong

Quick test for main grounding cable without raise your car & remove engine shield:
Remove the nut holding the interference suppression cable to chassis (P1), then lay aside the cable from the mounting point. Don't forget to temporarily reinstall the nut because there's another cable attached to it as well. Then try to start the car. Your car should crank and start, otherwise the engine grounding cable is bad.

Just tried this since I’m having slow cranks and sometimes no cranks, was wondering if it was starter or grounding and when removing this wire, car doesn’t crank, only clicks. So it is the grounding? No change it is the starter?

Edit: replugged the wire and it still won’t start, only clicks

[Ughandi]

I had these replaced on my own car. The guy over at Arc Terminator makes BEEFY grounds from welding cable (temp rated). I had him make upper and lower engine cables for my n52. Works fantastic, and I no longer worry about the electrical system. Good prices as well (not sponsored. Just a *very* happy customer)

Upper:

https://www.e90post.com/forums/attac...1&d=1700170106

https://www.e90post.com/forums/attac...1&d=1700170106

Lower:

https://www.e90post.com/forums/attac...1&d=1700170174


Link if interested in the lower ground. Contact him for the upper since it's not on his site (but he has the specs).
https://arcterminator.com/products/ground-cable-upgrade

[Tiberius73]

If you suspect a bad ground cable to be behind various low-voltage issues you can attach a jump-lead from your alternator, e.g. one of the bolts holding it to your engine block, to your chassis. On E90s, E70/71 (and actually a lot more BMWs) there are easy bare metal parts of the chassis near the alternator, but you will likely have to coil the jump lead a little to make it work semi-safely.

I say semi-safely because I'd NEVER recommend daily-driving this - do this ONLY to test! You shouldn't drive around a lot with this even to test if it's hot and you are likely to get stuck in traffic. A 10-20 minute test-drive on smoother roads should give you a clear indication if it's helping or not.

The other important thing is to make sure the jump lead is on securely and won't fall off when your car moves. The crocodile clips usually can easily and tightly latch onto the alternator bolts and are then held fairly reliably and you can attach the other end to a similar bolt, strut brace etc.
Don't use anything less than a jump-lead because if your actual ground strap is really bad then a really large number of amps can get pulled through that lead! Enough to instantly set fire to any house-hold grade electrical wiring.

[lordnikon]

I'm aware the car has two ground straps, upper and lower.

Just want to confirm:

https://www.fcpeuro.com/products/bmw...le-12427549393 (Upper)

https://www.fcpeuro.com/products/bmw...nd-12427578869 (Lower)

?