E36 Chasing an electrical gremlin, need help! 1995 M3

[NickRay]

Hopefully someone on here has had a similar problem and found a solution. I haven't been able to figure out anything that alleviates this.
The problem:
Car has difficulty starting, behaves like the battery has low voltage. Once the car is on, pressing the brakes immediately turns off the radio and also dims the interior lights (dash, OBC etc) pretty significantly. If I flip the headlight switch to fogs or headlights on, it also trips the radio, but once either the fogs or headlights are on, pressing the brakes will not turn off the head unit and the interior lights seem to dim less.
Background of recent repairs/mods:
Clutch replaced
ZKW headlights installed
Blower fan replaced
Interior leak from HVAC box (firewall side) which allowed water to get in the passenger side footwell fixed
H/K amp and rear 6x9s installed (from a 1998 M3), removed the H/K amp shortly after because this problem appeared

For about the last month the OBC has been saying to check the tail light circuit. Every time I checked, no tail lights were out. When I had the interior leak, a not insignificant amount of water got in the passenger footwell until I figured out the leak was from the HVAC box seal. The car starts every time but struggles and clearly has an electrical gremlin.

Help!

[NickRay]

Oh, I have also gotten the factory 3 button fob for the remote locking and started using that with the car for the past 3 months. The car already had the system installed from new but did not come with the remote when I got it.

[E36328Coupe]

Car has difficulty starting, behaves like the battery has low voltage.

Need a proper description instead of fuzzy words. For example do you mean the engine is rotated slowly when operating the starter (cranks as they appear to call it in the US and "turns over" as we call it in the UK)?

Google OBC secrets if you don't have a voltmeter, unlock the OBC and then you can see (and tell us) voltage before and after starting.

- - - Updated - - -

Once the car is on, pressing the brakes immediately turns off the radio and also dims the interior lights (dash, OBC etc) pretty significantly. If I flip the headlight switch to fogs or headlights on, it also trips the radio, but once either the fogs or headlights are on, pressing the brakes will not turn off the head unit and the interior lights seem to dim less.

This sounds like classic common earthing, where a poor earth from once circuit that is shared with another so the circuits ar more connected to each other than earth - you would get the ETM and trace to find common points


Alternatively you would make sure tail light earths and tracks / connectors are good, boot lid wiring in the rubber grommet is good and check all the battery positive and earth cable points.


Especially check anything you changed with wires that is common to the faults experienced - because if the faults did not exist before which ever change preceded the problem probably caused it, for example when the clutch was done did they undo the engine mounts, one has the earth lead on it

[NickRay]

Thanks for the reply! The motor turns over slowly, and sounds as if the starter is struggling to rotate the engine. The bendix engages fine. This issue came up after installing the H/K amp, and after I removed it the issue went away for a few weeks. During that time the car started perfectly. I changed the clutch, and the ground on the right side engine mount wasn't touched, but may have been stretched ir damaged from the engine leaning backwards when the transmission was out of the car. I replaced the clutch before installing the H/K amp and had no issues before/after putting the new clutch in. I agree that I need to check the bootlid wiring and I will test the car with the factory amplifier removed and tail lights disconnected.

[E36328Coupe]

So, do the OBC thing.

Also if a bad engine earth is suspected a jump lead from the lifting eye on the vanos to a good earth point on the chassis, such as strut mount nut, can be used to prove if it is engine earth.



Also if you changed the clutch worth checking the starter is securely attached and if you undid the power connections or they got strained making good

[Audi200tq]

There's a weakness of the wiring harness where it bends when the trunk opens. The wire covers wear away and the wires touch each other intermittently. This is a common cause of mysterious electrical gremlins. However, there's no way to know for sure if this is causing your problem without inspecting inside the harness.

[NickRay]

Update: checked the trunk harness and it all looks fine. Battery terminals were snug. Opened the hood and found this under the fuse box cover!!! This power line was super hot and is melting the plastic away. Battery is now disconnected. Please advise! lol14b459c7-ebdb-4e5f-98a7-79661006e88c.jpg

[hotdish]

You probably have a big short in the wiring. Pull each fuse one by one and check each circuit for a short. Also, check that + cable to make sure it didn't melt through the sheathing in another spot.

[NickRay]

Found the problem. Under the fuse box where the main power cable bolt goes, the nut sandwiching the leads together had backed out and was about to come off. The melting plastic must have been from arcs rather than a short. Tightened it up and the car behaves totally normal now. Hopefully this is useful if someone is ever chasing a similar problem. Really surprised/relieved how simple the solution was. If that nut backed all the way off, the car would have probably burnt to the ground. Thanks to everyone for the responses.

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