hello, this might be a long one but i will try to include all the things i'm dealing with at the moment! looking to see if anyone with some electrical knowledge of w203 thinks i have an electrical issue.

i have noticed recently my lights external and internal go dim when the engine is revved (headlights seem really dim anyway!) i also had my central locking module blow (which i have since repaired). another recurring fault is the "front right side marker bulb" which comes up now and then, i've gone through 3 bulbs this year but i'm not even sure its the bulbs! Dashboard/instrument panel will flicker half its lights too sometimes! also blown 2 rear dome/overhead light bulbs.

alternator is new (bosch refurb) and i have also tried a second alternator for testing. diodes measure out fine. alternator puts out 14.2v when driving. puts out 13.6-13.8 at idle. battery is varta silver which is about 18 months old. i have had the battery on charge whilst i tested with another battery, as soon as i put the charged varta battery back on it will go from 12.7v to 12.3v. sometimes battery voltage is as low as 12.1v as soon as i put the key in the ignition it will start so slowly drop!! cranking is slow. same issues apply with a spare battery.
whats also weird is if i press the accelerator, even just lightly, the voltage drops to around 13v sometimes lower to 12.5v which also makes the rpm drop and jerk. Is this normal for voltage drop on acceleration?

the only evidence of any electrical issues are.. Front Sam had some corrosion under some of the relays and fuses. tried my best to clean them up and put them back in (might take the whole sam out and check inside)
ground strap from the transmission is green/corroded



and a bit frayed (i will upload some pictures)
***continuity from alternator b+ to ground, chassis, engine. everywhere seems to have continuity from b+ (with and without battery in circuit)***

no EML, no codes. no codes when scanned with star with my local mb guy.

 

Do a test. Start engine, switch on all lights, heated seats and so on. With a multimeter switch to millivolts and measure from the negative pole of the battery to a blank part of the engine itself. The voltage drop must be below 40 mV or you have a bad ground connection.

 

It probably isnt the source of your problems but check the resistance of the ground cable from the battery to the chassis. I had weird electrical issues a while ago and although it looked completely fine on the outside when i checked the cable it was like 150 ohm and when i pulled off the ring terminal there was massive corrosion that had worked its way up and inside the wire.