I have a strange issue that needs help. Ever since repairing my coils, spark plugs, and valve gaskets, when I put my vehicle in key accessory II position (ignition, just before starting the engine), the dash fades a little and restarts. This happens a couple times and then it just turns off completely. It's an issue because I need this key position to run diagnostics.

I think this started after I reset my trouble codes using my OBDLink MX+ device, but not too sure. I also neglected to disconnect the battery during the repairs.

Any idea what the issue could be? My car seems to turn on and idle just fine.

Most likely a low (or bad) battery (the big one in the trunk).

you can't run diagnostics on these cars without adding battery support (unless you do it wrong with the engine running)

worked on mates one on monday (CL500, 2009) key on engine off - interior blower motor running it was eating 27 amps, fan off for the 60 mins I was looking at stuff it was eating 22 amps all the time

funny enough he was saying his icarsoft won't operate unless he starts the engine - then went on to add the engine stutters and messes about as the diagnostics run - which is exactly the point - its not meant to, as the comms around various systems is throwing curve ***** at everything all the time...

I notice on my father's e class tractor it almost stalls as it checks the engine management when the vehicle is running - its just the wrong way to try and work out what’s not behaving

Noted. Should I be trickle-charging the front or rear battery while running diagnostics?

In my current issue though, I am no longer running diagnostics but the ignition still continues to reset and basically fail. Do we think this is a bad rear battery like previously suggested?

trickle chargers don't really have the grunt - mine is a 40 amp device and its often using most of that on my car (which has more toys than his CL), I believe up to 37 amps is normal with the key on and door and interior lights wasting petrol.

the Merc methods is

twin battery (but I do the rear most of the time and it doesn't care)



single battery - note the non battery earth !

This helps so much, thank you. I just connected my CTEK battery charger PROPERLY and now it's at the very first phase of charging, which lets me know the rear battery is likely depleted!

Two more important questions. My batteries are genuine Mercedes. Are they lead acid or AGM? My manual states the rear is lead acid but mentions nothing of the front battery. Internet says front is AGM. This matters as a setting on my trickle charger.

For charging during diagnostics, do you mean i need something like ?

the front is only to start and is std battery

the rear should usually be a monster AGM thing

people get muddled with the later car with the silly big front one from 2008 (which I'd have would be an AGM) - they can have a tiny on under the dash

AGM is just a design of battery inside - it started life for the military - and rather than some lead sludge hopefully sitting there doing its chemistry in a puddle of **** - they wedge a slice of glass matting between each lead plate, so if you chuck it over the hedge the sludge can't fall off and short out against each other - and the puddle of **** is trapped in the glass mating so they don't leak - so they are far more durable and capable of high faster rates of charge and discharge

Got it! So does that item I showed look like the type of charger you're recommending? I had always thought the trickle chargers were enough, so i wanted to confirm.

that is incredibly cheap - the fact they claim it can do LiFePO4 but not latest lithium stuff is weird - likely cheap and nasty - but at that price might be worth a shot

I have this as my budget power supply https://www.sealey.co.uk/product/563...maintainer-40a

this element "13.5V DC power supply for battery support function providing support for the battery during prolonged electronic diagnostic checks". is instead of having a battery in circuit - not sure its any cop - if programming cars meant to have volts at 14.5

these guys make the real stuff but the prices are like $3000 upwards

The GYSFLASH 101.24 CNT FV is a high-power 100 A CONNECTED charger with inverter technology. It keeps a 6 V, 12 V, 16 V and 24 V Lead or Lithium battery at a perfectly stabilised voltage during diagnostic phases. It also guarantees optimum charge quality for servicing the most advanced vehicles. Its USB connectivity makes it 100% customisable.

https://www.gys.fr/prods-PFJ/CHARGER...PFJ/en?lang=en

That's a nice unit and I'm here in the states. Couldn't I just hook up the 40amp charger to my system while diagnosing or am I misunderstanding?

connect it as per the picture - set on auto for an AMG and see how it gets on

snap on do one that holds the car at whatever volts you want and winds up the amps as the load demands - my little one kind of does that too - both without pulling cables or or faffing about

the 13.5 volt bit is more for showrooms where the battery is left out and the kids can jump in, scuff the door trims, steal the ashtray and cigarette lighter whilst dreaming they'll buy one soon

Understood loud and clear. Thank you for the help!

For any searchers, @Jaap was right and it was the trunk battery that needed replacing. Once replaced, all was back to full functionality.