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2008 SLK280
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59 Posts
Discussion Starter · #1 ·
I suspect my wife's R171 SLK280 has a battery drain issue somewhere, which I really need to get checked out at some point. As an interim measure, I've been using a trickle charger to keep the battery topped up when it's going to be unused for a period.

We went on holiday for a week recently and when we came to use the car, there was enough juice to unlock it initially and put stuff in the boot but when I came to open the boot a second time, it wouldn't oblige. Because we had stuff in the boot that we needed to take with us and couldn't open it without connecting a power source, I decided to jump start the SLK rather than simply use my own car.

The jump start was successful and we then drove about 45 miles, which I thought would be plenty to top the battery up again (albeit my wife tends to have the heated seats and airscarf on max. constantly when the temperature's below about 25 degrees) but when we came to leave at the end of the day, the battery was dead again.

Another jump start allowed us to complete the journey home and I then put the car on the trickle charger, although I assumed the battery was knackered and wouldn't hold a charge. However, once the charger showed a full battery everything seemed to be fine again until last night (about a week later), when the engine wouldn't turn over and another jump start was required. My wife had mentioned over the previous couple of days that the engine was taking longer to catch but I hadn't thought much of it.

The battery is less than a year old and the fact that a 45 mile journey didn't refresh the battery but the trickle charger did has made me suspect either the alternator or the voltage regulator, with the latter obviously being the much cheaper option.

I've found what appears to be the correct Bosch VR for 拢32 + shipping here but it would be great to know if anyone else has used this part and I would also appreciate any guidance / suggestions in terms of working out whether the issue's down to the VR, the alternator or something else (unless it's just a case of replacing it to see whether it fixes the problem).

Thanks, in anticipation. 馃檪
 

Founding Member 2006
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Any after market kit?
HU
Parking sensors
Cameras
Alarms
Radar

Check sensor for bonnet closed a sneaky drain reported a few times before.
Check battery poles.

Once fully drained the battery will either need a recon (ctek systems et al do this) or replacing.
 

*Registered
2008 SLK280
Joined
59 Posts
Discussion Starter · #4 ·

*Registered
2008 SLK280
Joined
59 Posts
Discussion Starter · #5 ·
Any after market kit?
HU
Parking sensors
Cameras
Alarms
Radar

Check sensor for bonnet closed a sneaky drain reported a few times before.
Check battery poles.

Once fully drained tje battery will either need a recon (ctek systems et al do this) or replacing.
Hi Myk. I did fit a replacement head unit a while back plus a fibre optic adapter for the HK amp (thanks to lots of helpful advice on here) and that seems like the most obvious candidate, even though the only permanent feed is the one required for the HU's memory. I'm not sure how to check whether that's drawing too much current.

My charger does have a repair function but I didn't use that as the battery seemed to recover with a standard trickle charge so do you think that the problem might simply be that the battery needs a proper repair cycle rather than there being an issue with the car's charging circuit?
 

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2005 SLK55 AMG
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For goodness sake, before y'all get into 'what ifs' - check the charging voltage. First put a volt meter in parallel across the battery with everything off. Should be minimum 12.6v. Then have someone crank the car - the voltage shouldn't drop to below around 10v. Once running, you should see the voltage up to 13.8v-14.4v, with maximum output achieved by 1500/2000rpm, indicating that the alternator is charging.
The odds are more likely that a battery has failed than the charging system, despite that it's only a year old. By their nature, they tend to be more unreliable than other components on the car, especially if it's not a premium product.
 

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For goodness sake, before y'all get into 'what ifs' - check the charging voltage.
The odds are more likely that a battery has failed than the charging system, despite that it's only a year old. By their nature, they tend to be more unreliable than other components on the car, especially if it's not a premium product.
Once fully drained the battery will either need a recon (ctek systems et al do this) or replacing.
We know from past experience that new, off the shelf batteries are not always good.
Always worth charging fully before first use.

Heated seats, airscarf, roof all use power and are killers if only doing short journeys.
Not certain the following still holds, but I recall needing to cover 10 miles just to return power used starting.
Add in the other goodies...
 

*Registered
2008 SLK280
Joined
59 Posts
Discussion Starter · #10 ·
For goodness sake, before y'all get into 'what ifs' - check the charging voltage. First put a volt meter in parallel across the battery with everything off. Should be minimum 12.6v. Then have someone crank the car - the voltage shouldn't drop to below around 10v. Once running, you should see the voltage up to 13.8v-14.4v, with maximum output achieved by 1500/2000rpm, indicating that the alternator is charging.
The odds are more likely that a battery has failed than the charging system, despite that it's only a year old. By their nature, they tend to be more unreliable than other components on the car, especially if it's not a premium product.
Thanks for those specific points to check. If the voltage across the battery terminals is in the range 13.8 - 14.4 with the engine at 1500 - 2000rpm, does that mean the alternator and voltage regulator are working fine or am I being too simplistic?
 

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2018 SL400
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384 Posts
I had an 18 month old Varta battery fail in my old 350. It would look fine when charged using my CTEK 5.0, but after a couple of days would be flat again. Sent it back to Tayna (uk online battery retailer) and after testing they confirmed it had failed.
Many years ago I had a new battery fitted to our old trusty Nissan Vanette, through summer it was fine but at the first sign of bad weather it wouldn鈥檛 turn the engine over. On an old style charger it would appear fully charged and the charging system checked out fine. Turned out that the garage fitted the lower AH battery for the petrol model instead of the diesel.
Definitely worth going back to basics.Good luck getting it sorted.
Mark
 

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2008 SLK350
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Thanks for those specific points to check. If the voltage across the battery terminals is in the range 13.8 - 14.4 with the engine at 1500 - 2000rpm, does that mean the alternator and voltage regulator are working fine or am I being too simplistic?
As @Terminal said,yes. To simplistic. Charging of the battery by the alternator is controlled by the ECU and depends on several parameters.
For a proper test you need SDS I think.
Read the attachment.
 

Attachments

*Registered
2008 SLK280
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59 Posts
Discussion Starter · #18 ·
You need to do a parasite draw test.
Took me a month to find it on my slk... after fixing the fault all ok now.
Is that where you connect an ammeter in series with the battery to check the resting current then, assuming it's too high (over 100 milliamps I believe), pull the fuses one at a time to see which circuit's removal allows the current to drop to an acceptable level?
 

*Registered
2008 SLK280
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Discussion Starter · #19 ·
Or this procedure might give you some info too.
Thanks for sharing those docs Pete. Unfortunately the equipment I possess only runs to a simple multimeter and not a volt ampere tester or an oscilloscope, so I might have to resort to professional assistance after all. My concern is if I take it to a regular garage the answer will be "it's your alternator mate" on the basis of that being the most obvious candidate and I could then be throwing away a few hundred quid on something that's not the cause of the problem and doesn't actually need replacing. :confused:

I don't suppose anyone on here could recommend a good auto-electrician in the Reading / Wokingham / Bracknell area? 馃
 
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