Charging system issue?

BHAMDOC1973

MEMBER
2005 MERCEDES BENZ SL500
V8
2WD
AUTOMATIC
87,000 MILES

Bought this car which I believe has been parked for a while. Needed 2 jumpers to start it. I knew it had 2 batteries. The starting one up front and the one in trunk. When I jumped it and measured the rear it would read ok. So, I bought an OEM Benz battery for the front. Went to start it and nothing. Weird as the one in back is for electronics. Jumped the back one and it started. Changed the back one and it started right away. Red battery light on. Measured the vikts with my ohmmeter and it was around 14 in the trunk but not charging the battery at the front. Strange. How can one be charging if the charging system isn't working if say the alternator is bad. Am I missing something here?

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
42,961 POSTS

I would run a CAN scan on it because I have seen the front SAM module go out and cause this problem.

https://youtu.be/u-4syLc-ifQ

and

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

DANGER:
- Risk of explosion from electrolytic gas. Risk of poisoning and caustic burns from swallowing battery electrolyte. Risk of injury with caustic burns to eyes and skin from battery electrolyte or from handling damaged lead-acid batteries
- No fire, sparks, naked flames or smoking. Wear acid-resistant gloves, clothing and eye protection. Pour battery electrolyte only into suitable and appropriately marked containers.

1 Disconnect ground cables from batteries

NOTE: First the on-board electrical system battery in the trunk and then the starter battery in the engine compartment.

2 Remove starter battery
3 Move locking mechanism (1), unlock cover (2) and remove
4 Unlock and remove electrical connections (3) from front passenger-side fuse and relay module (K40/4)
5 Unlock front passenger-side SAM control module (N10/11) with front passenger-side fuse and relay module (K40/4) at the side (arrow) and pull slightly upwards out of the guides in the fuse and relay box
6 Unlock and remove fuse holder (4)

NOTE: Observe installed position, which is indicated by color coding and design.

7 Pull front passenger-side SAM control module (N10/11) with front passenger-side fuse and relay module (K40/4) slightly upwards out of the guides
8 Unlock and disconnect electrical connections (5)
9 Pull front passenger-side SAM control module (N10/11) with front passenger-side fuse and relay module (K40/4) slightly upwards out of the guides
10 Unlock and disconnect electrical connections (6)
11 Open locking lever (7) as far as the stop and remove battery cable (8)
12 Remove front passenger-side SAM control module (N10/11) with front passenger-side fuse and relay module (K40/4)
13 Install in the reverse order.

Check out the images (below). Please let us know what happens.

Image (Click to make bigger)