I recently wrote about inside air filtration and what might happen to a car that sits too long without being driven. Usually, such problems are a simple fix once you’ve unraveled the root cause, but not this time.
A customer brought in a 2003 Mercedes-Benz SL500 last month because the battery was going dead, the key remotes did not work correctly, the air-conditioning blower motor barely blew and the instrument cluster had several warnings dimly lit.
During the initial test-drive, we saw that the battery light was on and that most of the nonessential items in the car were not operating properly. After returning to the shop, we put the car on the scanner and pulled codes. There were at least 60 codes, most of them for low voltage. We then tested the main battery and it came in at 8 volts at 200 amps. The minimum voltage for the battery is approximately 9.6 volts at 350 amps. We then checked the alternator output. The alternator was putting out 14.2 volts at 55 amps. Obviously the first step was to replace the main battery.
After replacing the battery, we went through all of the relearn procedures and basic engine adaptations. We relearned the air-conditioning fan speeds, steering angle adaptations, window set points, engine idle adaptations, etc.
Once that was done, we checked the key remotes. The batteries were weak in both keys, so we replaced them. We then programmed the keys to the car.
The next step was to clear all of the existing codes and test-drive the car. During the second test-drive, the battery light was still on, but all the nonessential systems and the charging system to the main battery worked normally.
Blowing a fuse
When we got back to the shop, we pulled codes again. Instead of having 60 codes, we had one. It was code B1829 (component fuse for starter battery circuit is defective) in the battery control module.
We removed the covers in the trunk and passenger-side footwell to gain access to the power transfer module and fuse boxes. The power transfer model is in that footwell. There are two large amp fuses in a junction box that power this module. There is a 250-amp fuse that is used in conjunction with the start battery that was intact and a 100-amp fuse that is used to charge the start battery that was blown. Due to the design, you cannot just replace the fuse – you have to replace the entire junction box.
Once we replaced the junction box, we had to relearn the steering angle sensor and then drive the car. Finally, the battery-light warning was off. It seems that in this car’s design, if either the main battery or start battery is having a problem, the instrument cluster shows you the battery-light warning.
The reason I found this so interesting is that this was a combination of many problems. We had a main battery that was dead, we had key batteries that were weak and we had a blown fuse.
It’s possible that the main battery and the key remote batteries were the original problem. If a key cannot communicate with the car properly, it won’t start the car. This could have led to an unnecessary jump-start that might have blown the start battery charge fuse. Then once the start battery got the car running, it could not run normally because of a dying main battery.
While that is one scenario, it is still unclear what blew the start battery charge fuse.
Either way, make sure to always read your owner’s manual before trying to jump-start your car. Because so many cars now use a battery control module or body control module, it is not always safe to jump directly to the battery. Many of today’s cars use auxiliary jump connectors that are in the engine bay. You just need to know where they are and how to use them correctly.