LanceDal

Just got a used SL500 and already ran into this problem. Been researching the last few days and can't find a solution.

Problem:
- After starting the case, a message displayed "Battery Voltage Too Low. Please Start Engine or COMMAND will shut down in 3 min". After 3 minutes, the COMMAND shut down. Within that 3 minutes, everything works fine. Radio is good, Roof can close/open etc.

Background:
- New rear battery. 12.75V @OFF. 14.2V @ON
- Front battery: same voltage read as rear

It seems the alternator/voltage-regulator are fine as the voltage at batteries are good.

At first, the problem came few days after the rear battery was replaced and its voltage was read at 11.7V when car is OFF. It is possible that as due to some parasitic issue. I bought a battery-tender and charge the battery to full (see the voltage above), but the problem persists.

Since the battery was low once, do I need to dis-connected and re-connect it after fully-charged to reset something?

Lance

LanceDal

Found something that is interesting: In the Dyno Mode, the instrument show 10.5V when OFF and 11.5V when started. At the battery, the voltage is actually @12.5V and 14.5V.

It appears that the system read wrong voltage. The fact that it is lower may indicate a bad wiring?

Anyone know how the system read the battery voltage?

pmercury

iTrader: (2)

don't you have 2 batteries ?

LanceDal

I do. It only shows 1 reading so it's possible that the system read both together.

LanceDal

Got it fixed. The system read correct voltage now and no more Low Voltage warning.

HOWEVER, I'm not sure EXACTLY what is the culprit.

There is a black-box next to the rear battery. It has wires run into it and F78 7.5A fuse. The fuse read 0.3ohm, so with a 7.5A, the voltage drop would be 2.15V, which is the difference between what the system read and the actual voltage at the battery. Replaced this fuse and it's good.

HOWEVER, F78 is for the steering, per the fuse chart I found online :-(. So I'm not sure if the fuse was the problem or because I opened the clamp and a loose-wire was corrected.

In any case, if you have Low Voltage problem, be sure to check the system reading vs. actual voltage. It may be something there.

pmercury

iTrader: (2)

what I mean by that is that it is a sophisticated system
not just one reading for all

bobterry99

I think that fuse is actually F33, and it powers the ignition switch, N73.

Austinsec

How did you get it fixed?

LanceDal

I replaced the 7.5A fuse.