Rudeney

MY2011 SL550. I've had a few instances of sluggish cranking over the last month. Tonight, tried to start the car and it was especially sluggish, and did not start. Now I have no starter rotation at all. Everything works except there is no operation of the starter when I turn the key (or tap the KeylessGo Start button). Consumer battery is 12.1v and the starter battery is 12.8v. The consumer battery drops to 11.8v when trying to start, but the starter battery does not drop at all. There are no errors - none on the MFD, and none in SDS scans of the EIS, ECU, or BNS. The only sound I hear when trying to start is a brief hum from the rear, which I assume is the fuel pump. Any ideas?

moretech

I'd pull the starter batt and get it load tested at autozone or somewhere. Got to be sure starter batt is good and STRONG before I'd mess with anything else. Batteries can be tricky and a test is an easy thing to do, usually free too. I'd do the same for the trunk batt if a new starter battery gets you going. Keep us posted on your solution. This is a good thread.
moretech

Rudeney

Yeah, I'm thinking the same thing. It's just odd that it doesn't drop a volt - not even 100th of a volt, then I turn the key to the start position.

next2pool

It sounds like the trunk battery is trying to assist the starting while the front starting battery has a poor connection--it should show a voltage drop. Are you checking the front battery voltage at the terminals or on the battery studs?

Rudeney

The onboard voltmeter in the "hidden" menu of the MFD shows the consumer (rear) battery voltage. The power supply module will runt he starter and comsumer batteries in parallel when there are undervoltage conditions (10v or less):

http://benzbits.com/R230/BatterySystem.pdf

I put a voltmeter directly on the starter battery to monitor its output.

My problem is the starter. I checked and voltage is supplied to the relay and the relay is working. I checked power at the solenoid and it is energized when the key is in the start position, but there's no action. I tapped on the solenoid with a wrench, and got one "click" out of it when i turned the key and that was it. I suspect it just needs a solenoid, but I'm going to replace the entire starter. Now I just need to find one. None of my local retail parts stores show it in stock, but I will call tomorrow and see if they can find one nearby. Worst case, I order online.

next2pool

I agree they will work in parallel if needed and if the rear battery voltage goes down, that indicates it is in shunt mode, but why doesn't the front battery voltage go down while attempting to start?

Rudeney

In my case, the starter solenoid was bad, so it never engaged and thus there is no drain on the starter battery - it was doing nothing.

secondchance550

Thanks for sharing solution to this particular scenario. SL550, hands down, is the most complex car I have owned so far. I am learning about this car through reading issues and solutions.