wankel8

Hello mbworld, diagnosing a 2011 ML350 w 100k-miles. A new battery that load-tests 'good' out of the car will then fail the test when installed with all connections clean and tight. The test is done through the posts in the engine bay and this was done with 2 new batteries.

The car always starts but it also always cranks slow like it has a weak starter. I want to dig into the power cables and connectors but I don't know where to start or if there's any other special things to check with these advanced German vehicles. Something is causing a problem between battery and battery-posts under hood as well as to the starter.


The car has virtually no corrosion and overall drives like new. Relays, power cables, ground. I should mention the tester uses the current not just the voltage for a pass fail
​​​​​I can see the positive cable harness that runs from the battery area to the positive post, little distribution block, alternator.

Tsumi

Slow cranking on these cars are more a symptom of a dying starter. I would try the starter first before trying to chase down electrical problems.

Also, that post might not have a large wire going to it. It's an emergency jumpstart post designed to use in conjunction with the existing battery. A better test will be off the starter solenoid.

wankel8

I hear ya but a dying starter doesn't explain how the same battery passes a test when direct but fails when in-car

Tsumi

Again, I don't think that post was designed to carry that much current. If you really think the wiring is an issue, test it on the starter solenoid, not the post.

expl0rer

I have a GL 550. I bought it with a brand new battery installed by a dealer who sold it (not MB). For about a year the car was lazy starting. First crank was slow, then it would sometimes start after 3-4 more cranks, and sometimes 10 or more cranks. I thought it was the starter as I had a new battery, but it turned out that it was not the correct battery for the car. I installed a new Interstate battery, correct for the 550. The car starts on the 3rd or 4th crank every time since. Of course, saying that I practically jinxed myself. HA!

I did not replace the starter. I was waiting for warmer weather to get to that, but in Feb the battery died after I used the rest function for about 20 min. That prompted me to look closer at the battery and that's when I discovered that it was not the right one. On the plus side, now I have a second spare battery for the boat. The one I pulled out has held a charge well even though I drained it.

All that said, you may still have a starter on its last leg.

Maj. Dundee

You have no alternative but to bench test the starter which calls for the removal of the starter.

If your starter has a braided cable & has corrosion (see arrow), the starter must be replaced.

If not, remove starter and bench test.

DavidLN

Measure the battery voltage during cranking. You want to see >10.2V.

Dave

WI06ML350

What ever happened here? I’m having the same issue on my 06 ML350 3.5L V6 with the battery passing when directly connected under the passenger seat but fails even tested under the hood. Also has slow cranking at startup.
thx

wankel8

New starter fixed her right up. I was being a bone head testing battery from engine bay posts