W164 battery replacement
 

Just found out my battery was about to die in the ML. The cranking was getting more and more sluggish.

To check your battery voltage, do the following:
All doors, hood, tailgate closed
Key in Pos I
Navigate to the odometer display using the left hand lower left steering wheel button.
Press the R button on the cluster 3 times.

The voltage will stay on the screen while you glow and crank the engine.

My voltage was 11.9 V with key off, 9.5 V glowing, 6 V cranking :eek:

Off to the dealer to get a new battery. Dealer wanted $600 including labor, said roadside assistance will not swap M class batteries in the field.

So for about half that I walked out with a new battery. It's really not that hard to change yourself.

The battery lives under the front passenger seat. You do not have to pull out the seat. There is enough range of motion in the seat to bring it all the way up and forward enough to reveal the battery chamber. Mostly.

After you have brought the seat up there is a cover for the heat vent. Pull that straight up to remove.

You will see the corner of the battery. You have to cut the carpeting to find the rest of the battery. Don't worry, the cut you make is hidden under the seat in its normal driving position.

You need to make 1 cut about 4 inches straight forward from where the opening ends. The Owners Manual tells you to make 2 cuts but you really only need 1 cut, then you can peel the carpet aside.

Now you will see the air duct. It is held in place by one of those push pin fasteners. Remove the push pin and lift out the air duct.

Now you should see the entire top of the battery.

The battery is held down by the short sides. The forward end of the battery slips under a ledge in the battery tray. The back end has the hold down clamp, secured with two 10 mm nuts. You will need a 10 mm and a long extension. A "magic fingers" grabbing device is essential to remove the nuts and the hold down, there is not enough room to get your hands in there past the battery.

Be sure the key is off and totally removed from the switch. If you have keyless go, pull the button out.

Now disconnect the power connections, - first then +. Be sure the cables are held back so they don't touch the battery posts.

Disconnect the vent tube from the side of the battery.

Now lift the old battery out. Slide back to free the battery from the front retainer. It's heavy and awkward.

Before setting the new battery in place, there is a vent plug on the plastic cap covering the + terminal. Break the plug off and insert it in the hole in the + end of the battery. Set the battery in the well and slide forward to engage the front retainer.

Using the "magic fingers" set the hold down bracket in place. Either using the "magic fingers", a magnetic socket, or by taping the nut to the socket, install the 10 mm nuts to secure the hold down.

Attach the vent tube to the hole on the - end of the battery.

Reconnect the + and then the - battery terminals. Put the red cap on the + terminal.

Reinstall the air duct and the push pin. The air duct just slips into the air chamber opening below the center console.

Reposition the carpeting and the heater vent grill.

Put the seat back to normal position.

If you have Comand with GPS, the cluster clock will do the "time warp" and go to the right time automatically once you turn the key to Pos I. If not you will need to go into the menu to set it.

The windows need to be retrained. Push the window switch down but not past the click. Now pull up but not past the click and hold there until the window stops. Count to 3 and then release the switch. Test the window, make sure it goes fully auto up and auto down.

Also the sunroof needs to be retrained. Open the sunroof then push the switch forward but not past the click. Hold until the roof closes plus the count of 3 then let go. Now push the switch up to raise the rear of the glass. Pull down to close and hold until it is closed plus the count of 3. Test the sunroof, it should auto open and auto close fully.

I also don't have conclusive results yet, but the tailgate also seems to not be acting weird any more. Sometimes it would not open by itself especially on my sloping driveway. Since I changed the battery it has worked every time. Don't know if it had to do with resetting the computer or the better voltage.

Great write up. Thanks. I will save this info.

If you attach a battery charger or other 12V power source to the terminals to maintain system voltage, you won't have to monkey around with the retraining and resetting issues.

If you use the jumping terminal in the engine bay, the leads won't be in the way.

That is true. However bear in mind the + cable will be live with respect to the chassis so take precautions when the cable is disconnected that you don't short it to the frame. A thick plastic bag and zip tie will suffice.