Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
I'd start by checking for worn suspension joints - jack the car up so the wheel is off the ground (you’ll need to do this for all 4 corners) and shake the tyre from top to bottom and from side to side (with someone holding the steering wheel on the front) there should be no play in either direction. Any play in 1 direction will usually require a joint to be replaced, if there’s play in both directions then the wheel bearing may be at fault. If theres play only up and down its the strut top mount - to find this one you have to shake the suspension strut it self up and down with the wheel off the ground - so you may need to remove the wheel to check this one.
Now with the car jacked up on that side with the wheel about 2 inches off the ground use a steel bar under the wheel and lever the wheel up and down slightly, you should be able to look at the end of the axle and see if the suspension arm is moving up and down separately to the axle.If so replace the bush
To feel if there's a worn strut top bearing then lightly hold the spring by hand and have your assistant move the steering from left to right with the wheel on the ground - a worn bearing will feel 'notchy' through the spring. The wheel needs to be on the ground for the test to load up the bearing
Also check for worn drive shaft joint - typically these won't produce any vibration unless badly damaged but they can be noisy or 'clonk' for a long time beforehand, turn the steering to full lock and look at the drive-shaft rubber boot there should be no rips or tears in it and the suspension should be clean and dry of any grease. If you reverse at a reasonable speed with the steering on full lock a worn joint will 'click'.
And how should I diagnose the front differential vs transmission? What is throwing us for a loop is that it is not constant. Some days it does it, and some days it doesn't.
a differential noise will tend to only occur under load , so if its diff noise then you tend to get a whine when accelerating
So, we jacked it up. When the other tech put the vehicle in drive, it began whirring. I put the ears on the mount, no vibration. I put them on the rear end, and mad vibration, so I'm thinking we're looking at a bad rear end. Your thoughts?
I'd next suggest to drain the oil from the rear differential, straining the oil through some paper towel as it empties
this will catch any debris
if you find lots of metal then yes you may want to consider a replacement diff
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