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HOST has mechanical question

Mr_ShiftrightMr_Shiftright Member Posts: 64,481
edited March 2014 in Mercedes-Benz
Some advice from the M&R Senate, please.

One of my cars is a Benz diesel. Great car but parts replacements are very expensive.

I seem to have a noisy CV joint in the rear swing axles somewhere. I get a clicking and sometimes a binding noise when I back up, for a few seconds--then the CV joint must load up and it's gone.

No noise forward, no noise on the road---EXCEPT---I have heard the characteristic raprapraprap noise when making a hard, sweeping, high speed LEFT turn. This noise co-incides perfectly with wheel rotation and speed.

Question: Since there are two axles and four joints, each costing HUNDREDS of dollars a pop, I want to GUESS which one is bad.

My conclusion is that it is the right side swing axle, and I'd like to get a good one from a wrecker and save money that way.

Do you all think this is a good guess?

NOTE: Can't duplicate noise on a lift because wheels are hanging. Tough to isolate noise just by walking alongside the car when it's backing up. Noise travels under there, can't be certain.
I guess I could back over a friend but thus far no volunteers.
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Comments

  • 0patience0patience Member Posts: 1,712
    Are you certain that it isn't a wheel bearing noise?
    If I'm not mistaken, they have the hub style wheel bearing?
    Just a thought. :)
  • Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    Well, would I get a sharp cyclical rap or knocking, like a hammer banging on a lead pipe? Or that kind of "boing" going in reverse?

    Seems like a bearing would only growl in a monotone, no??
  • 0patience0patience Member Posts: 1,712
    Oops! Sorry, I misunderstood.
    Does that have a limited slip differential by chance and have you checked the diff fluid if it does?

    I think you are probably correct, but if it has a limited slip differential, I have seen them do some strange things when the additive has given up.

    I know, not much help.
  • swschradswschrad Member Posts: 2,171
    get an old kiddie phonograph, and pull the ceramic phono cartridge and tone arm out of it.

    duct-tape the arm so the needle rests lightly on the half-shaft or housing (you don't want this rotating underneath the needle) on the side you suspect. you are going to have to put some bubble wrap or thin foam under the arm so the needle isn't squashed flat against the metalwork and won't have any flex left.

    rig an extention cable to go into a boombox line input jack, and set it to record. put headphones on so you can monitor it, and back up.

    it's a remoted mechanics stethoscope, and that should either transmit the noise clean as a bell, or not. if not, tape it to the other side.

    I've tried to isolate noise with a microphone before, but road noise and air movement drowned it out. it REALLY gets drowned out with my homebrew 45 khz ultrasonic listening rig. this ought to work. the ceramic phono pickup should be a half to 3/4 volt output, so it's line input level.
  • alcanalcan Member Posts: 2,550
    Find someone who has a Steelman Chassis Ear, secure two of the pickup leads to the outer knuckles, two to the inboard side, and take it for a drive. Might be a trick keeping the leads out of harm's way but it should be doable with velcro or nylon zip ties.
  • Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    I have a friend who has a chassis ear actually but he was skeptical that we could rig it, as that axle is, of course, a spinning part.

    So you guys don't think that the "loading" of the CV joint is a clue? That is, a rear CV is more likely to knock if the car's weight is thrust to that side?
  • swschradswschrad Member Posts: 2,171
    the noise is being transmitted to the chassis, right? so methinks getting soundings off the hub area is perfectly acceptable, and perhaps even closer to the actual problem, so the noise would be louder.

    there is a chance that there's binding on the links someplace, too, and moving a pickup around there could isolate the assembly.
  • Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    Well at $650 per side + labor (if one were crazy enough to buy new at the dealer), one had better guess correctly.

    I can get an axle from the wreckers for $100 but they are rather hard to bench-test unfortunately, unless they are totally wiped out.

    Big monster joints, seem more suitable for a panzer tank. You could use them in the weight room at the gym. Why they put them into service with a less than 100 HP diesel engine I don't know, unless they planned for interplanetary travel distances.
  • swschradswschrad Member Posts: 2,171
    which means they might show up in the next little dinky mopar, since DC is very keen on the idea of parts-sharing.
  • burdawgburdawg Member Posts: 1,524
  • Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    It's a W123 dawg!
  • burdawgburdawg Member Posts: 1,524
    What's the condition of boots over the CV joints? Are they cracked open? Funny, usually the outer left side wears faster (due to the proximity to the hot muffler and tailpipe). Are you sure it's not in the brakes? The rear brakes on my W116 cars, which are about the same, have been noisy while backing from time to time (especially when new, and yes I use the shims). Also, the parking brake gets neglected quite often, can the shoes be sticking, hence the binding you hear? As for the noise when making left turns at speed, are your rear subframe mounts worn flat? Just food for thought, but diesel and 6cyl cars don't usually have axle problems unless the boots are bad and the joints have been allowed to dry out, get wet, etc.
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