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#1
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Tracking a floorboard vibration at ~40 mph
In my 1982 w123 300CD.
New tires, good, tight suspension, no steering wheel play. Tires (15” bundts with 195/65r-15 Cooper CS5 Ultra) we’re roadforce balanced. One is showing 34# of roadforce balance, the rest are all 8-12#. Tires all roll smooth, the one that’s the worst is the drivers front tire, and there is minimal to no shimmy in the steering wheel. Car tracks slightly to the right, but the tiniest bit of adjustment keeps it straight and true. Tires do not appear to be wearing funny. At around 40 mph, I feel a rumble and vibration from the floorboard/seat. Minimal to none in the steering wheel. Harder to notice when at faster speeds. Sometimes more notable when decelerating. Using a vibration meter, at 60MPH, and knowing the tire revolutions/mile, I can see that the third harmonic is as bad as any tire motion. Usually tire motion (14Hz) should be way higher. This points to the driveline leading up to the diff. So I took a look underneath this afternoon. Everything looks straight and perfect. I had done subframe bushings last year: Squeaky subframe bushing Diff mount looks good: As does the driveshaft: As does the csb as far as I can tell. Can’t really shake the driveshaft and get any motion out of it. I guess I could get it up while on the kwik lift and cause the wheels to spin, car in drive. Maybe have a helper take it to 40 or so and see. But what else should I be looking for? Is the best bet to just remove the driveshaft and check things from there, with it out and able to be moved independently? Thanks!
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) |
#2
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I see both sides of the center mount bushing, and the rear driveshaft guibo/flex disc…
But I don’t see a pic of the front guibo…
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread "as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do! My drivers: 1987 190D 2.5Turbo 1987 190D 2.5Turbo 1987 190D 2.5-5SPEED!!! 1987 300TD 1987 300TD 1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere! |
#3
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Try shimming the center support bearing away from the tunnel and see if it changes anything. I had a W124 start a vibration at 38 MPH felt in the drivers seat going away on the highway. A shim between the nose of the diff and the floorboard changed the pinion angle and the vibration completely stopped. W123 diff is set-up/suspended differently.
Good luck!!!
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer |
#4
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Quote:
And the other one…
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) |
#5
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Quote:
Does your driveshaft have a u-joint? I’d suspect that in your case, maybe the slight change in angle caused the bearings to sit and roll slightly different. It seems like my best option is to pull the driveshaft (I’m assuming shimming would just be some washers on the bolts?), versus dropping the thing. But isn’t it prudent to check all the components for binding?
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) |
#6
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The fundamental looks to be below 10hz with 2nd and 3rd order harmonics occurring around 14 and 42 hz respectively. Provided that these measurements are accurate they are too low to be either tire or driveshaft related. I believe you are dealing with an aggregated suspension harmonic issue due to excessive play. Id check all joints for tightness and double check alignment.
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#7
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If you need a visual youtube heavy duty ford death wobble.
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#8
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The 3:1 ratio of frequencies of peak amplitudes (42:14) suggests that the driving force is in the driveline and an axle shaft is being driven, or contrariwise.
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#9
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Quote:
Yes a washer as a shim between the body and the center support bearing hanger would work as a test. Pull everything to check it? You could, but I'm not sure if it is worth it. Good luck!!!
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer |
#10
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Quote:
So I’m not seeing how 14 and 42 become the 2nd and 3rd harmonic, but that’s just me and my cursory math. The 34lb roadforce tire, though on the front axle and not causing steeering issues or wobble in the steering wheel, could be in play, as could anything else I guess… It just seemed obvious to me that if a tire vibrated once per revolution, and a driveshaft spun three times per tire revolution, that the math works out…. And was a pointer. Everything feels tight which is also what confuses me. No funny play or movement anywhere….
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) |
#11
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Quote:
What do you mean by “being driven” and “contrariwise”? There is some slop in the differential, and I did get new axle washers. I had this thread sometime back with some video and pics… W123 Axles: What do I have and what is this play?
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) |
#12
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Quote:
[Just as a curiosity, a 3.07 final drive has tooth counts of 15 & 46.] The frequencies that have been observed are way below the first order (natural) resonant frequency of any of the driveline components, let alone the frequencies of 2nd or 3rd order harmonics. |
#13
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Quote:
I get the driver or driven aspect now, thanks for that. It just seems like the relative magnitudes would be the indicator. It could be that there is play in the diff, a beating someplace, the driveshaft, the one iffy tire… whatever. Question now is how to isolate and move stuff. I have a lift that puts the car 20”+ up in the air at the bottom of the tires. It’s a nice height for diy without a pro lift, the downside is to get the height it is roll-on, so I need to jack the rear on the lift somehow. Thank you for your explanations!
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) |
#14
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I’m interested how this gets solved: I’ve got the same/similar problems with new-looking guibos and CSB. I expected to find a misalignment of the arrows on the driveshaft halves but they look correct.
I did isolate my vibration to driveline by putting the car in neutral at about 40mph; the vibration remained and slowed with speed irrespective of engine idle rpm. Sent from my iPhone using Tapatalk |
#15
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2nd, and 3rd harmonic because I see a spike in the single digits which could be the fundamental or spurious.
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