dar

Trying to figure out a strong vibration coming from transmission/driveshaft. Curious if the symtoms pinpoint anything specific.

Car is a 1972 Dodge Charger RT clone. Recently bought the car so some of its history is sketchy. Has 383 from a 1968 Charger and brand new TCI Automotive Torqueflite 727 streetfighter trans. Both were put in about a year ago from what I was told. The trans was a kit that came with a new torque converter, I believe. I do not know the details beyond that the car had the engine and trans installed within a year of my buying it. Also has 8 3/4 rear end with 4.11 gears.

At 55 mph a strong vibration starts, seems to be coming from trans or drive shaft. Gets worse, builds to the point where its really shaking the car at about 65 mph. At no time is the handling of the car affected, drives straight and true, no issues. But anything loose in the car is rattling like crazy, its not a subtle vibration. sounds like the car is coming apart. I can then "drive through" it - if I accelerate to 70 mph it goes away completely.

I noticed the following. RPMs range from about 2800 to 3200 when the vibration is happening in 3rd gear (only), then at 70 mph I am at about 3400 RPM and it feels like the trans/torque converter engages and vibration is gone. I can feel something engage and the car seems to take off a bit, but its rather subtle. According to the specs on the trans and torque converter, the stall speed is rated at 3000-3400 RPM. I am not sure if this is connected.

Front U-joint was bad, had it replaced along with yoke from trans, thought this would solve issue initially but no change.

As a side note, sometimes (but not always) speedometer needle bounces wildly (20+ mph fluctuations). Different minor issue, but curious what is causing that.

If any of this makes more sense to someone than it does to me, I would appreciate the insight. I have some guesses but I am not an expert in trans/drivetrain issues.

Thanks in advance.

RacerHog

Did you have the Driveshaft checked for balance while it was out?
Also, you know tires and wheels have been checked and don't have issues? Rite?

Drag Pak

My brothers Model "A" street rod had that issue . Turned out to be as simple as too much rear tire pressure. Good luck.

Archer

dar -

First rule out what Bob suggested.
Probably need a new speedo cable. Could be the gear, but less likely.

Last thing to look at would be the torque converter. (Defects/poor installation.)

Archer

TVLynn

drive shaft its the easiest to check,also check the drive shaft angle/ phasing , after that I would check the Trans converter see if it has any un needed weights,and the bolts are tight, then check the front balance r.. Could they have put one from a 400 by mistake ?? Something like that can drive you crazy. Could also be an unbalanced engine ??? was it ever rebuilt ?

dar

Thanks for all the suggestions. I was aware tire/wheel balance could cause vibration but I concluded since it very quickly disappears at about 70 mph this might not be it, assumed it would get worse, but the viration abruptly disappears above that speed, maybe I am wrong on that.

I do not believe drive shaft was checked when pulled. Long story, but I was up against deadline (30 days) to have car MD safety inspected - they are very strict here, so just had yoke and u-joint replaced and put back together. At the time I was doing about a dozen other things myself to get it to pass, from new brakes to fixing the horn (which did not work for 4 different reasons, it was funny). No historic plates, its now registered like a daily driver. The mechanic did comment that the exhaust was run too close, one pipe appears to slightly rub on the driveshaft, he noticed a mark, a thin line scribed into it. Is that the obvious problem? It just does not feel like that type of vibration which is so specific to the RPMs and the final torque converter lockup at about 3400. Idk.

I wondered also about the match to the engine and balance. Had the car since October, and some of its history I have had to put together. For example I have no idea why it has 4.10 gears and very old air shocks in the back. It was a 318 car someone built up. Hood had 340 emblems but the seller who only had the car for a year told me they pulled a 318. Replaced with a 383 from a 1968 Charger that spent its life most recently in a baracuda show car but the guy pulled it since it was wrong for the car, then sold it to the seller of this '72. I saw pictures where the new trans and engine with new headers were being dropped in. All the numbers check out, the engine is a 383 from 1968. The engine appears to be in good condition, but I have not torn it down yet because it runs fine. I just know very little about internals.

What I wondered about was when the mechanic was putting this all together, was the fact that the engine was from '68, the car is from '72, and the new transmission, ordered from Jegs, was everything matched up? Not an expert in the balance issue, just aware of the issues somewhat. Then there is possibility something was assembled wrong. The seller and mechanic, who I met briefly when I picked car up 3 states away, seemed honest about the history, no games, but maybe mechanic made mistake. I did notice he forget the dust cover on trans, I had to pick one up, but I did not think much of it.

From what everyone is saying, I think I need to have drive shaft checked, and if that does not fix it, then might have to have the torque converter checked, and then check the balance issue. Trying to avoid pulling things apart too much, wanted to enjoy the car this spring before tearing the engine down for mods, but maybe its enevitable I will need to pull it. Its a shame, car runs nice. Well, except at 60 mph when it feels like its coming apart, but then at 70 mph it runs nice again.

afastcuda1970

A bad wheel bearing on an 8.75 axle can also cause a vibration at random speeds.