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Discussion Starter · #1 ·
Posted about this a while back and did some investigating and still can't figure this out.

My 1990 W150 is pulsing on braking and after a 15+ minute drive it gets worse and begins to shake over 40 mph when coasting or braking.

As recommended I replaced the drum brakes, all brand new, mostly parts from Napa. This didn't work.

I took the truck to a drive shaft & diff shop and had them test drive and look around. They did not see anything out of the ordinary. I watched the test drive and as the truck went by it looked sort of like the axle was rocking/jerking around but I'm not sure. Going to stick a GoPro on the frame to get a better look.

I am having the shop replace the Ujoints and balance the shaft and take another look around next week. Until then, just seeing if I can get any further opinions... Thanks!
 

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How are the leaf spring bushings? (axles jerking) are the U-Bolts tight? Motor mounts? Cross member tranny mount? Have you said anything about alignment, tire balance, and tire pressure test checking for hard spots in the tires? I think you said they were new IIRC.
 

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How are the front rotors? Pulsating upon braking is a sign of warpage/uneven thickness.

Bucky
 

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Diagnosis by parts replacement should be a last-ditch effort. Have you got 4 heavy duty jack stands or access to a lift? Support both axles, put in 4 Hi to get all the wheels spinning and see if you have the shaking. If so, take a pair of hose compression pliers and squeeze off the rear brake line. If it continues block off both fronts, if it stops do the fronts individually to try to pinpoint the cause. If it doesn't shake with all 4 tires off the ground, swap to a different set of wheels/tires. I'd guess a rotor warped or with a high spot. Driveline vibrations typically occur under load and smooth out under decel/braking...
 

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Diagnosis by parts replacement should be a last-ditch effort. Have you got 4 heavy duty jack stands or access to a lift? Support both axles, put in 4 Hi to get all the wheels spinning and see if you have the shaking. If so, take a pair of hose compression pliers and squeeze off the rear brake line. If it continues block off both fronts, if it stops do the fronts individually to try to pinpoint the cause. If it doesn't shake with all 4 tires off the ground, swap to a different set of wheels/tires. I'd guess a rotor warped or with a high spot. Driveline vibrations typically occur under load and smooth out under decel/braking...
And if you have them up on stands, you can pull the tires, put a metal rod across two more jack stands and press the end of the metal rod up against the rotors to check for warpage. I did this with a car once and it worked out very well.
 

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Anyone checked the front wheel bearings? you'll get a heck of a vibration if loose or worn. Its good maintenance and probably required at this point to look in each front hub and inspect/repack bearings/replace seals.
 
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a good shop would find frontend problems [ BJ's wheel bearings ] l wanna know why change axle u-joints and balance the front shaft when it does NOT spin while driving , if it IS spinning then THATS your problem
 

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Discussion Starter · #8 ·
These are all good points, thank you guys.

The problem is definitely in the rear. The pulsing is not felt in the brake pedal. None of the issues can be felt in the steering wheel.

Here’s something to chew on. When I have a load in the bed, everything smooths out. (as if the driveline geometry is corrected when the rear is weighted down?)

some side notes, there’s a bit of a thump when going from a coast to accelerating. Also a loud clunk when shifting to reverse.
 

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Well, maybe... If the drive line shop didn't see anything obvious on the outside, could it be that the diff or the internal bearings are bad? Maybe sheered teeth on the ring or pinion? Just guessing...
 

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Since you clarified it wasn't in the front then I would do the jackstand test mentioned above.
Since it disappears with a load in it then that may point to a driveline angle issue, or something in the suspension that gets "corrected" carrying a load.

How's the mount under the transmission/transfer case? If that's sagging then it could throw the angle off.

Bucky
 

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These are all good points, thank you guys.

The problem is definitely in the rear. The pulsing is not felt in the brake pedal. None of the issues can be felt in the steering wheel.

Here’s something to chew on. When I have a load in the bed, everything smooths out. (as if the driveline geometry is corrected when the rear is weighted down?)

some side notes, there’s a bit of a thump when going from a coast to accelerating. Also a loud clunk when shifting to reverse.
More information is always good. Consider the way the rear axle rotates the pinion up or down when accelerating or braking: on accel, pinion noses up, rear of vehicle rises, opposite when braking. Shifting to reverse is the same as braking, pinion going down. Check front eyes of rear springs, spring front mounts, u-bolts, etc. Have someone hold the brakes, trans in neutral, and check how far you can rotate the driveshaft. The "no problem loaded" definitely makes me suspect bushing/mount issues: when the suspension is pre-loaded with no free-play, you don't notice the problem.
 

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warped front brake rotor , no back feed to brake pedal .
 
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rear drive shaft splines are shot or drive shaft not fazed correctly and or 9 1/4 clunk pinion nut is loose and yoke is falling off
 

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Discussion Starter · #14 ·
Just for fun, as well as diagnostics, I made a video of the axle while test driving.


One thing I realized is the shake is MUCH worse when braking downhill and more weight is taken off the rear end.
 

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you have a broken spring or leaf spring bushing or center pin as you can see the rearend MOVE at minute 1;58 and you can HEAR a LOUD clunk
 
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on the right side
 

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Yes it does look jiggity. But I haven't seen many similar videos to compare it to.

I guess if you can't figure it out just drive it loaded lol (the truck not the driver)

Bucky
 

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Discussion Starter · #19 ·
I went ahead and let the driveline shop do some things. They replaced the u-joints, balanced the drive shaft and changed the diff fluid but they didn't find anything out of the ordinary. After a test drive they suggested trying a proportioning valve for the rear brakes or possibly adding a traction bar.

I had another mechanic take a look and he pushed the truck back and forth in park and he felt like the leaf springs were flexing too much. We figured they could be worn or simply those particular springs don't work well with the 4" lift blocks.

I was thinking of upgrading to a 4" lift spring an no longer run blocks.
 

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I was going to suggest looking at your springs. My truck had a similar issue like yours and after I installed my new Skyjacker springs all of the rear-end problems went away. I went with the 2.5" lift springs and was the best investment ever. My truck drives like a brand new truck. I strongly recommend Skyjacker springs because they ride sooo nice on the road and articulate awesome off-road also. Plus they love Jesus 🙏
 
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