Engine/Transmission (1994 - 1998) 4th Gear Light Throttle Vibration

[MMcCallie]

Tuesday night I noticed some bucking during light acceleration in 4th gear. The whole truck was vibrating very quickly. It seemed to shake from the floor up. The steering wheel was not shaking but even the hood was shaking with the vibration. It seemed to be worse on rough roads and only occurred while accelerating very lightly around 40 miles per hour.



Today, Saturday, I was towing near 7,000 lbs worth of trailer loaded with Highland coos around the Ozarks and the shuddering was worse. I started while the trailer was empty, only weighing around 4,000 lbs. The shaking now occurs at higher speeds roughly 65-70, though it also happened once in the mid 50s. Never happened at lower speeds today but throttle input was greater due to the load.



The shudder has become far more aggressive with the whole truck shaking violently. As far as noise, the sound of the whole truck shaking is quite loud so I couldn't aurally pinpoint a source, just a loud roar. We pulled over at one point to check things out since the problem was so consistent and scary. 4 wheel drive is not engaged, the rear driveshaft seems normal with no slop in the u-joints when shaking by hand and watching it with lots of testing in a lot.



I'm suspicious of the transmission. I would expect that if the transmission is slipping that the temperature would increase. It did seem higher than usual, around 145 on the interstate though it never exceeded about 171 degrees even when pulling through muddy pastures, water holes, and steep hills off road. The transmission used to run under 135 even in the summer. Ambient temperature was about 63 degrees today so it was not exceptionally warm.



Fuel pressure is mostly in the upper 20s but is not varying enough to be the cause, I think. Tires look perfectly smooth while driving. Using a laser thermometer gun my rear differential only got up to about 180, as were my rotors. I have sticking caliper squeal from the rear but I feel no pull. I find it hard to imagine a sticking caliper that is not slowing the vehicle could cause such a harsh vibration.



The way the truck is bucking it feels like a loose throttle spring. I say this primarily because the shudder is usually triggered by rougher terrain such as a bridge, around 70 mph, under 25-35% light throttle acceleration. The bucking continues after the initial vibration until I either accelerate or decelerate.



I've tried downshifting into 3rd gear while maintaining speed and the vibration ceases. This again makes me suspicious of 4th gear. There are no changes in shifting behavior, timing, or feel. The only slippage indicated by my PCS TCU is when I shift between 3rd and 4th gears for just a second during the shift. The transmission oil is full and still has great color, no odor or debris. Brian of BTS told me not to worry about changing the filter or oil until it looked bad so I never have in the years I've been using this 4R100.



I'm also considering the problem could be the driveshaft going out of balance. Or perhaps my engine's timing or GSK has slipped. There is no engine smoke and vehicle speed and engine RPMs do not bounce with the truck, which surprises me since the vibration is so harsh it is impossible for me to keep my foot steady on the throttle.



I'm hoping someone may have had a similar experience and found the cure. Otherwise, does anyone have any other ideas or steps I should take toward the diagnosis? I am not sure if I should try to find a good diesel shop or drivetrain shop. I don't know of a good one of either in the area but money is extremely tight so it would be best if I can figure this out and repair myself. For the moment, I'm puzzled.

 

[NIsaacs]

After you drive it a few miles, use your laser on the u-joints. It sounds like they are dry and resisting their rotation. If they are bad they will be hot.



Nick

 

[MMcCallie]

Today I used my laser gun on the u-joints and they were actually cooler than the ambient temperature of 48 degrees, being about 43 themselves. I also got temperature readings from all of my hubs, tires, and rotors. One rear rotor was about 118 degrees compared to the other side around 80. Probably the one with a sticking caliper. Tires were all between 68-80 something degrees on the tread surface. Hubs were ambient.

I had difficulty duplicating the vibration during over 100 miles of mostly interstate driving today. Sometimes I felt a light vibration around 57 mph, the worst all day was at 70 mph again but nowhere near as severe as yesterday with the trailer. Though the shaking was mild today I still noticed it occurring with the throttle position between 20 and 35% on my laptop. I tried recalibrating the line pressure in 4th gear so it was on as strong as possible and noticed no change other than firmer shifts. Transmission temperature during interstate driving today was 115-120 degrees at the hottest, running at 118 as an average during cruising.

I'm leaning away from a transmission problem for now. I figure I should try installing a new throttle cable since this one pops off the engine. I got it from a salvage yard off a V-10 Super Duty a few years ago. It has had paper towel stuffed in the socket to stay tight but still pops off occasionally. I don't think that looseness is the problem because it hasn't popped off in months and if it were that loose with the intense vibration, it should have come off the throttle mount by now in the last couple days. I'm more suspect of the throttle return spring built into the throttle cable. When the vehicle is stopped I have no way to replicate the speed and vibration but since the cable should be replaced anyway it seems a logical first step.

I'm also going to get my tires rotated and balanced again. The Toyo Open Country M/T 37x13. 5" tires are down to about 9/32nds with around 20,000 miles. I had tried an internal bead type balancer several months ago that shook the truck very similarly right around 61 mph and sometimes slower. Went back to sticky strips and no more vibration. I have a few thousand miles on this balance and rotation. I bought the tires from Les Schwab with a free lifetime balance and rotation on their Hunter GSP 9700 road force balancer. They advised that these particular tires lasted longer and rode smoother with a static balance and rotation every 1,000 miles. Unfortunately they have no stores in Missouri so I'm going a few thousand miles per balance and rotation. But I think it's got to be drivetrain related.

 

[NIsaacs]

It sounds like you were empty today, with a lot less vibration. So maybe it is drive shaft angle related. Different angle with a load? Has the rear axle turned from original, the u-bolts loose or the spring perch twisted? Motor mounts dropped somehow, changing the engine angle? Is the drive shaft a one piece?



Nick

 

[MMcCallie]

Nick, that's a great question. I have airbags in the rear and when I first observed the vibration they were low. While towing and today they were filled. This makes for a difference of I estimate about 6 inches in the height of the rear of the vehicle. I've had this system for a year and a half and have never experienced this vibration due to change in air pressure or height.

Even with my airbags at their maximum 100 psi this small 7,000 lb trailer load did drop the rear of my truck at least a couple inches. The vibrations occurred with the trailer empty and loaded, as well as without a trailer at all. 6 months ago I grossed near 30,000 lbs all the way from Alaska to St. Louis and never experienced any such bucking.

What I did notice the other day when checking the driveshaft was some slight rocking of the suspension assemblies within the axle u-bolts. I visually inspected all of the nuts and didn't see any signs of them backing out. When having someone shift and brake from drive to reverse and from a stop there was just enough rocking to visibly notice from underneath. The leaf springs, blocks, and airbags would all make one initial rocking motion during an intentionally aggressive change in vehicle direction.

I have never expected that I have axle wrap, especially with the airbags. I've never heard of axle wrap occurring at high speed. I'll get an impact on those u-bolt nuts to make sure they're torqued down tight. Is it not normal to see a slight rocking when shifting from drive to reverse?

The rear driveshaft is a one piece with double cardan using 1350 u-joints at the transfer case and a 1410 at the differential. The 1410 went out last spring and caused very similar vibrations, shaking the whole truck from below. When that joint went bad it was loose enough to feel slop by shaking the driveshaft by hand. It very well could be another u-joint. If so, I'll be very disappointed in the lifespan of this 1410. I has been greased.

As for motor mounts, they're difficult to access so checking them will be a little more involved. They had backed out a little a couple years ago but got a light application of Loctite before being tightened. I'll just have to check.

 

[BHaden]

Check the trans output bushing. If it is beaten out the driveshaft output yoke moves in an eccentric motion and does all manner of weird things to u-joint angles and the driveshaft itself flails around.

 

[MMcCallie]

So you're suggesting I remove the the transfer case to check the transmission output BHaden? If there is such flailing wouldn't it create friction and heat and register on my laser?

 

[BHaden]

The one time it happened to me it caused a high freq vibration through the entire truck while under power and smoothed out when I back off. It depended on how the loads were put to it. You can see it by just pushing up and dpwn or side to side on the shaft where it comes out of the tailhousing. I don't know whether it would heat up a lot. That would be logical, but maybe not. It's a free check.

 

[MMcCallie]

OK. So no need to pull the transfer case to check that? I've never tried shaking between the transmission and transfer case, but the transfer case has always moved a little when I push everything hard by hand. The driveshaft is of course tight. I've wanted for years to build an extra brace for the transfer case because it has always worried me that it is not stiffer. If feels no different now than ever.

 

[Dieseldemon]

Check the center ujoint that will cause the problem you are having (frozen joint) If you can run it on a lift you will see it, it will look like a bent dr shaft it moves so far

 

[MMcCallie]

Driveshaft looks as smooth as always. By center u-joint do you mean the furthest back double cardan 1350 at the transfer case? Haven't been able to duplicate the vibration to the original severity, probably because of slower speeds on the country roads. In the 50s I get a little sense of it with an overall jiggly feeling in the truck.

It was warm enough today to drive with all of the windows down and I noticed my tires sounded more choppy than usual. They are due for a balance and rotation. When I spoke with a local tire shop about the shaking they said it sounds like a tire issue. I've bent 3 wheels in the last few years, that could be it as well. Guess these fancy aftermarket wheels aren't durable enough for heavy towing.