One of our regular customers brought in their 2006 Dodge Ram 1500 4WD with a 5.7L engine mated to a 545RFE transmission. The owner's concern was a very bad shake when accelerating while having the 4WD engaged.
I proceeded with an evaluation of the issue. We have detailed procedures for this process, including but not limited to a battery/charging system analysis, complete module scan for DTCs, road test, visual inspection, and TSB search. During the road test I was able to duplicate the customers concern. On acceleration in 4WD, the truck had a pronounced wobble, or shake. When the truck was driven in 2WD the concern was not there, eliminating some of the possible causes for this issue. The under-car inspection did not reveal anything that I would consider abnormal for the age and mileage of the truck. The fluids in the transmission, transfer case and both differentials were in good condition, but the front differential was a bit low due to a small axle seal leak. However, no clues as to what could be causing the issue.
There was some evidence that someone had worked on the front differential previously, or at least had removed an axle. The customer has had several other vehicles into our shop before and we had a good relationship with him so I had the manager call him and see if there had been any other work done, especially to find out if anyone had tried to repair the truck for the shaking problem before it came to us. From that phone call we learned that the truck was purchased at auction about 30 days prior to the shop visit, and therefore had no known previous repair history.
After the customer consented to some diagnostic time, I decided to start with the easiest thing I could do and removed the front driveshaft. Both the single front U-joint and the double cardan joint felt fine with no binding or play in either of them, so I left the shaft out and went for another road test. With the front driveshaft removed the wobble was gone, even in 4WD. I fully expected this since there was no load on the 4WD components.
Once back in the shop, the truck was placed onto a two-post lift so the front end components could be examined with the suspension unloaded. I will start with saying that this truck was not in perfect condition, and while inspecting the front end components there was a little bit of play in the tie rods and the pitman arm. While not very bad, it was still something that I could not rule out 100% at this point. I checked the tire circumference with a stagger gauge and it checked okay. Prior to removal of the front driveshaft, I had also driven the vehicle in a straight line while in 4WD with no evidence of any type of binding concern, so I knew that I was not dealing with a gear ratio difference between front and rear differentials.
Getting back to the wobble, I can best describe what I felt as similar to a shake or vibration caused by loose front inner CV axles in a front wheel drive vehicle; when the inner CV joints get loose it can cause side to side type of sensation that we usually refer to as a "wobble". This truck felt very similar to that, but since this is a 4WD vehicle and power is supplied to both front and rear, there is just enough difference in the feel that I was hesitant to condemn the CV axles as the culprit. While there were a bit of play in both of the inner CV joints, it did not seem to be enough to be the cause.
I sent an email to some other diagnosticians within our company to see if someone had dealt with a similar situation. The responses I received targeted either front-end steering components, or a bad axle. While I was waiting for response from my fellow associates, I rotated the tires front to back to see if I could "move" the sensation but yet again was unsuccessful in pinpointing the cause.
Since I could feel some play in the inner CV joints, I decided to replace both of them with reman axles from one of our parts suppliers. Yes, you guessed it. The wobble was still there. I do not think it changed even a little bit. Disappointed but undaunted, we reinstalled the customer's original axles back into the truck and continued the diagnosis.
Fortunately around this time we had another 2006 Dodge come into the shop, but this one drove fine in 4WD with no signs of a wobble or vibration like the problem truck had. At least now I had something to compare our subject vehicle to. We only have one drive-on lift available in the shop, so it was hard to do a side by side comparison of the two vehicles. Nonetheless, I still could not really see a difference in the CV axle angles or driveshaft angles between the two trucks, but I was still convinced there had to be something I was missing. Since there was some play in the pitman arm and tie rods we replaced those parts and had the truck aligned at a nearby general repair facility, but yet again did not fix the problem.
When I returned to the shop I parked the truck in the back lot alongside the other 2006 Ram and went in to speak with the manager to tell him the news. I am really frustrated at this point because I am having difficulty fixing this truck. When I go to the back lot to test drive another vehicle I look at my nemesis sitting there and I noticed something: the front end of the problem truck is sitting a couple inches higher in the front as compared to the truck that does not have the issue. Neither of these trucks has a lift kit installed, but both have stock-sized tires; so why is the ride height different?
I took a closer look at the front springs and noticed that there was more space between the spring and the upper spring perch on the subject truck than on the comparison truck.
I do a little research and find that there are companies selling a "leveling" kit for these trucks that is just simply a block to increase the installed spring height intended to raise the front end ride height, but without addressing the increased front CV shaft angle.
Now, to answer the question: why does this have such a big effect on the way the truck drives when it only adds a couple of inches to the ride height? The differential is attached directly to the motor mounts and moves with the engine. Because of this, the differential rotates upward on the passenger side when the engine's torsional forces are active upon acceleration. This, combined with the increased CV angle from the spacers (without a differential drop), causes the CV joints to bind under a load.
We removed the "leveling" kit which should be renamed to, "Change your driveline angle kit". Predictably, the wobble was gone and the truck drove like new again; well maybe not new, but you know what I mean. It is worth mentioning that there are other brands of leveling kits available that will raise the front end of the truck the correct way to get rid of the factory rake these trucks are built with, yet do not change the driveline angle. These kits are more $ than the $50.00 - $100.00 kits, but the results would be well worth it. This is just another example of how aftermarket parts can deal us fits!