Thursday, April 27, 2017 Detailed Auto Topics
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After coming to a stop it feels like someone hits you from the rear

You turn and look, but no one is there. If you drive a†General Motors or other rear†wheel drive truck or SUV, you are not alone. A loud clunk after stopping is a frequent complaint on these vehicles. You may have even†been told it is normal.† It is not and it can normally be fixed without†much expense.

GM truck with clunk noise on stopping

The culprit is†often the†yoke on the driveshaft. When the vehicle stops, the wheels slow but the body momentarily remains in motion. This motion is absorbed by the rear leaf springs. As a result there is the movement of the differential. The driveshaft also moves forward and "plunges" into the transmission. This is normal and allowed for in the design of the vehicle. The rear shaft of the transmission is splined. The yoke on the driveshaft is also splined and the†shaft slides into the yoke.

Driveshaft plunges forward on stopping

Whenever there is movement between two pieces of metal, lubrication is†needed.† It appears GM and others†intended the transmission fluid to provide this lubrication. If both components were smoothly machined this would likely be more than adequate. From my observation this is not the case. The machine work on†GM splines appears extremely rough. The driveshaft yoke plunges in and binds. When the rear†springs relax, the tension becomes sufficient and the yoke is yanked†back into position.† This is the cause of the†loud clunk.

Driveshaft slip yoke in rear of transmission

We have had very†good results by lubricating the splines and driveshaft yoke with a synthetic grease. The grease seems heavy enough to allow the yoke to move smoothly, despite the poor†machine finish. In order to lubricate the splines, the driveshaft must be removed. The splines are then thoroughly coated with synthetic grease. Care must be taken to avoid over lubrication. There is an air space at the end of the splines that must remain clear. If grease is allowed to fill this gap, the driveshaft will lock when it plunges forward and damage to the transmission†could result.

Grease splines on GM driveshaft yoke

Left unattended the transmission, driveshaft and rear differential take a beating. Properly lubricating the splines takes under an hour and will normally quiet the noise. Depending on use, the process may have to be repeated in time. If your GM or other†rear wheel drive vehicle has driveshaft clunk, let AGCO fix it for you. AGCO, itís the place to go!





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