Things That Go Bump..Posi Diff
Thread Starter
Intermediate
Joined: Feb 2004
Posts: 27
Likes: 0
From: Greer SC
Things That Go Bump..Posi Diff
My '74 has a 3.36 Posi-traction rear diff. When I first bought the car, I noticed a "bump" from the rear suspension when starting off, especially when turning right, like at a traffic light. Sort of like hitting a series of little pot holes in the road with the back tire. I removed as much of the rear gear oil as could with a turkey baster, put in new 90w gear oil and a container of Posi additive that I got at my local independent parts house.
Seemed to correct the problem, at leat for a time. Now it has started again. It is a bump that can be felt through the seats. It does this worse if have someone else in the car with me. And I only notice it at low speed, low RPM, just starting off from a stand still.
Is there a better oil I should be using, and is there a special posi additive that I should use. And should I have the diff checked for damage ?
Thanks all !!
Seemed to correct the problem, at leat for a time. Now it has started again. It is a bump that can be felt through the seats. It does this worse if have someone else in the car with me. And I only notice it at low speed, low RPM, just starting off from a stand still.
Is there a better oil I should be using, and is there a special posi additive that I should use. And should I have the diff checked for damage ?
Thanks all !!
Team Owner
Joined: Jun 2001
Posts: 23,375
Likes: 1,065
From: Virginia USA
If it went away with a gear lube change then it may be internal. Externally you should check the front Diff mounting bushing. There is a little bracket with a rubber bushing that bolts to the frame. When it goes then you hear the "bump". Thats where I'd start.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,500
From: Little Rock AR
I would add an additional container of GM additive. The GM stuff is best. Then drive in a figure 8 in a vacant parking lot for a few laps and see if it goes away. I needed 2 jars in my Camaro. It would "snap" in gradual turns (ex:off-ramps) without the second jar of additive.
-Mark.
-Mark.
Burning Brakes
Joined: Oct 2004
Posts: 985
Likes: 23
From: Florence SC
The GM additive is OK but I've found BFL gear lube by lakewood in the yellow labled bottle to work even better. It can be bought from JEG'S p/n 620-32970 @ 11.99 / quart. Once you put either the BFL or GM additive in, do some slow figure 8's this will get the oil in the posi unit. When driving the rotation of the ring gear and unit tend to throw the lubricant out of the clutches in the unit. Figure 8's rotate the clutches and they pick up the lubrication need to stop the poping. 
