Help ID that clunk... 1970 edition

[GeorgeF]

I am having trouble identifying the source of a suspension or driveline clunk I experience in my 1970. It is a soft impact that hits within an instant of stepping on the gas and also when I am braking, at the last second before the rear suspension re-loads after stopping.

- There is no accompanying vibration, so I have ruled out a worn driveshaft ujoint or pinion/output bearings.
- There is no hard/sharp metallic sound, its just a thump, kinda like someone hit the tub from underneath my seat with a rubber mallet.

I suspected the rear leaf spring bushings had slop that was allowing the rear axle to shift back and forth, but I can't find anything obvious. As far as I know the spring bushings are all OEM still. What else should I check?

It has 2" lift blocks that the PO put on, and they aren't moving around.... ? Thanks in advance for suggestions.

[SeventyOne]

The posi in my 12bolt sounds like that...

[GeorgeF]

Would a 4WD have posi? Probably should have put that in the original post.

[SeventyOne]

my 4x4 does...do you still have the options sheet in the glove box?

[GeorgeF]

No option sheet for me... I am used to the way a full Detroit locker feels with a Dana 60 rear, V8 and 37" tires.

... this clunk happens every time I accelerate and brake, which makes me wonder every time I step on the gas if the driveshaft is going to drop, rear end going to blow etc... should a posi feel like that or is it ready to blow and take the rear end with it?

[SeventyOne]

Unsure...you may have a timebomb waiting to happen but I know that I notice/hear and feel my posi engage and disengage when I start and stop just how you described

[drunkbus]

Have you checked the motor, transmission and transfer case mounts? Might be loose or worn out mounts.

[cparman]

Agree with drunkbus, The clunk on acceleration might be a bad motor mount. If you get someone the look for you, and you foot brake the engine, IF the drivers side motor mount is bad when you do this the engine will raise up, and when you let off the gas it will fall back to a relaxed position. If you can get under the truck with a pry bad, you could put the bar undet the tailshaft of the transmission and raise it to see if the rear trans mount is toast.

[SeventyOne]

where does the sound come from?

[DANTES JIMMY]

Sounds like a mount to me

[Critter]

I have the same issue in my 72. The 12 bolt is worn and the gears have too much slack in them. With the driveshaft out (which has new Ujoints) I can grab the pinion and turn it freely 5 to 10 degrees of back and forth rotation. I am not sure if it can be fixed with a new bearing and shim kit or if the gears are actually toast. I haven't been too worried about it being as I am going with an altogether heavier axle. I would suggest you drop the driveshaft and see how much slack is in the pinion, mine was making the exact noise and "thud" feeling that you described.

[Critter]

Oh, and you may or may not have limited slip axle. They were an option on our Blazers. If you don't have the option sheet, you might as well pull the rear cover and just see what's inside.

[MJN]

Had a similar problem with our '72 Blazer that had posi. The clutches in the posi were sticking and making the clunk when they engaged and released. We chased the problem down to not using the GM friction modifier in the differential after we put the rear end in the truck. We tried using the stuff from Napa for limited slips that says it's good for posi units but it doesn't work. The GM stuff comes in a 4 oz. bottle and costs about 8 or 9 bucks from the dealer. Add it after the old gear oil is drained and your ready to fill the diff again. Then fill the diff with gear oil. It should start to work after the first few times the clutches engage and release.

[GeorgeF]

I'm guessing its the fluid in the rear diff... the PO said he had the R&P done a few years ago so I would be surprised with less than 80k that the R&P was worn again. The more I drive it the more I suspect something in the rear end and since its not getting worse over the ~150 miles I have put on it since noticing the bump, I bet its the friction modifier.

I could also have a busted trans or motor mount easily... just gotta trust my wife enough to not run me over when I check those out for sure

[old man jimmy]

Not sure about the 70's model year. But mine's a 72 and it has an Eaton posi. Since new it has had the old posi clunk. Whale oil use to solve it when you could get it. Now they have an additive that seems to work pretty well. It's like the clutchs get glazed over, and the temp. cure i used to use was to go to an emty paved parking lot and make 2 or 3 tight circles in each direction. I'm not talking 90 per. Take it slow. Makes the clutches do their job. If your clunk goes away for awhile, you know what it is. If not, look at mounts. When you have a problem that can have many answers, I'm a big believer in starting with the easiest, and cheapest answer first. This is free and you don't even have to get your hands dirty. Good luck guy.

[k-nuts]

My 70 Jimmy 4x4 does the same thing when stopping. I let off the gas and then right before, or right as I stop I get that little blunt "thump" too. IIRC sometimes it happens after I stopped completely. My glove box says:
"4G80AM REAR POSITRACTION A" Haven't verified it's still that way, but truck seems pretty un-molested. So might be coming from there. Almost feels like a weird hard downshift sometimes. But the truck does NOT slow or jerk when the noise happens so probably not a downshift problem.

So I'm listening to possible answers too.