Help diagnosing drivetrain issues?

[freeborn]

Hey, team. I have a ‘78 40. It has a couple of things going on that could use some attention from someone with more experience than I have.

Issue #1:
In 3rd gear the shift knob gets a vibration going around 2k rpm under load that smoothes just under 3k and any time I let off the gas. Shift into 4th yields slightly less vibration under load. Even without my hand on the stick, the vibration is audible through the rattle of my transfer case shift lever.

Issue #2:
In 4wd I get a grinding, gravely noise intermittently. It usually shows up on deceleration 20-30 mph, but not exclusively. It happens without the hubs locked. I put the rear end on jack stands, put it in 4wd and 3rd gear to listen with a stethoscope, but it was pretty inconclusive. The front axle and diff were super quiet. The tcase was a little more noisy, but not terrible sounding. I think I figured out that the drive shaft has some slop in the slip yoke, but that didn’t seem like a candidate for the noise I’m hearing.

Worth noting:
- Rebuilt original 2f 4 years ago, Sniper and large cap dizzy added 2 years ago
- rebuilt transmission <1 year ago
- rebuilt rear diff last summer
- rebuilt tcase + tranny 4 years ago (by Redline, who I trust about as far as I can throw them)

So…Anybody wanna go for a ride and see what you think? I’m in Highlands Ranch. Could meet up just about anywhere with enough heads up.

 

[rckhound]

Pull the front drive shaft and rotate the front output of the tcase by hand and see if it is smooth. If its rough that could be your problem. As far as you tcase shift goes I have had this on a couple cruisers, maybe missing a washer or worn bushing in the linkage. Take a look and make sure it matches the fsm.

 

[DaveInDenver]

How about motor and transmission mounts?

 

[freeborn]

How about motor and transmission mounts?

Installed new ones when I rebuilt the motor ~4 years ago. Doesn’t mean they’re not bad. But it seems unlikely. Eh?

 

[DaveInDenver]

Installed new ones when I rebuilt the motor ~4 years ago. Doesn’t mean they’re not bad. But it seems unlikely. Eh?

Yeah, wouldn't think it's #1 to explain grinding. Oh, also, lifted suspension? There may not be just one thing that fixes all the symptoms. Could be a combination of wear in bearings along with driveline alignment and geometry. One thing masking or enhancing another.

 

[freeborn]

Just realized I need to update with things [I think] I have learned...

Issue #1 (vibrating gear shift in 3rd):
It felt like a misfire condition to me. I ended up running a heavier-gauge wire with a more direct connection from the battery to the HEI distributor and things smoothed out a TON. I replaced plugs and wires for good measure. There's still some vibration at the top end of 3rd and cruising in 4th. It feels more related to wheel RPM than engine.

Issue #2 (front drive grind):
I think I figured out that it's the slip yoke in the front drive shaft...It has just a bit of lateral slop. I have a replacement drive shaft waiting to go in to test the theory. Still wide open to other things to look into.

To answer the question above: The vehicle does have a 2.5"(?) OME lift and I'm running 33" tires.