front driveline "thunk"? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

71Blazer · 11-21-2003, 08:21 PM

What would cause a front driveline "thunk" that increases with speed when 4WD is engaged. The transfer case also has a tendency to pop out of 4Lo.

I've replaced the u-joints at both wheels in the front axle and the entire front driveshaft out of necessity. I half hoped that might take care of the mysterious "thunk" too.

It's still there, any ideas?

Scrub · 11-21-2003, 10:42 PM

What T-case do you have? Is it a full time axle? Do you have a lift? Are you sure the noise is comming from the driveline?

I would suggest you jack it up and see if you can find the noise by spinning the tires. You may have to jack up the rear also if you need to spin it in 4wd.

Possible causes:
Broken tooth on a gear in the t-case or differential
bad bearing somewhere
The popping out of gear tends to support that idea.

Why can't life just be simple?

71Blazer · 11-22-2003, 02:35 AM

Yes, simple. Why not.

I have a 4" lift and 33" tires but everything else is stock as a rock right down to the original locking hubs.

Thanks for the suggestion and by the way that K-20 sounds like a beast, should come in handy around central Or.

SWEET7T · 11-24-2003, 03:55 AM

Your t-case popping out of LO could be caused by it not getting all the was into gear. Maybe your shift linkage is a bit out of adjustment or binding and causing it not to get all the way in. I had it happen on an old Jeep.

Thunking, I would first say a u-joint. Is it your output shaft on your t-case? Check the gears inside your diff for a borken/bent tooth.

chevypete396 · 11-29-2003, 11:53 PM

Sounds to me like it might be time to go through that t-case. Its usually a sure sign when it starts poping out of 4-low or 4-high. That usually means the collar is bad and there is to much slack. If it "thunks" too, then that sounds like what it is to me. Choose your shop wisely... Chevy Pete.

Mudder · 11-30-2003, 12:11 AM

Could be the angle of the front driveshaft. Does it do it when its free wheeling? Hubs unlocked and transfer in 4Hi?

71Blazer · 12-05-2003, 03:48 PM

I haven't tried free-wheeling in 4Hi.
The angle of the front driveshaft was checked when the front driveshaft was installed.

I think I'll put transfer case rebuild on my to do list. Am I brave or ignorant to tackle it myself?

Thanks for the input.

Scrub · 12-06-2003, 01:40 AM

I don't think it is that hard.

Try the online rebuild manual at

http://www.okcnetworks.com/bronco/205/np1.htm

chevypete396 · 12-06-2003, 01:49 AM

That all depends on your skill level. In the military I use to build heavy equipment T-cases and you usually don't need very many special tools but a press would be nice. They are spendy but if you have access to one that would help.

I didn't rebuild my NP205, I had someone else do it and they used the wrong seal... still, I am glad I let them do it.

If you decide to do it yourself go slow and pay close attention to how it comes apart... Chevy Pete.

11-21-2003, 08:21 PM	#1
71Blazer Goodbye Darlin' Join Date: Nov 2001 Location: Coupeville, WA USA Posts: 330	front driveline "thunk"? What would cause a front driveline "thunk" that increases with speed when 4WD is engaged. The transfer case also has a tendency to pop out of 4Lo. I've replaced the u-joints at both wheels in the front axle and the entire front driveshaft out of necessity. I half hoped that might take care of the mysterious "thunk" too. It's still there, any ideas? __________________ 1971 Blazer CST SOLD!

11-21-2003, 10:42 PM	#2
Scrub Collector of rusty Items Join Date: Dec 2001 Location: Sisters,Oregon USA Posts: 731	What T-case do you have? Is it a full time axle? Do you have a lift? Are you sure the noise is comming from the driveline? I would suggest you jack it up and see if you can find the noise by spinning the tires. You may have to jack up the rear also if you need to spin it in 4wd. Possible causes: Broken tooth on a gear in the t-case or differential bad bearing somewhere The popping out of gear tends to support that idea. Why can't life just be simple? __________________ Scrub Sisters, Oregon - Home of the Sisters Rodeo. 70 GMC 1,000,000 + miles 72 K-20 project, 456 Dana60 front, Corp14 rear w/locker, 265R19.5 tires 20-ply. Warn 12k winches both ends, Cross-over steering with raised tie-rod, Powerbox steering, 4500 watt 120-AC power, Air, Hydraulic aux power, 4 inch lift, 5000 lb air-bags both ends.

11-22-2003, 02:35 AM	#3
71Blazer Goodbye Darlin' Join Date: Nov 2001 Location: Coupeville, WA USA Posts: 330	Yes, simple. Why not. I have a 4" lift and 33" tires but everything else is stock as a rock right down to the original locking hubs. Thanks for the suggestion and by the way that K-20 sounds like a beast, should come in handy around central Or. __________________ 1971 Blazer CST SOLD!

11-24-2003, 03:55 AM	#4
SWEET7T Try spinnin 4 rear tars Join Date: Sep 2003 Location: Salt Lake City, Utah Posts: 757	Your t-case popping out of LO could be caused by it not getting all the was into gear. Maybe your shift linkage is a bit out of adjustment or binding and causing it not to get all the way in. I had it happen on an old Jeep. Thunking, I would first say a u-joint. Is it your output shaft on your t-case? Check the gears inside your diff for a borken/bent tooth. __________________ SWEET7T 1970 C10, 2wd, LB, 307/TH350/3.08. PS, PB

11-29-2003, 11:53 PM	#5
chevypete396 Registered User Join Date: Mar 2002 Location: Fernley, Nevada, USA Posts: 540	Sounds to me like it might be time to go through that t-case. Its usually a sure sign when it starts poping out of 4-low or 4-high. That usually means the collar is bad and there is to much slack. If it "thunks" too, then that sounds like what it is to me. Choose your shop wisely... Chevy Pete. __________________ 1968 Chevy - 3/4 Ton - 4X4 - Longbed - 383ci - TH350 - NP205 Building the motor, painting the engine bay, rewireing everything... (775)-527-1199 / ryanspeterson@msn.com

11-30-2003, 12:11 AM	#6
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,339	Could be the angle of the front driveshaft. Does it do it when its free wheeling? Hubs unlocked and transfer in 4Hi?

12-05-2003, 03:48 PM	#7
71Blazer Goodbye Darlin' Join Date: Nov 2001 Location: Coupeville, WA USA Posts: 330	I haven't tried free-wheeling in 4Hi. The angle of the front driveshaft was checked when the front driveshaft was installed. I think I'll put transfer case rebuild on my to do list. Am I brave or ignorant to tackle it myself? Thanks for the input. __________________ 1971 Blazer CST SOLD!

12-06-2003, 01:40 AM	#8
Scrub Collector of rusty Items Join Date: Dec 2001 Location: Sisters,Oregon USA Posts: 731	I don't think it is that hard. Try the online rebuild manual at http://www.okcnetworks.com/bronco/205/np1.htm __________________ Scrub Sisters, Oregon - Home of the Sisters Rodeo. 70 GMC 1,000,000 + miles 72 K-20 project, 456 Dana60 front, Corp14 rear w/locker, 265R19.5 tires 20-ply. Warn 12k winches both ends, Cross-over steering with raised tie-rod, Powerbox steering, 4500 watt 120-AC power, Air, Hydraulic aux power, 4 inch lift, 5000 lb air-bags both ends.

12-06-2003, 01:49 AM	#9
chevypete396 Registered User Join Date: Mar 2002 Location: Fernley, Nevada, USA Posts: 540	That all depends on your skill level. In the military I use to build heavy equipment T-cases and you usually don't need very many special tools but a press would be nice. They are spendy but if you have access to one that would help. I didn't rebuild my NP205, I had someone else do it and they used the wrong seal... still, I am glad I let them do it. If you decide to do it yourself go slow and pay close attention to how it comes apart... Chevy Pete. __________________ 1968 Chevy - 3/4 Ton - 4X4 - Longbed - 383ci - TH350 - NP205 Building the motor, painting the engine bay, rewireing everything... (775)-527-1199 / ryanspeterson@msn.com



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