Engine/Transmission (1994 - 1998) cyclic 'hum' in lock-up...

["Mad Max"]

... for a while now I've been trying to determine the source of a cyclic hum-hum-hum-hum-hum-hum coming from either the engine or transmission of my truck. It's a '93 Ramcharger, 6BT, 47rh transmission, 241 t-case, d60 front, d70 rear. It only happens with the converter locked up - it goes away immediately when I disengage the lock-up. It is noticable under power and slightly discernable when simply maintaining speed.



At 60 mph the hum is about every half second.



Naturally I thought it was a bad converter (rebuilt unit), so I installed a DTT unit thinking that would cure it; no change at all - same vibration. Next I thought it was the rear shaft, so I pulled it completely out and ran on the front shaft - still there. Thought it was something wrong with the rear diff itself (bad bearing or something); replaced it with another I had - no change.



It appears to be coming from either the engine or trans, and I have no idea what would cause this.



Has anyone encountered something like this and could shed some light? My next move is to go to my local chassis dyno shop and get it up to speed with a stethascope and pinpoint the location of the noise... and go from there.



Anyone?



Many thanks,

- Sam

 

[GHarman]

Mad Max, Just one thought from past experience. Not that I am at your level as a mechanic. Check out your cooling fan bearings, alternator, AND the water pump bearings. I know this sounds off beat. However, it is a simple check and sometimes the simplest things can get slid under the carpet. Hope this helps? Good Luck! GregH

 

[cerberusiam]

What rpms are you runnign when the hum occurs?



Can you change the noise by speeding up or slowing down?



My 92 with the 47RH is TERRIBLE around 1800 rpms, TC locked, and under load. The torsional harmonics are really bad. Move the rpms to 2000 or above and it smoothes out.



It is cyclic growl when loaded and a hum when its not loaded.