Cummins 4BT & Diesel Conversions Forums banner
1 - 10 of 10 Posts

·
Registered
Joined
·
45 Posts
Discussion Starter · #1 · (Edited)
I built an 01 suburban for a guy. It's a 4btaa, NV4500, chevyauto 4X4 T case with an exhaust brake. In the bown shift when using the brake, some times after it holds back for 20 secounds something that sounds like metal on metal lets loose and it starts to freewheel. It is a mechenical noise in the area of the bellhousing. When he pushes in the clutch it will grab again when it is released. The clutch never slips under lots of torgue. He took the trans out to replace the sincros and could not find any other problems. So he opened the T case to check it and it is all in order. So we thought it was the auto 4X4 contols so he disconnected the shift motor to keep it in the two wheel mode but it is still giving the same thing. We have come to a conclution that it is in the clutch. But to me it is stange. Please help me think strait, what are we overlooking.
 

·
Registered
Joined
·
3,391 Posts
I don't know much from your description. There's an awful lot of different "metal to metal" noises, like clank, clunk, grinding, squealing, pots & pans rattling, train wrecks etc. etc. etc. Some further info would help, maybe...

You say it has an exhaust brake. That's what slows the vehicle on a downshift. If it quits slowing on a downshift and freewheels, well, sounds like an exhaust brake failure. You didn't say which exhaust brake, or how it's actuated, like vacuum, hydraulic, pressurized air, mechanical, electric etc. Is it automatically actuated whenever the vehicle decelerates or what?

Whatever, my wild guess is the exhaust brake isn't holding its actuation, and re-actuates when clutch is depressed and released again.
 

·
Registered
Joined
·
45 Posts
Discussion Starter · #3 ·
Thanks Jimmy that is a good suggestion. It is a BD vaccuum controled brake,and maybe the vaccuum has a problem and allows the butterfly to fluter under pressure. I didnt test it myself becuase it is a freind that lives 6 hours away. Im just taking his word for it. But I will defenetly have him check his backpressure to see if it fluters.
 

·
Administrator
Joined
·
4,180 Posts
Don't the engines that have the exhaust brakes also have a minimum requirement for the exhaust valve springs tension? There is the possibility you are "floating" the exhaust valves and the sound is being amplified by the bell housing which is also the closest point to you. That metal to metal sound could be the exhaust valves slapping the pistons.:eek:
 

·
Registered
Joined
·
3,391 Posts
Yeah, good thought, or I mean, bad thought, well, whatever, Bob might be right! I'm thinking BD units call for a 60 lbs exhaust spring to prevent engine damage. That sound from too weak of exhaust springs would sound like the valves need adjustment. Could toast the engine...
 

·
Registered
Joined
·
45 Posts
Discussion Starter · #6 ·
I've spent some time on the phone with my friend and learned of some changes he has made with his exhaust brake. We had put in the 60psi valve springs but the backpresser was higher than that so he placed a presser regulator in the manifold to let presser into the vacuum line to deffet the brake and control backpresser. This I believe is the reason for the noise.
 

·
Administrator
Joined
·
4,180 Posts
Did you install an exhaust brake designed for a 6BT or was this engineered for a 4BT?
 

·
Registered
Joined
·
3,391 Posts
Some exhaust brakes use a bleed orifice [hole] in the shutter valve/butterfly to allow excess pressure to bypass, others rely on spring pressure at butterfly actuator. Jake brakes and some others also spec the use of 60 lbs exhaust valve springs. With a spring pressure bypass at butterfly, designed for a 6BT's exhaust pressures it might not work right on a 4BT, but theoretically the 4BT might not need that much bypass, hard to say. Whatever design is used it could be modified to allow more exhaust pressures to bypass by drilling one or a few holes in the butterfly.

I calculated the Pac Brake butterfly's bleed orifice [hole] at about 20% of total surface area of butterfly, but of course E-brake housing diameter has lots to do with it. I think Pac Brake is 4" housing, but not hard to find out by checking their website. Might also modify linkage so that butterfly doesn't close as tightly, causing more bypass.
 

·
Registered
Joined
·
1,462 Posts
Ever figure out what this was? Sounds exactly like a trashed input bearing in the t-case to me. The new chain drive throw-it-in-the-dumpster t-cases of today tend to wipe the cheezball input bearing/bushing when run low on ATF. Under power everything seams normal, but usually heavy decel will push the input forward enough to slip the teeth and you get a sound like you're grinding gears. It always sounds like it's coming from the transmission too. Probably because the sound is transmitted through the mainshaft and into the bellhousing area.
 
1 - 10 of 10 Posts
Top