Transmission Hose Noise - Allison 740

GnarlyBus · June 27, 2019, 03:39:26 PM

I can hear a noise that I've tracked down to the transmission. It sounds kinda sounds like a Turkey caller "errt errt". I tried to shoot a video of it but it's tough to hear over the engine. Maybe I'll try again.

It can be heard easiest when the engine is on slow idle especially on start-up. The transmission is in neutral and parking brake on. I climbed into the engine compartment through the passenger side access door and traced the sound to the transmission. I put my hand on the hose going from the trans to the filter (oil and filter replaced about 2500 miles ago). It sounds like it's coming from the hose but not sure. When holding the hose I can feel a surge that coincides with the noise. Do transmissions have an oil pump? The surging can be felt on the return hose but about 50% as strong.

The sounds seems to quiet down when the engine is warm.

Any thoughts?

chessie4905 · June 27, 2019, 03:52:26 PM

Low fluid or pump drawing some air will cause that type of noise. As the fluid warms, it expands and can cause the noise to subside

GnarlyBus · June 27, 2019, 04:01:01 PM

I checked the oil and its actually an inch high on the dipstick. I'm not totally level and leaning to the drivers' side, though. I think I'll open up the fill plug when I do the differential fluid soon and let some out.

luvrbus · June 27, 2019, 04:49:50 PM

A bad seal ring on the pickup and filter inside the transmission,remove the pan ,the internal filter clean the filter replace the o-ring refill with new fluids and external filter and you should be good to go

GnarlyBus · June 27, 2019, 07:33:43 PM

Thanks Cliff! So I read the manual a bit and I've attached a pic. This is on a 750 but this is the seal ring you're saying needs replaced, right?

Also, I'm seeing cork gaskets and paper type ones. Which one's better?

luvrbus · June 27, 2019, 07:48:43 PM

Same as a 740, the gasket depends on the pan that is why you have 2 options just replace it with the same gasket material

GnarlyBus · June 27, 2019, 11:40:25 PM

Right on, I'll report back when I get it done. Thanks for the help!

chessie4905 · June 28, 2019, 07:11:36 AM

So you changed the external filter? Was this sound there before or after? Is the cooler line against the engine or an accessory transmitting the noise? When you pull the pan,inspect that interface between filter and transmission, mainly the o ring for cracking or broken. Also any significant debris in pan or on filter. Hopefully the noise isn't from a more serious problem. When you changed oil and filter a while back, was it to hopefully take care of some issue with trans, or were you just doing a normal service?

GnarlyBus · June 28, 2019, 10:46:29 AM

I changed the oil and the external filter because I'd had the bus for 3 years and hadn't done it yet. I had put maybe 10k miles on it in that time. We changed it just for PM. The oil looked good on the dipstick but I was changing the engine oil anyway so I fig'd I'd take care of it. We had to dig a pit under it in the desert to access all the drain plugs and filters. That was an adventure. Now I'm a lot more prepared for this kind of stuff.

Next time we fire the bus up to move (about 10 days) I'll try to get a better video of it.

The sound really does sound like air in the oil to me. Kinda similar to power steering noise but pulsing. It doesnt sound like grinding or anything like that.

It'll probably be a little while before I can pull the pan. I'm hoping to get it done before we move south in the fall.

HB of CJ · June 28, 2019, 12:30:29 PM

Sounds like it might be tiny bubbles of air trapped in the transmission oil? Tiny amounts. What you are hearing is the noise when the air bubbles get compressed and collapse. You might be sucking air somewhere. Does not take too much. Run the bus until warm then take a small tranny oil sample. Look for foam or very tiny bubbles. The cause has already been discussed. Good luck. You might have the very best automatic tranny.