Transmission Chatter at Idle

J

JackH

New Member
Feb 18, 2007
21
Bear, DE
Boat Info
370 Sundancer 1995
Engines
454-7.4L Merc, Hurth trans
Have a 95 370DA with merc engines and hurth v-drives. The starbaord transmission seems to chatter at idle when in gear. (forward or reverse) This just cropped up over a weekend cruise yesterday.

Problem goes away when RPMs go slightly above idle. After a search of the message base I found one note that mentioned a similar symptom that could be related to the transmission not being fully engaged in gear, and could be solved by adjusting the linkage.

Does anyone have any expereince with this symptom? Any input would be useful.

Thanks,
Jack
 
Odds are your idle speed has fallen below 650 rpm and that is the cause of the chatter. If your transmission was slipping in and out of gear at a rapid rate at very low rpm and then it is fine when you add only about 50 to 100 rpm then I would bet you just need to adjust the idle. Also of interest would be the hydraulic pressure being fed to the gear box. If it is low for whatever reason you could have a problem at very low rpm. Also what about the condition of the trans fluid? Is it full and did you check the fluid right away after shutting the engine down(frothy or bubbles in fluid)?
 
I have a 1988 30ft Sea Ray Express. I fish mostly and have my engines idled down pretty slow. My starboard engine is a little lower than the port engine and I also get a bit of chatter when trolling. That engine is a hair under 500 rpm at idle, if I bump the speed just a little the chatter goes away. Had my marina mechanic check it and it's not a problem. I agree with bluebelly, if you bump your idle speed a little that should resolve it. After having said that though,check your fluid level after your engines are warm and immediatly after shut down. If fluid level is good, if you increase your idle speed (which I believe should be around 650 rpm but check your manual) and you still have chatter, have your marina mechanic have a quick. Could be some slight wear or you may need to re-allighn your engine with the shaft.
 
Jack, what you're describing is a common problem with Merc V drives. What you're most likely hearing is the drive plate between your engine and transmission making a rattling sound. I had one on my boat that was so annoying that I replaced it only to find the new one did the same thing just not as bad. Any of the research I did told me that as long as the sound goes away when you start to put a load on the trans that you shouldn't worry about it. When I removed the old one from my boat it was in perfect condition and looked like it was brand new but I was able to see and hear how it was making that sound just by playing around with it while holding it in my hands. It's a bad design but not necessarily a problem.
 
Same sound here also but I don't recall from what side. It sounds almost like a grinding/rattling noise that isn't consistent. It kinda rattles rattles stops rattles stops rattle rattles. I say don't worry about it unless it gets worse.
 
VelvetDrive.jpg
 
OK something I have experience with..

I have the straight shaft "HURTH" gear on mine. This is very much the same gear, except its not a vdrive config. I removed the transmission to replace the "flex plate" because of the rattle noise you describe. (prob. overkill)

The flex plate or damper plate is how the motor is connected to the transmission. It has a circle of springs or the new one has rubber shocks to cushion the coupling (motor/trans)..

Some noise /rattle is normal at idle. Once above idle speed the motor produces nice smooth constant torque and the noise gos away. At idle the torque is rough and the flex plate and trans. will make some noise. If this is a new noise with your boat, read on.

***Make sure your motor is tuned up. A bad cap,rotor,plug or wire may amplify the noise in the transmission area. It is hard to tell if a V8 is firing on all cylinders at idle by ear.***
TRY THIS ON THE GOOD MOTOR: pull one wire off the cap and start..now shift into FWD listen, now REV listen..what a racket the flex plate and trans make because of the uneven or rough torque that is being produced on seven cylinders.

On a gas inboard powered boat, one bad part in the ignition system will have you chasing nonexistence gremlins in the drive line.

Don't ask how I know,
Mark.
 
Last edited:
Mark,

This is great info. I had a problem with the engine on this side cutting out the previous day. I cleaned all the contacts in the black main harness connector and the engine has been running fine, save the trans chatter. Based on your input as well as others' input about idle speed, there seems to be a good correlation of this symptom to the engine problem I had earlier, which I thought was unrelated to the chatter. Will focus on the ignition next to see if that takes the chatter back to a normal level.

Thanks much,
Jack
 
At the very least you'll wind up with a tuneup! Do the wires, cap and rotor. Whats there to lose!
 
Jack, you can always take the boat out and push the throttles all the way up. If she has a clean bottom and hits 4400 to 4800 RPM you probably don't need a tune up. Wide open throttle is one of the best diagnostic tools I've found.
 
All,

Good news - just spoke with the service manager at my marina. They had a tech look at the boat this morning who confirmed a tune-up should clear up the problem. They will do the plugs, wires, cap, rotor, etc. Should be done later today. Thanks to all for the advice.

Jack
 
Yes, keep us posted and if note if after the tune up what is your rpms on your engines and was that also instrumental in cleaning up the chatter. I would really like to know.
 
Same problem for me, only on the port engine. It appeared on the survey back in july '04 and has been rattling ever since, despite numerous tranny fluid changes (no burnt smell and nice red color) and full tune up (plugs, wires, dist cap, rotor, ignition coil - all service done by yours truly).

I'm due for new dist cap and rotor (4 full seasons) this spring and i'm going to swap out the plugs with AC MR43Ts. We'll see if that makes any difference. For what it's worth, I have always been able to attain proper WOT.
 
Per 3 mechanics comments to me (here comes Gary....) this is normal and is usually an indicator of a need for an engine tune-up (timing, carb, idle, plugs, wires, cap, rotor, etc..). Doing this cleared up our noise.
 
Well, i'm going to do said tuneup and i'll let you know - but ya gotta wait til the spring for the answer!
 
Air O'Nautical said:
..............

***Make sure your motor is tuned up. A bad cap,rotor,plug or wire may amplify the noise in the transmission area. It is hard to tell if a V8 is firing on all cylinders at idle by ear.***
TRY THIS ON THE GOOD MOTOR: pull one wire off the cap and start..now shift into FWD listen, now REV listen..what a racket the flex plate and trans make because of the uneven or rough torque that is being produced on seven cylinders.

On a gas inboard powered boat, one bad part in the ignition system will have you chasing nonexistence gremlins in the drive line.

Don't ask how I know,
Mark.
Click to expand...

My transmission noise / rattle turned out to be a burt hole (Exhaust valve) and thus only running truely on 7 cyl. After we fixed it, trans. noise went away!
 
The tune up was probably a good idea, but these trannies do rattle a bit if your idle is set too low. I bumped up the idle on my week ender which had Velvet Drives and the sound went away never to return again.
 
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