Slight transmission shimmy @ 1800 rpm – known issue?

[Sebago]

I have a 2008 G35xS with a 5AT. I’ve noticed that at about 1800 rpm ~38 mph, it has a little “shimmy”. It goes away with either increased/decrease speed. I mentioned this to an Infiniti service manager and he immediately knew what I was describing and without hesitation said the transmission gets a little confused under that condition – put it DS mode to lock it in.

I did this and he is right – the shimmy goes away. Anyone else notice this?

[pfarmer]

I have a 2008 G35xS with a 5AT. I’ve noticed that at about 1800 rpm ~38 mph, it has a little “shimmy”. It goes away with either increased/decrease speed. I mentioned this to an Infiniti service manager and he immediately knew what I was describing and without hesitation said the transmission gets a little confused under that condition – put it DS mode to lock it in.

I did this and he is right – the shimmy goes away. Anyone else notice this?

Try this with the cruise control on. There are a couple of places I do this on known radar trap roads near by. Make a pass in D and and another in DS. You will notice the whole car 'hunting' not just the transmission. Putting it in DS seems to tame it a lot. I think it has a lot to do with lockup and it puts the whole works into a cycle. I don't know of any machine that doesn't have a place in its loading that doesn't have a sort of resonance. Some machines like a very large industrial boiler I worked with had roughly a 10 minute cycle at full load. Parts of it such as pumps, turbines, fans, etc. all had theirs separately and the way it was explained to me is that all of these tend to have some point in time where the stars line up, the 10 minute cycle.

I think the same thing happens in cars and in our case that is why we see. At a certain speed and load the transmission does its thing. Then the engine responds. The suspension reacts as well. The transmission does its thing again in a different direction, the engine responds,.............. the cycle is in play. In my cruise control example you can see this with it off, but with it on both lags in trying to respond holding a steady speed and over responds when it finally does. This is also present at higher speeds as well when it comes to the cruise control but I believe it gets caught up in the rest of the other cycles and introduces one of its own.

What disappoints me is that there are ways to dampen things like this out so they become basically unnoticed by the end user and this was either not caught or considered as not that important.

For example with the cruise control in my 1999 300 m in the same situation it will make an earlier attempt to control speed but will also not over respond if it can't do so immediately. Many controllers have a deadband where they will allow the machine to drift + or - some value before doing anything. Then they go into a mode where the amount of response is based on how far off the target they currently are.

I think we see this and you and I are seeing the same thing in different situations.

Basically the Infiniti could be compared to an Industrial DCS (digital control system) with distributed controllers. The individual controllers operate to a point on their own and talk to the central unit (and or other controllers). What is important is that they all work together and their individual responses are trimmed by what others located in the system are doing. This is an example (your shimmy, my hunting) where this was not carried out properly.

Perry

[kmckis1029]

on my 05 if you in manual mode and you down shift quickly into 1st gear at a low speed... like making a slow right hand turn... there is a dead zone where the accelerator doesnt respond... and if you continue to hold the accellerator and wait... then after the computer finally agrees to down shift to 1st you will rocket first gear by a mistake...

no other gear does that and it only does that with a quick down shift to first if you decellerating at low speed... its like it gets confused...

[pfarmer]

on my 05 if you in manual mode and you down shift quickly into 1st gear at a low speed... like making a slow right hand turn... there is a dead zone where the accelerator doesnt respond... and if you continue to hold the accellerator and wait... then after the computer finally agrees to down shift to 1st you will rocket first gear by a mistake...

no other gear does that and it only does that with a quick down shift to first if you decellerating at low speed... its like it gets confused...

This sounds familiar but in a different gear. On my 08 a couple of weeks back I was merging on the freeway and was in an outside lane. I made a move towards the center lanes and at the same time a rig behind me swapped lanes and I thought I was going to get hit so I floored it.

Then engine responded and so did the transmission. However the response nearly got me hit anyway. What it did was go through a whole bunch of downshifts which resulted in a large rpm excursion in the upward direction with little beneficial change in actual forward speed. I would have been far better off if it simply stayed in the 5th and didn't downshift at all, at least I would have accelerated some.

In your case I notice something like this but I think it is from 3rd to 2nd at about 20 mph. I enter into a round about at about 20 mph. It will downshift and tends to slow down. Not a big deal but that will sometimes be followed by an upshift and an acceleration. This would not be a big item but it tends to do this at the wrong time. A situation that may well be close to yours is I will slow down for this one particular corner between a highway and a side street. When in the corner the car will then downshift and rapidly decrease in speed. This sort of surprises you the first few times especially since this corner has a slight dip in it and that seems to where it decides to downshift. I think the system is either not smart enough or too smart for its own good.

Perry

[kmckis1029]

too smart lol...