Another newbie M5 trans. question: falling out of gear

[Sean Jackson]

So I’ve got the electrical side of my M5 gearbox working as it should again with very little effort thanks to you guys, but now I’ve discovered a new problem (and hopefully the cause and solution). 
 

In short: is it possible for low oil level in the transmission to let the car fall into a “neutral” state while driving in high range?

I went for a test drive to see how the solenoid circuit was working and it passed the test. However, upon coming back up the road I noticed this:

The car shifted just fine up into 4th at first, but a few moments later it seemed to drop into a neutral state. The engine would race without any of the power reaching the wheels. I pulled over and it went back into 3rd just fine, but then after taking off again it didn’t want to upshift (classic low oil symptom, right?) 

As soon as I got home I checked the level of the oil through the filler cap and found that it is definitely low. Obviously the next step is to top it off with proper SAE 10W, but my question is if this dropping to neutral behavior is a normal symptom of low oil pressure behind the direct drive piston?

As I visualize it by looking at the diagrams, the oil pressure is all that’s holding the piston in place and keeping the shift fork forward when driving- and if the oil pressure is too low the pressure of the spring on the front of the piston could overcome it and let it push backwards. 
Is there enough room between the gears for this to happen? Or am I dealing with a more mechanical issue here? I’m visualizing the drive gear just spinning in the gap between the 3rd and the direct drive gears. 
 

I know the obvious answer is to top it off and try again, which I’ll do soon, but I just want to see if my issue is a common symptom or if I need to prepare for some more serious repairs. 

Thanks for your patience, I’m slowly learning how this thing is supposed to work!

-Sean

 

[James_Douglas]

The way this thing works is that once the load of off the trans AND the piston is pushed back by the spring due to the fluid dumping OR a lack of fluid it will drop out of gear. make sure it is filed up and NEVER drive a M5/M6 without full fluid.

 

Go over to the Imperial Club and read over all the service booklets on the trans...

[Sean Jackson]

44 minutes ago, James_Douglas said:

The way this thing works is that once the load of off the trans AND the piston is pushed back by the spring due to the fluid dumping OR a lack of fluid it will drop out of gear. make sure it is filed up and NEVER drive a M5/M6 without full fluid.

 

Go over to the Imperial Club and read over all the service booklets on the trans...

Thanks James. 
I won’t move the car until I’ve found some 10W for it. I’ve only put about ten miles on it in my ownership, but hopefully it wasn’t driven for too long BEFORE me with low oil. 

[chrysler1941]

There are other possibilities other than oil level I'll explain but first a question, do you experience the same situation with 1 and 2 ?

[Sean Jackson]

19 minutes ago, chrysler1941 said:

There are other possibilities other than oil level I'll explain but first a question, do you experience the same situation with 1 and 2 ?

I haven’t experienced it in 1 and 2, although it seems a little more hesitant to shift from 1 to 2 when I’m on an incline. On level ground it does the 1/2 shift just fine. I haven’t driven it much in low range. 

[chrysler1941]

There is also the possibility transmission relay contacts or solenoid intermittent power fault thus cutting off oil flow to piston.

Could also be low oil pressure from pump. There is a plug for measurement. Over 40  PSI

[Sean Jackson]

Thanks for the input everyone! 
After draining and refilling the gearbox with fresh 10W it shifts exactly as it should and stays in gear. 

[Dodgeb4ya]

The clue was going up a hill and popping back into third.

Low oil level...oil pump sucking air.

The trans won't burn up low on oil...reason is the car isn't really drivable maxing out at 25 mph in 3rd.

People park the car until the trans issue is fixed.

Glad you filled it up and it upshifts and stays in 4th.