Bad Grounds make for Crazy Transmissions

[Shanehsmp]

Long read for no reason - but maybe it may help someone.

For about 4 months now my '06 M35x has been going in and out of Limp Mode - it started after I had it sitting for a few weeks due to a broken AWD propeller shaft. After buying a 2 different shafts that didn't fit - I said screw it and pulled the AWD fuse so that I could at least drive it for the summer.

Shortly after I started driving it again the car started going into a weird limp mode - I had all my gears, but the acceleration was gone. It was intermitent, and there was no rhyme or reason as to when or why it would come on - Not a SINGLE CODE was thrown. I started thinking it was my speed sensor in the transmission but I still could get up to 5th gear in manual mode so that wasn't it...it wasn't until a week ago I read that a bad ground somewhere could cause limp mode.

Welp, this is where I feel like an idiot - when I had the car sitting waiting for a new AWD shaft, I didn't charge the battery so it died. I ended up taking it out and charging it....when I put it back in, the ground battery connector wouldn't tighten on the post - so I used a small shim on one side of the post, thinking that it wouldn't affect anything. Needless to say, electric circuits need a proper ground or things go nuts.

Ended up cutting off the old terminal and putting on a terminal - for a year I thought I had a faulty transmission, NOPE - just a crappy ground terminal. This thing shifts like a brand new car now, no more high revs from 3rd to 4th without a shift.

Moral of the story, check your ground wires/connections as they might be some of the causes for a few posts I've seen here.

[Ilya]

Yep. I'm 100% into cleaning grounds. I do it annually or semi-annually.