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7 Series - E38 (1995 - 2001)

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  #1  
Old 04-03-2013, 08:46 PM
89HondaLegend 89HondaLegend is offline
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Location: Australia
 
Join Date: Apr 2013
Posts: 1
Mein Auto: BMW 540i
2001 735i holding gears/chaning down gears

Good Afternoon to all the BMW gentlemen and gentlewomen on this forum.

I've recently got into a couple of BMW's, a 540i and a 735i (both sport models). Both nice cars but the 5 has a bit more poke and the suspension on the 7 is pretty tired.

I'm posting with a problem, as so many people do. I've searched around which will make some of you happy to hear. The problem is as follows:

Basically the 7 refuses to change gears. It is usually OK first thing in the morning but after it warms up the transmission decides to change down/back gears and will not change up. For example, cruising at 60 MPH the transmission will change back to 3rd and refuse to change back up to 4th or 5th. The only way to get it to change gears after coming to a stop is to take it to redline after which it will shift to the next gear and stay in that one. When you slow down it changes back to gears as soon as it can.

I've checked the transmission fluid at the specified temperature and fluid came running out. The actuall transmission had a service a few years ago and is very smooth when it changes gears which leads me to believe it is an electronic problem.

The odd thing is that my 540i will do the same thing if I take a particular route home, otherwise it does not exhibit the persistent problem of the 7.

So do any of the guru's know what might cause the transmission to hold back these gears? I've checked for codes using the PEAK tool. I've got a bad auxilliary fan and an electronic MAP thermostat code. I've kept an eye on the temperature through the digital display and it never goes above 110C which I would think means that the transmission wouldn't be overheating..(I read that if the transmission overheats it will disengage the torque converter and change back gears to keep it cooler...)

Hope some of you may be able to help me. Look forward to having a look around the place.

All the best.

Kind Regards,
Steve
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  #2  
Old 07-15-2013, 04:51 AM
Podmore Podmore is offline
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Location: Adelaide South Australia
 
Join Date: Feb 2008
Posts: 151
Mein Auto: 2001 BMW 735i
I know it's been a long time since this was originally posted, but I've had a slack day with time on my hands and went for a surf through about 20 pages of posts and found this one unanswered. I have the answer, in case you haven't already found it - maybe it will help someone else out there anyway.

The problem is associated with a temperature sensor which is built into the wiring harness inside the transmission - No 13 in this diagram:
http://www.realoem.com/bmw/showparts...25&hg=24&fg=15

It's a reasonably rare problem but I've now seen it pop up on forums a couple of times - and there's my personal experience to back it up. The trans will not upshift gears until getting to 2500 rpm / 3500 rpm / 4000 rpm depending on which gear it is in, then will change down as soon as the revs drop below the threshold rpm limit. I had this happen after a AUD$5000 rebuild, so as you might guess I was not happy! It went back to the shop 4 times till it was diagnosed - as it turns out, by me, through a process of elimination and deduction. Eat your heart out Sherlock Holmes.

The temperature sensor trips an overheating mode, which forces higher rpm to pump more trans fluid through the oil cooler (and also reduces the load on the transmission). Change out the wiring harness (or get a new sensor spliced into your original one) and your problem will go away.
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