E65 I - (N62 32 VALVE - V8 ENGINE)

2003 BMW 735



December, 21, 2011 AT 12:51 PM

I have a 2003 model BMW 735Li E66 with the N62 V8 engine. I also have an identical car which I have been using for spare parts. My question concerns the various ECU modules used on these vehicles. I have read so much conflicting information I would like to get some clarification from an independent BMW expert.

For example, 10,000 miles ago the valvetronic ECU on my car developed a fault. I went online (including various BMW owner forums) to see if I could use the ECU from my donor vehicle. Some said that I couldn't, as the valvetronic ECU was 'married' to the vehicle and the DME control unit (main ECU) wouldn't recognize it. Others said that there should be no problem if the donor vehicle was identical. As my vehicle was permanently in 'limp home' mode I decided I had nothing to lose and installed the Valvetronic ECU from my donor vehicle (I also swapped the Integrated Supply Module for good measure).

Hey presto! Fault cured and vehicle running fine. So my question is this. Which ECUs are 'married' to my vehicle and which aren't? (I now know that Valvetronic ECUs can be used from identical vehicles without any programming issues).

I assume the main DME ECU which stores all of the vehicle's electronic data, is married to the vehicle and only a new unit programmed by a BMW dealer can be used, but what about other ECUs?

I want to use the transmission (ZF 6HP 26/1) from my donor vehicle but what about the transmission ECU? ZF themselves (and most transmission specialists) say that there shouldn't be a problem if the donor vehicle is identical. Others say use the original ECU or a new programmed ECU(very expensive) to be safe. Who is right?

And what about the various other ECUs? At the moment, I also have a problem with the Safety Restraint/Airbag system. I haven't had the problem diagnosed yet. It may not be the Airbag ECU but if it were, could I use the ECU from my donor vehicle? And what about the other various sensors and switches in the safety restraint system?

Help! I have owned this vehicle (my first BMW) for just 1 year and it has suffered a multitude of electronic problems. The onboard fault notification lights up more often than the headlights! (Should have got a Lexus. Japanese electronics never go wrong!)

3 Answers



December, 21, 2011 AT 1:27 PM

Doing swaps in a BMW is difficult with the myriad of electronically controlled systems. Kudos to you for getting things to work as well as you have, that is no simple or easy task.
The DME and transmission control unit are not, "Married" to the ECU as they are seperate control untis and for the most part can be called, "Stand-Alone". However, take that with a grain of salt becasue they will be looking for certain voltage signals in the amount of voltage sent and the pulse length in some cases. Since the engine was close enough that the DME would switch over, there could actually be some variance between ECUs and the DME could still work, because the voltage signals were the same from the same pins. This is something that all car companies will tend to follow as a cost effective practice as having more than necassary ECUs to cater to engine configurations that are near the same would cost money in manufacturing, repair and replacement parts. So, you did get lucky in some sense as there is one huge problem that you will have trouble overcoming unless you get lucky or shell out hundredds of dollars for a factory BMW service manual. They will have the ECU PIN OUT chart that should also come with test specs for checking the ECU. This way you could check the ECU to see if the pins would be putting out the correct signal and from the right location. BMW will not release these otherwise and I am not even sure if the service manual will go as deep as you need, even though it must so ECUs can be tested. Then you run into another issue, do you need to see another pIN OUT to compare ECUs Do You need the PIN OUT for the connector for the DME or transmission control unit? You may need to knwo what the connectors are and there position for the DME harness as you may be able to move a terminal to another position in the connector for it to work.
As far as the transmission control, it is seperate as the DME is but there is something that makes this even harder to deal with than the DME. Automatic Electronically controlled transmissions are incredibly picky and difficult to work on and diagnose. Even BMW will generally just replace the tranny if it seems to be at fault in a warranty situation as diagnosis costs more than replacing the unit. So, you may get lucky as the configuration is pretty much the same, but will other parameters, from the DME or other sources, keep the tranny control working correctly?
It is nearly impossible to say. If you have a transmission specialist that really knows there stuff, I woukld tend to go with that information as it takes years and years to learn how BMW does things and then it takes access to information and then it takes the talent to implement the work at hand. BMWs are a different beast and automatic transmissions even from a 1977 Oldsmobile are difficult to work on without the electronic control. You have to consider that the DME is isolated in the fact that most of its inputs are from other sensors in the DME system. The transmission control takes input from Engine, Load and DME circuits and sensors as well as signals from its own system. It is worth trying as trying to get it to work is about the only way to see if it will. You might callsome high performance BMW shops, I don't think you can get in touch with the BMW personel who make 500 series into M5s, and that might be a good source. I would not take information you find in enthusiast forum to heavily unless you can find someone who has done the same thing and they have a good and long reputation on the forum.
You are kind of taking a fairly well educated shot in the dark, but that is the new age of hot rodding, dealing with the on board computers. Thank goodness they aren't Windows based.
Good luck and I hope it works out well.



December, 21, 2011 AT 2:51 PM

Thanks for the information, very helpful.

With regards to the transmission. The transmission in my working car is running very rough(generally noisy with bumpy downshifts). It's been diagnosed by a transmission specialist as being caused by general wear and tear in the transmission drive train(brake bands & clutches)and more specifically problems in the valve block(the bumpy downshifts).

The transmission in the identical donor car is low mileage and was working perfectly smoothly (before the car was 'totaled' in an accident). So I plan to use it to replace the faulty transmission.

Here's my dilemma. Which transmission ECU do I use? The ECU in the worn out transmission is probably fine but no guarantee. The ECU in the good transmission from the donor car is definitely OK as that transmission was working fine. As the ECU is inside the transmission, making the wrong call and having to swap the ECU after installing the transmission is a whole load of extra hassle.

After my success with using the valvetronic ECU from the donor vehicle, my instinct is to use the ECU in the transmission from the donor vehicle which, as previously stated, is absolutely identical; same year, same model, same engine. What do you think?

Regards - Ian Brown



December, 22, 2011 AT 2:17 AM

I would use the one from the donor car. It has memorized the inputs from that transmission.

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