I have an 2003 540i/6 and I recently put in a Dinan 3.15 rear differential to increase performance. The only drawback of this modification is that the cruise control now operates incorrectly and shuts itself off. I understand that this is due to an engine rpm/vehicle speed discrepancy and the car thinks that the clutch is slipping and disables the cruise. My question is can the cruise control module be recoded with whatever information it needs so that the cruise control will operate properly?? Would the dealer be able to recode this control module/unit?? The part number I found for the control module (65718375497) is the same for all MY E39s and I believe the early 540i/6 cars already had a 3.15 rear diff (as do the later 540iA cars). Any information you can provide is greatly appreciated. Best regards, Ned Vogler Tolland, CT 2003 540i/6
3.15 cruise problem I've been looking into this for over a year now, the only fix is to pony up some big bucks to get a software guy to hack the code. On the 6 speed the DME keeps track of the engine speed and the wheel sensor speed. (It must get compared to a lookup table in the DME) With the 3.15 gears, you are on the hairy edge of it working, a bump in the road is sometimes all it takes to kill the cruise. When I change my diff from 2.81 to an M5 3.15 I had the stock Dunlop 255/40 rear tires. I now have PS2's 265/40 on the rear, which are slightly taller (0.3"). With these taller tires the engine speed vs wheel sensor speed is slightly different and I find the cruise works better, but it still can drop out from time to time. The 3.15 gears in an iA don't have this problem because the tranny has an input and output shaft sensor to detect any slipage in the tranny. I assume you spoke to Dinan about this?? What did they say? Let me guess, they never heard of this problem before? I called them a year ago hoping to see (and pay for) a fix. I was told they never saw this problem. Good luck.
Jim - Thanks for the reply. Your reply (along with replies from other people that I've spoken with) seems to confirm the wheel sensor/engine speed DME lookup table theory. I did speak to Dinan about this issue, and the tech said that the wheel sensor was how the car determined speed and set cruise control, and that mine should be working properly with the new 3.15 gears. He said to check for fault codes in the DME. I spoke with someone in the dealership as well, and they wanted me to bring the car in to check for codes as well. I put a Peake code reader on it, but didn't find anything helpful. I am currently running the stock 265/35R18 M-Parallels in the back (235/40R18 in front), but will soon be putting on 245/35R19s all around. I'm wondering if the cruise functionality will be improved at all, as the tires (like yours) are a bit taller than stock. I'm sure there are many people (and I've corresponded with several) with manual E39s that have done the 3.15 diff swap and lost their cruise control. Since Dinan seems unwilling to even acknowledge the problem, I'm wondering if anyone at BMWNA would be able to help us out. Do you think there's anyway to "fool" the DME into thinking the car is an iA? Comments and suggestions appreciated! Best regards, Ned Vogler Tolland, CT 2003 540i/6
I spoke to the dealer and their techs many times, they can only download the OEM software based on the cars VIN with their GT-1 or MoDic computer. The only way I can think of to get around the problem is to take the signal from the 2 rear wheel sensors, put them thru a circuit that will change the pulse width to make the DME think the rear wheels are going slightly faster to match the higher RPM of the engine, but not too fast so the DSC module doesn't start to activate. Don't you just love all the software in these cars? FYI, pre 99 6 speed 540's don't have this problem, their cruise circuit and throttle plate control is via cable, not drive by wire like ours.
Do a lot of people have this issue? I could make a circuit to do the frequency shift, but it's not as simple as it sounds cause you have a hall effect sensor, not simple volages. If it's one or two people with the problem, you guys are out of luck... If it's more, maybe I can help.... If you really want a electronic solution to this, e-mail me at mattr@ix.netcom.com. Matt
