Towing advice to protect the transmission for xDrive35i?

[RhythmMachine]

UPDATE:


We were able to take the trailer out this past weekend after doing:

• coolant change (without flushing or draining the engine block)... the coolant was nasty and I'm sure was not very effective
• transmission oil change and replacement of the pan and filter, going with the standard ZF plastic pan

There was almost no change on the operating temperature. Towing up some moderate hills in mixed weather (about 22 C going out and a cool 8 C coming home) with the transmission in manual mode, primarily in sixth gear with torque converter locked the temp was pretty consistently around 103 in the warmer weather and high 90s in the cooler weather.



So it doesn't appear to be running much cooler, but I'm not sure it's a problem... it seemed to hover around 103-105 C but didn't continue to rise. I'm going to do more research on the actual expected operating temperature, because even in daily driving it's often around 100 C, so either there's a problem with my cooling system or my expectations of typical range are out of whack. I need to do more research on this.



A few other things I discovered along the way:


In attempting to fix my shifter, I broke it, but it was because I didn't understand which part I would need to remove in order to prevent the gate from keeping the shifter out of M/S mode. There is a small, white, plastic tab that slides horizontally, near the bottom of the shifter unit that does this, and it is very easy to remove (it's actually tough to keep in place when working on the internal mechanism). I ended up breaking the ribbon wire connector as I was trying to replace another part I mistakenly took out, but if anyone else encounters the problem where the gate is keeping the shifter from moving over into M/S, pop it open, remove the white tab, put it back together and I bet it will work. Be careful with the ribbon connector. I found a replacement shifter unit (including the parking brake module and mounting plate) at a wrecker here for $120 (about $90 USD), so it was frustrating to not be able to have fixed it myself but a pretty cheap mistake on a part that isn't considered serviceable.



In manual mode with our little camper (which is actually closer to 3200 lbs dry, so likely around 3600lbs loaded) we can handle most hills just keeping it in sixth gear, even if they're quite long. On a couple steeper ones it bogged down a bit and we slowed from around 100km/h to 90 (cruise on), but unless it's quite steep it seems like 6th would be fairly set it and forget it.



We got pretty decent fuel efficiency locked into 6th as well: we were 20L/100km on the trip there and 19L/100km on the way back, travelling at around 100km/h through fairly hilly southern Alberta terrain (11.7mpg out, 12.4mpg coming home, average speed around 63mph).


Next trip out I'm going to try it out in Sport Auto mode, rather than shifting manually. Since the weather and terrain will be different it will be hard to make a direct comparison, but I'm curious to see if there's a notable difference in either transmission temp or fuel efficiency by letting the car manage the shifting.

[andrewwynn]

I did not have to use manual shift much at all in the mountains towing. I just used sport mode and it figured it out. Only had to use manual maybe 10% of the time.

Steady state driving with the 35i the usual temperature goal is 109c. Drops to like 94 for hills. If you are seeing temps just over 100 it's doing its job to keep temp lower and oil thicker to protect the engine under load.

Killer price on the gear suggestion knob. I see them used for over $300 USD on eBay.

[RhythmMachine]

Originally Posted by andrewwynn

I did not have to use manual shift much at all in the mountains towing. I just used sport mode and it figured it out. Only had to use manual maybe 10% of the time.

Steady state driving with the 35i the usual temperature goal is 109c. Drops to like 94 for hills. If you are seeing temps just over 100 it's doing its job to keep temp lower and oil thicker to protect the engine under load.

Killer price on the gear suggestion knob. I see them used for over $300 USD on eBay.

Yeah, I'm going to try it out in Sport mode next trip and see what the difference is. If the temp and fuel efficiency are similar, then it will be easier to keep it there so it can downshift when the hill is just a bit too big.

[RhythmMachine]

I had another interesting discovery. On the way home from work (stop and go city traffic, not towing), I decided to try the Bimmerlink app again to check the transmission temperature, just to see if I got the same result as with Car Scanner Pro.

Turns out that the "Transmission Temperature" reading from Bimmerlink (73 C) was 8 C lower than the "ATF Temperature" reading from Car Scanner Pro (81 C). I haven't tested enough to know if the difference is constant (8 degrees) or relative (about 10%) or consistent across different temperatures, but if the transmission temp is always 8 degrees cooler than the ATF temp (or even always similar to that in operating range), it means that I've been peaking in the mid-nineties, which is right in line with expectations and leaves a more significant buffer before I'm approaching dangerous temperatures.

[andrewwynn]

I'll pull up transmission temp next time I have foxwell attached and compare.


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[RhythmMachine]

We got out again last weekend with flatter terrain, but I gave it a try with the transmission in automatic sport mode.

Fuel efficiency may be a bit better: I had it in Sport Automatic for the first half of the trip to our campsite and got 17l/100km for the trip there (13.mpg). The temp, though, was significantly higher, getting up to 109 C on my gauge for ATF, which should translate to 101 C for actual transmission temperature.

I switched back into manual shift mode for the remainder of that trip and most of the return and it pretty consistently stayed between 98-103 C on my gauge for ATF (which would result in transmission temp of 91-96 C). Mileage on the return trip was 18l/100 km (13mpg), so still pretty good.

We have an upcoming long highway trip without the trailer, so I'm curious to monitor and see what the temps are like on that to determine how much impact the trailer actually has on the operating temp.

[RhythmMachine]

Another update after a trip without the trailer. Not as many hills on this one, but some high temps on the return (up to 34C, 93F). In lower temps, regular automatic mode, transmission temp was around 90-95C or 194-203F (showed as mid-90's to around 103C on my ATF gauge), with engine oil temp just over 100C (212F) and fuel economy just under 9L/100km (26mpg).

On the return, with higher ambient temps, engine oil was hovering around 115C (239F), with transmission temps in the high 90's C (about 208F). Fuel efficiency was just about exactly the same.

So my conclusion is that the temps that I'm seeing when towing carefully (going into manual mode, getting torque lock-up in sixth gear, and keeping speeds around 100km/h) are pretty much in line with "regular" highway driving. Since expected range seems to be 80-100C and there isn't really cause for concern below 120C, things look good.

What I'm not sure about is how much more headroom I have in the transmission cooling system... if transmission temps in the high 90's (C) are still within the comfortable operating range, there may be additional cooling capacity that isn't being used (if the fan doesn't kick in until higher temps than I've reached, for example), in which case my caution while towing may not be gaining me anything. I'll try to test this further over the summer, but don't want to risk overheating/damaging the transmission in an effort to find out how hot is too hot.

[RhythmMachine]

I wanted to close the loop for anyone referring to this thread down the road... we had another trip in the Alberta foothills in moderate weather (25-30C) last week, and on climbs I was having trouble maintaining a reasonable speed in sixth gear in manual mode, so decided to give it another try in Sport Auto mode.

Coming from the Mercedes diesel as our last tow vehicle, it was a bit shocking hearing the N55 downshift and rev to keep speed (just over 100km/h) on longer or steeper hills, but it was certainly able to keep pace.

While climbing, engine oil temp got to around 115C (about 240F) and ATF fluid temp to just under 110C, meaning that the actual transmission temp would have been about 102C (215F). After climbing, though, it would quite quickly regulate to around 105C (220F) for engine oil temp and 95C (200F) for transmission temp, so both within the acceptable range. Fuel efficiency for the trip was around 20L/100km, so not significantly different than using manual mode, but certainly easier for the driver.

My conclusion after all of this is:
- There may be some benefit to using manual mode and locking in on sixth gear when towing on fairly flat terrain - it will keep wear and tear on the transmission to a minimum
- On trips with more hills, using Sport Auto mode simplifies the driving, allows me to keep the speed up on cruise, and manages the engine and transmission temps adequately, even if there is a slight decrease in fuel efficiency

[andrewwynn]

Almost exactly matches my towing in mountains experience. You can and I have used cruise and manual shift. Helpful for long downhill sections.

Diesels have a lot more torque lower RPM explaining why you need to downshift with a gas turbo engine that needs to be at a higher RPM to get similar torque.

As an aside: my Watergate controller failed and I lost boost during a mountain plus towing trip! Manual shift saved the day! Also basic basically prooved you could do it with 30i model.

[RhythmMachine]

I was curious about what the Dinan Stage 2 Tune (software only, no parts changes required) would do for our towing experience. Based on projected fuel efficiency savings, I was able to talk my wife into approving the purchase while the tune, with self-install kit, was on sale for about $300 USD, if I recall. While the fuel efficiency gain isn't huge, my estimate was that it would pay for itself in about 4 years, assuming we could resist the urge stomp on it regularly after installing.


Well, install was relatively smooth (and I didn't brick the ECU, which is always a nice outcome) and the tune, which promises an increase of up to 55 HP and 74 lb-ft of torque at 3900 RPMs had an immediate impact in day to day driving. The X5 doesn't feel like my M3, but it now has WAY more responsive acceleration off the line or overtaking, and the fun-factor is much higher than stock. I've also noticed that the car doesn't need to downshift nearly as much when driving tamely up hills.


What's interesting is that the torque gain may be highest at 3500 RPMs, but it's still very significant lower in the power band. I had hoped it would make towing feel more diesel-y, and it does. Quicker to get up to speed if I'm in a hurry, far less "frantic" feeling (revving high and shifting frequently) if I'm not, and now I really can just drop it into 6th in manual mode and handle any incline I've run into in the Southern Alberta foothills without needing to downshift.


In our most recent trip in fairly hilly territory, I got around 16.8 L/100km fuel efficiency, which is about 15% better than a similar trip last year.


So if anyone is considering the Dinan Stage 2, I'd highly recommend it. Slightly better day-to-day fuel economy, WAY more fun if you don't feel like worrying about the gas, and a much nicer towing experience.