Need towing advice - managing transmission temp (petrol engine)

[RhythmMachine]

We took our 2012 35ix (ZF 8HP45 transmission) out with the trailer (about 3500lbs loaded) this weekend for the first time. My previous tow vehicle was a 2009 MB ML320, which was a diesel with a 7-speed transmission. Between the lower torque with the X5's gas engine (about 100 less) and the 8-speed transmission, I was concerned about how the transmission temp would be. I also have an intermittent issue where sometimes the gate will open up to let me shift into sport/manual mode, but often it won't, in which case I'm stuck using automatic mode and the transmission is shifting as often as it wants... which is really often.

I had to be in full automatic mode for most of the trip, but was able to get into manual for much of the return. Ambient temps were around 20-25C, engine temps were usually 100-110C, and my transmission temps were usually 100-108C. From what I've read, normal operating transmission temp should be 80-100C, with the max allowed for short duration (like climbing a large hill) is 135C.

Interestingly, after I dropped the trailer off and was driving home the transmission temp had dropped about 10C while I was sitting, but once I started driving it rose to the same levels I had while towing and stayed there... so I'm not sure if it's actually getting any hotter while towing than not.

I expected to see a significant difference when I was able to get into manual mode and reduce the number of shifts, but it didn't have much of an impact.

So, I have a number of questions that I'm hoping those of you with more knowledge and experience will be able to help me with.

1. Should I be concerned about my current operating range? I was consistently running a little above the the high end of "normal", and while there were a number of significant hills (southern Alberta), the ambient temperature was lower than it will be when we're doing most of our towing in the summer. I'm not sure if I was over the normal range because the car couldn't cool it adequately or if it was "intentionally" running in that range, since it didn't continue to climb.

2. I'm at about 160K km on the car and the transmission fluid hasn't been changed (I bought it this past winter), so I have an appointment with my trusted indy shop to get that done at the end of the month. Will fresh fluid impact the temperatures much?

3. When I get the transmission fluid changed, I opted to go with the VF OE oil pan and filter, which is plastic, but several hundred Canadian dollars less than an aluminum model. I've read lots of people saying that the aluminum pan "should" have an impact on lowering the temp, but no real-world input from people who have used both with the same car/similar circumstances. Does anyone know if upgrading to an aluminum transmission oil pan will have a material impact on keeping the temp down and so may be worth spending on?

3. I got a VEEPEAK OBD dongle and Car Scanner Pro to read it to see when the torque converter is locked up, but it doesn't seem able to pull that value even though it lists a sensor for it. Bimmerlink doesn't list that sensor, so maybe it's not possible to monitor. I am able to monitor the transmission's output shaft speed and turbine speed, though... am I correct in assuming that if those are the same, the torque converter is locked? Running in 6th gear at around 100km/h both values were often sitting at 288 rpm, but having it there didn't seem to impact the operating temp much. I also read somewhere that when the fuel efficiency gauge is at about 1/2 way or lower (which, on a Canadian model would be 10L/100km), that indicates that the torque converter is locked, but I couldn't find any way to keep it locked in that zone at highway speed with any kind of incline. If keeping the torque converter locked is important, how can I do that and tell when I'm doing it?

Thanks!

[X5Sport]

Hello and welcome.

ZF recommend that all transmission fluids are replaced every 8 years or 50,000 Miles - so yours need doing. The oil change kit for the gearbox should include a new pan and filter assembly (comes as part of the ZF kit). The standard pan is plastic and on mine is hidden behind a metal underbody shield. I’m not sure what if any advantage an aluminium pan may give, but you need to watch out for differential metal electrolysis where the pan and box weld together. That would be bad! The ZF kit also includes new pan bolts. NEVER re use the old ones!

What is the weight of your trailer with load? The E70 for Europe came with two towing setups so with you being in the US, there may be a subtle difference.

For Europe, one option has a maximum load of 2,700kgs (5,952lbs UK) whilst the other (a different factory option) raised this to 3,500kgs (7700lbs UK). The second option included a secondary radiator mounted in the front wing. Both Factory fitted towing equipment options also added a larger electric cooling fan (800W in place of 600W). Do you have either of those? I know towing kit is a challenge in the US as BMW rarely homologate with each State having its own regulations.

It doesn’t sound as if your towing load is anywhere near either factory limit, so it may be that in your ambient temps you need the larger fan (if not fitted already).

Torque converter locking is automatic with no manual overide option. It happens above certain rpm. An incline will almost certainly cause an unlock. The technical description that I have simply says:

<Starts

A hydrodynamic torque converter is used in the 8-speed automatic gearbox. Its basic design with impeller, turbine wheel and stator remains unchanged. However, the three-way converter is a power-optimised development where the converter lockup clutch is actuated by an individual oil line. This provides the following advantages:

The torque converter is optimally supplied and cooled even if the converter lockup clutch is closed
Better activation of the converter lockup clutch is possible in all driving situations.
Line 1 is for oil supply.

Line 2 is for oil return.

Line 3 is for pressurised oil supply of the converter lockup clutch.

To decouple rotational vibrations from the engine to the gearbox, this type of torque converter can be combined with known damper systems:
- Turbine torsional vibration damper
- Dual damper system

The converter lockup clutch is for suppressing slip during torque transmission. It therefore contributes towards reduced consumption. The new three-way converter actuates the converter lockup clutch by a separate oil line. This means it is independent and disconnected from the turbine chamber. As before, the converter lockup clutch has a control range, i.e. an operating range in which a defined slip between gearbox input and output end is permitted. Mainly the transmission during opening or closing. This slip reduces rotational vibrations that are transferred from the engine to the gearbox. Improved control results from the possibility of considerably smaller mechanical slip in many operating ranges. An open converter lockup clutch was previously required for comfort reasons. Previously opening and closing the converter lockup clutch was performed by the pressure regulation of the gearbox. The direction of oil flow in the converter was influenced. Depending on the direction of oil flow, different pressure stages developed on both sides of the piston. This was how it was moved to either open or close. The improvement of the new gearbox is that independent activation results in improved control. This enables more operating ranges to be operated with a controlled instead of an open converter lockup clutch. Additionally, the flow of the converter can be optimised to suit the respective requirements at any time (e.g. cooling).

>Ends

Available sensor circuits
- Turbine speed sensor
- Output speed sensor
- Position sensor to detect the parking lock position
- Gearbox oil temperature sensor
- Substrate temperature sensors


Clean fluids will help and from my own experience my 8HP70 as fitted to the 40d (3.0L twin turbo diesels) was a much happier box after my fluids were replaced a couple of years back.

[RhythmMachine]

Hello and welcome.

ZF recommend that all transmission fluids are replaced every 8 years or 50,000 Miles - so yours need doing. The oil change kit for the gearbox should include a new pan and filter assembly (comes as part of the ZF kit).

What is the weight of your trailer with load? The E70 for Europe came with two towing setups so with you being in the US, there may be a subtle difference.

For Europe, one option has a maximum load of 2,700kgs (5,952lbs UK) whilst the other (a different factory option) raised this to 3,500kgs (7700lbs UK). The second option included a secondary radiator mounted in the front wing. Both Factory fitted towing equipment options also added a larger electric cooling fan (800W in place of 600W). Do you have either of those? I know towing kit is a challenge in the US as BMW rarely homologate with each State having its own regulations.

It doesn’t sound as if your towing load is anywhere near either factory limit, so it may be that in your ambient temps you need the larger fan (if not fitted already).

Torque converter locking is automatic with no manual overide option. It happens above certain rpm. An incline will almost certainly cause an unlock. The technical description that I have simply says:

<Starts


>Ends


Clean fluids will help and from my own experience my 8HP70 as fitted to the 40d (3.0L twin turbo diesels) was a much happier box after my fluids were replaced a couple of years back.

Thanks! Curious... I wasn't aware of the different towing capacities. From what I've been able to see, the US/Canadian models are all listed with a 6,660lbs capacity. After doing some research, I see European and NZ drivers talking about option 233 (increased towing capacity). I'm not sure if I have the 600 or 800W fan, but I'll see if I can find a part number or sticker on it that gives me a hint. I'm also seeing that aftermarket options are available for adding a transmission cooler if one wasn't included in the factory options.

That gives me a couple of things to look into if I don't see an improvement from the fluid change and new filter. Thanks!

[X5Sport]

The other thing you can do to check is run your full VIN against a tool such as mdecoder on line. It will give you your ‘as built’ spec from the factory. That might give you some clues as to what the car left the factory with.

[RhythmMachine]

The other thing you can do to check is run your full VIN against a tool such as mdecoder on line. It will give you your ‘as built’ spec from the factory. That might give you some clues as to what the car left the factory with.

Actually have it open in another browser tab! I have the factory pre-wiring (which, now that I've installed the actual wiring, I realize is a cruel joke, especially the lack of any wiring up to the dash for the brake controller!), but no other mention of any trailer-specific options:

Vehicle information
VIN l756266
Prod. Date 2011-09-05
Service History !! CHECK HISTORY !!
Type X5 35iX
Series E70 (X Series)
Body Type Sports Activity Vehicle
Steering Left Hand Drive
Engine N55
Displacement 3
Power 225kw / 306hp
Drive awd
Colour Platin-grey metallic (A68)
Upholstery Leather, "Nevada"/black (LUSW)
Check mileage !! CHECK HISTORY !!

Comfort and interior equipment
S402 Panorama glass roof
S4NE Blow-by heater
S496 Seat heating, rear
S464 Ski bag
S494 Seat heating driver/passenger
S430 Interior/outside mirror with auto dip
S488 Lumbar support, driver and passenger
S4AZ Fine-wood trim bamboo grain dark
S4NA Interior mirror with digital compass
S431 Interior mirror with automatic-dip
S441 Smoker package
S4NB Autom. climate control with 4-zone ctrl
S417 Roller sun visor, rear door
S456 Comfort seat with memory

Multimedia
S6VC Control, Combox
S620 Voice control
S639 Preparation f mobile phone cpl. USA/CDN
S655 Satellite tuner
S6AA BMW TeleServices
S6FL USB/audio interface
S6NF Music interface for Smartphone
S677 HiFi system Professional DSP
S6AB Control, Teleservices
S697 Area-Code 1 for DVD
S6NR Apps
S609 Navigation system Professional
S645 BMW US Radio

Driver assistance and lightning
S508 Park Distance Control (PDC)
S575 Additional 12V sockets
S502 Headlight cleaning system
S521 Rain sensor
S563 Light package
S5DK Side view camera
S548 Kilometre speedo
S524 Adaptive Headlights
S544 Cruise control with brake function
S5AC High-beam assistant
S522 Xenon Light

Wheels and drive
S248 Steering wheel heater
S245 Steering wheel column adjustment,electr.
S212 Equipment, brake syst. country-specific
S2RX BMW LA wheel, star spoke 210

Environment and safety
S823 Hot-climate version
S853 Language version, English
S838 National version Canada
S8TL Dummy-SALAPA
S386 Roof railing
S8SL Trailer coupling preparation
S8SM Car ident. number visible from outside
S876 Radio frequency 315 MHz
S8KC Dummy-SALAPA
S8S4 Decoding, variable light distribution
S8SC Telematics access request,country-spec.
S302 Alarm system
S319 Integrated universal remote control
S3AH Reversing camera and Top View
S316 automatic trunk lid mechanism
S322 Comfort access

Individual equipment
S761 Individual sunshade glazing

Other equipment
S1CA Selection of COP-relevant vehicles

[X5Sport]

Can’t see any reference to anything trailer related other than the wiring option - and that just adds a pigtail without anything else. BMW Dealerships in North America allegedly won’t install tow bars without that option.

S823 - Hot climate - option actually gives you the larger 850W electric fan that is also supplied where a factory tow hitch is included.

[RhythmMachine]

I have an interim update: the car goes in for the transmission oil change in a couple of days, but I did my own coolant change this past weekend. The coolant that came out was dirty and not blue... looks like it may have been yellow at one time? In any case, it was a pain getting in and getting the hose off, and although we haven't had hot weather or a chance to tow since just doing the coolant change, it does appear from initial monitoring that the transmission temp, which previously stayed quite close to the engine temp, is now running much cooler (on a drive after the change the engine was 107 and the coolant was 70).

The hose I removed to drain the old coolant is the one that feeds in to the transmission oil cooler. The hose fits over top of a nozzle coming out of the transmission oil cooler, so I wonder if there was a chunk of something in there that was preventing the coolant from flowing in at the required rate. I didn't sift through the coolant after, but based on the shape it was in I wouldn't be shocked.

I was hoping to be able to get the trailer and do a test with the coolant change, before the transmission oil was changed, to isolate the impact of each. It looks like that won't be possible, but I'm hopeful that the improvement I'm seeing in a couple of drives without the trailer will carry through to an improvement with the trailer, and that if the transmission oil change also has an impact I'll be worry free under most conditions.

[X5Sport]

I can’t remember the coolant colour in mine, and it isn’t with me today, however BMW engines must only use the right coolant in order to avoid issues such as corrosion. It’s possible the wrong one was put in at some point and went bad so changing it for the right one will be helping.

BMW used to do coolant changes every two years but removed it as a service item back in the mid 2010s. I don’t think it’s been changed in either my X6 or her E46 3-series in more than a decade.

May be yours needs another change to flush any other rubbish out after it has had a day or two.

[Leslie]

In the UK the coolant or certainly the replacement stuff we get should I think be pink, blue here is short life coolant (2 years rather than 5) .It may be different were you are though.
I'd say an aluminium pan would make zero difference and id stick with oe spec plastic. The oil cooler does the cooling .
The increased tow capacity here was a very rare option few took when new

[X5Sport]

It was expensive to add it to the X6 and working from the ‘never exceed 80% of the weight of the tow vehicle with the trailer/caravan’ guidance, I left it.

Having been responsible for closing the A1 southbound 28 years ago with a Disco towing a 3.5T trailer (and no towing training), I have unwanted experience of just what can happen when it all goes wrong. No one hurt, but it shook both of us up badly. Funnily enough my then employer got everyone who needed to tow full trained!

[Leslie]

Its not usually an issue in the uk with it's cool climate but in other countries id maybe try to get the bigger fan if doing a lot of towing , Ive had what I calculated to be about 2.7 ton gross on the back (Including car transporter trailer) ,the max allowed before but it was a cool day , you certainly know its there though ,stopping distance is greatly increased and that was on a brand new trailer with good brakes it feels ok on the dual carriageway but no way id ever exceed 50 mph as the tail could easily wag the dog

[X5Sport]

Having had the factory tow bar added, that automatically included the upgraded fan.