X5 Towing- Problem with Reverse
 

Hi all,

I recently bought a 2012 35i with about 80k on it, and tried to tow my boat for the first time. We live on a moderate hill, and I was trying to back the boat up the driveway. The X5 would not move the boat- with the pedal to the floor. The engine just bogged down a lot. One one attempt, I was able to inch up the hill just a bit with the engine revving higher than the other attempts. I stopped after a few seconds though. The trailer weighs maybe 5000 lbs, so within the capacity of the X5. Plus, it seems to tow okay moving forward- I was able to go up the same hill forward towing the trailer with no problem. I do know I may have something wrong with the transfer case (front tires skip when turning tight at low speeds), but I haven't read anything about a transfer case causing the symptoms I have with towing. Problems with the trailer have been ruled out- I had the same issue with 2 separate trailers.

I'd appreciate any ideas. Hoping not to have to fix both the transfer case and transmission.

Ken

Are you sure your trailer wiring is turning off the trailer brakes when in reverse? Check for voltage at the correct pin (may be the center one) while in reverse.

Hi smassey321
Yes- problems with the trailer have been ruled out- The boat trailer worked fine with my old suburban (used it to tow when X5 couldn't). I also have a utility trailer without brakes and the saw the same issue with a 4,000 lb load.

Are you saying your front tires skip when driving normally in forward on dry pavement? Issues with the transfer case can cause the engine to limit torque but you would get a reduced power message along with a stored code.

I wouldn't be so quick to rule out the trailer brakes depending on your brake controller.

How are the trailer brakes on your trailer setup? Do you use a separate controller?

Is the trailer able to be reversed when you on level ground or on a slight incline?

Surge brakes on a trailer can fool you. Some vehicles send the proper signal and allow the system to electrically dis-engage. I had one boat trailer that required me to physically lockout the surge system when backing down a ramp or up any incline.

I believe trailer brakes can be ruled out. I have the same problem with a trailer without brakes.

I'm not sure how much hill you're talking about, but trying to backup a boat with a 300hp single turbo motor is asking quite a bit. Are you sure that's all the boat/trailer weigh? I have found that people seriously underestimate the weight of what they're pulling. My Malibu Wakesetter with the trailer, full of fuel and gear on a scale is about 6500lbs.

I have a 35d which makes significantly more power and torque than your 35i and it has to work to get the boat up the slanted curb into my driveway.

Yes, they skip when turning sharply at low speed, but power seems fine under normal driving. The dash is not showing any errors- not sure if any codes are thrown.

Hmmm, yeah I've never weighed the boat/trailer combo, so it could be close to, or over 6000 lbs. The boat should weigh about 3900 lbs dry, and I think the trailer is about 1000 lbs. Add 70 gallons of gas ~ 420 lbs, plus gear 500 lbs?

The gear ratio of R is actually a bit taller than second. I suppose I should see if I can go up the same hill in second. Thanks.

Ok sitting here wondering if I bought the wrong car... However, doing some math comparing the suburban, which backs up just fine, and the X5, it appears that the X5 should actually have higher torque at the wheels in reverse than the 454 suburban.

Assumptions:
R X Final X Torque = Wheel Torque (is this right?)

Suburban
R:2.085
Final: 3.73
Total Ratio: 7.78
Torque: 385
Wheel Torque: 2994

BMW
R: 3.295
Final:3.154
Total Ratio:10.39243
Torque:300
Wheel Torque: 3117

Is that math right?

This ZF box will not handle uphill tow in reverse - you are lucky it was not damaged.

Well that was a minor detail I overlooked. Thanks all for your input.

Elaborate, where are you getting this info?

+1.

More info from my digging... Talked to a ZF-certified transmission shop owner, and he thought I was describing a properly operating transmission. He's never heard of any issues with reverse like I was describing on the ZF8HP. Since the RPMs were staying low, that is an indicator that the transmission wasn't slipping.

I also removed "X5" from my google searches on backing uphill with trailers, and it is not an uncommon problem with vehicles that do not have a 4wd-low gear range.

Now trying to figure out how to fit that circular driveway in our front yard.

On this 8HP45 car - in reverse if the load is high as determined by the DME - engine torque is reduced (engine rpms reduced as you experienced) to protect the transmission...had to look up my notes about this complaint from another car.

Is this the same for the 6 speed tranny?

I dont think so. I back-up my race car and trailer up to a short, relatively steep driveway and never felt power reduction. Perhaps there was not much room for the reverse gears and the 8 speed and they made them really tiny and fragile.

Thats my experience as well, I have backed up my boat with no noticeable loss of power.