This time of year our tow vehicle, the Mercedes GL320, generally rests in the carport. We log about 14,000 miles each summer between May and October, mostly towing, and that’s a lot of use. So in the off-season I try to give it a break, except for occasional cross-country trips. This allows the car’s years to catch up with the miles somewhat. It’s a 2009 and already it has 56,000 miles on it. By the time we get back from travel this summer, it will have about 70,000 miles.

A few weeks ago I had the car out for a little trip and the Check Engine light popped on. This is becoming a familiar sight, unfortunately. We’ve had about five incidents of Check Engine lights since the car was new, and all of them have been related to the Adblue (a.k.a. Diesel Exhaust Fluid, or DEF) system. This system is a big part of why the car’s emissions are legal in in all 50 states. It injects a spray of DEF into the exhaust stream, which combines with the exhaust gasses in a special type of catalytic converter and results in the nasty smog-causing oxides of nitrogen turning into harmless water vapor and carbon dioxide.

It’s a brilliant system when it all works, but our 2009 model was the first year for Mercedes to install this technology, and there have been a few bugs. Mercedes seems to have worked them out with a combination of software updates (yes, like everything else on modern cars, this process is entirely controlled by computers) and upgraded components.

This time the Check Engine light was indicating that a heater for the Adblue (DEF) was failing. The heater is needed so that the fluid doesn’t freeze at low temperatures. Replacing the heater is a labor-intensive job that requires complete removal of the Adblue tank. And this is where the nightmare began …

You see, back when we first bought the car, we had to do some extensive modification of the factory receiver hitch, in order to make it suitable for our Airstream Safari. The key modification was the addition of a “third leg” that spread out the tongue weight of the trailer. You can see this “leg”, made of 2-inch square tubing, in the photo at left. It was welded to the rear suspension crossmember and to the factory receiver.

When this solution was proposed, I had two misgivings. First, that this would take up too much ground clearance. This turned out not to be an issue, as the car still has 10″ of ground clearance at this point even with the tube installed. My second concern was that it was blocking access to the black tank you see above, which is the holding tank for the Adblue fluid.

After considering for a while, we decided that replacement of the Adblue tank was highly unlikely, so we went ahead and installed the third leg. It has functioned perfectly ever since, taking up stress from the receiver so that we can get good weight distribution without overstressing the rear end of the GL’s frame.

So when I got the call from the dealership’s Service Advisor telling me that the tank had to be removed, my heart sank. We had to cut the third leg of the hitch off (where indicated with the orange line in the photo above). I dragged the decision out a few days by asking the dealership to do an individual component test on the Adblue heater to double-check that it really had failed, and to try to rule out the possibility of another software problem. They did that, but the news was unchanged: we have to remove the entire tank in order to replace the heater.

I feel very protective of my receiver hitch. We went through a lot of trouble to get it modified just so, to suit our particular needs. We first had reinforcements (not visible in the photo) welded on here in Tucson, and then drove 2,000 miles to Can-Am RV in London ON (Canada) to have the final reinforcement added. I inspect the receiver at least monthly, and do an annual crawl-around-on the-ground-with-a-flashlight inspection at least annually, along with wire brushing and repainting. Any receiver can fail, and since a failure can result in your death, it’s a piece of equipment worth taking seriously. So I didn’t want anyone touching it, and I especially didn’t want anyone coming near it with the intention of cutting it off.

But in this case there was no choice. Andy Thomson at Can-Am was very helpful in marking up the photo above, which I gave to the dealership’s body shop to show them exactly what to do. The hitch was cut, the Adblue tank and some other components were replaced, and I got the car back a week later with the hitch re-installed—but sliced right through the third leg. I drove it 50 miles and the Check Engine light stayed off, so the next step was to get the hitch repaired.

Obviously we didn’t want to weld it back, since there’s always the possibility that we’ll need to remove the hitch again, so after discussions with Andy and other consultants we came up with a plan to add some heavy plates and bolt the two ends of the cut tube together. This was done locally at a qualified welding shop. You can see the result below. Sorry for the lousy iPhone photos.

The bottom line was $49 to the dealership body shop, and $200 to the welding shop that installed the bolt-up re-attachment. The Adblue tank was covered under warranty, which was good since the estimate for that job was a whopping $2,200. I do like the Mercedes as a tow vehicle, but the cost of parts and repairs can be astronomical. I’ve already started a maintenance fund for repairs after the 100,000 miles warranty has expired. As I tell people these days, it’s the best tow vehicle I’ve ever owned, and it’s also the least reliable tow vehicle I’ve ever owned.

But I’ll cut it some slack since we really use the heck out of it. There’s a chance that this replacement of much of the Adblue system will resolve the persistent issues we’ve had with it in the past. Discounting the Check Engine lights, it has done well for us. We bought the GL320 because we wanted a long-term tow vehicle with a durable diesel engine, and overall it has worked out well.

Realistically, there are no perfectly reliable vehicles, just different compromises. At this point the car still feels and drives like new, so my original goal to get 250,000 miles out of it has not wavered. In that long-term context, this little bit of receiver work seems well worth the expense. It is just part of a long-term investment in safe and happy traveling.