When I bought the Mercedes GL320 in 2009 to be our new tow vehicle, I knew I was taking a big risk.  At $66,000 (out the door, tax included), it was almost double the price of the most expensive vehicle we’d ever purchased.  Mercedes has a reputation for expensive repairs and maintenance, and their dealer service network is small compared to just about any other brand.

The justification for taking this risk is complicated, but the major factor was the diesel powertrain.  At the time, only the European brands (Audi/VW, BMW, Mercedes, Land Rover) offered diesel SUVs, and they rack up impressive performance stats.  In 2009 when we made this purchase, we were planning on many more years of Airstream travel, so it made some sense to invest for the long term. I felt confident the Mercedes 3.0 liter turbodiesel could last for hundreds of thousands of miles while carting as many as 7 people in comfort and delivering fuel economy (not towing) in the upper 30s.

In the past decade the European diesels have also been impressive for their emissions improvements and quietness. I can start my diesel dead cold in the morning at a campground and hardly anyone will even notice the sound, while the exhaust is scarcely more offensive than baby’s breath.

That’s all very nice, but there is one thing that a tow vehicle must be able to do to justify its existence: tow.  Our GL320, despite having 127,500 miles on it, has done as good a job of that as it ever has—until this week. One tiny problem this week managed to cripple it, rendering the GL entirely worthless as a tow vehicle.

We had a gentle rain on our last night at Beachside State Park on the Oregon coast.  Over a period of hours, a drop or two of water managed to work past the gasket on the right rear taillight, wick through some insulation on the inside, and drip down to a black plastic cover below.  This cover has thin vent slits in it because it houses a very expensive electronic device called a “Signal Acquisition Module” (SAM).

The water dripped through the vent slits and down to the exposed circuit board inside.  When the SAM gets wet, it behaves like any other electronic device when wet: it malfunctions spectacularly.  This SAM happens to control most of the functions in the rear of the vehicle, including trailer lights and brakes. Just one tiny drop of water in the right spot means no trailer lights or brakes.

This has been a recurring problem.  It first cropped up in February 2015 at Alumafiesta after a heavy rain, with the symptoms being taillights that didn’t work for a few hours. I didn’t find the cause until May 2015 at Alumapalooza when it happened again.  I dried the computer with a hair dryer and took it to a northestern Mercedes dealer in June.  The dealer service tech glopped everything up with black sealant and pronounced it fixed, which it wasn’t.

Water hit the SAM again in January 2016, so I dried it again and took the car to another Mercedes dealer (this time in the southwest) and they replaced the right taillight, noting this the leak was a known problem. They said the magic words that they say every time I have to buy an expensive replacement part: “This is an upgraded design, so it won’t have that problem again.”

That was a nice warm and fuzzy thought, but three weeks later the SAM decided it had suffered enough from the prior repeated water intrusions, and it died without warning—while towing in downtown Castro Valley, CA.  Imagine the fun: suddenly, no brakes and no lights on a 7,500 pound trailer in heavy traffic.

This time the hair dryer trick wasn’t going to work.  I had to tow the Airstream through city traffic for a mile with no brakes, signals, or lights to a Walgreen’s parking lot large enough to dump it.  Then I had to convince the manager of the pharmacy to let me leave the Airstream overnight.  Then I had to find a Mercedes dealer and pray that they had the part I needed—on a Friday afternoon. Fortunately Mercedes Benz of Pleasanton had the part and installed it the same day, for $1,300.

All was well until last week, when mysteriously the upgraded taillight assembly let in just a couple of drips during an Oregon sprinkle, and our expensive new SAM got wet for the first time.  You can imagine my reaction when I got into the car and the dash lit up with five warning messages—and of course, no trailer brakes.

After I ranted for a while, Eleanor and I got to work.  About 45 minutes of the hair dryer treatment got everything working except the left turn signal and taillight.  We decided to start towing toward Eugene OR (nearest dealer location).  Two hours later the left turn signal began working again.

In Eugene I had a friendly chat with the service tech, in which I explained that intermittent lack of brakes and lights means the car can’t tow. That triggers what I would call an existential crisis for the Mercedes GL320.  If it can’t tow reliably after a light rain, I can’t use it. He understood the conundrum, but had little to offer other than tearing apart the interior of the car to look for other possible leak points—at $140 per hour, my expense.

I talked with Andy Thomson of Can-Am RV about possible replacement vehicles. Andy has been a very reliable source of information over the years, despite his tendency to terrify Americans with his non-truck towing suggestions. He listed the Audi Q7, BMW X5, and the new Durango (based on the Mercedes platform) as possible replacements, but pointed out that my GL has relatively little trade-in value.

Worse, there are no suitable new diesel SUVs available.  VW screwed us all on that one. Friends at Mercedes dealerships have told me that Mercedes has quietly suspended shipments of new diesel SUVs to the US. Audi and VW of course are out of the question, and BMW’s X5 might be available but it’s too small for us.

So we’ve taken the path of least resistance. The taillight assembly has been replaced again but I’ll never trust it.  We are going to rig up a plastic shield over the SAM to block the water droplets. It’s a low tech, easy fix that will probably work just fine for the life of the car.

And, despite my momentary lapse of confidence, I think we’ll stick with the GL.  Hopefully the SAM will survive this one episode of water intrusion. To be sure I’ll test it a week or two before every trip. I still want to see the odometer turn over 200,000 miles before we re-consider getting rid of it, and more miles would be nice.

On balance the car has been everything I hoped it would be: a comfortable, confident driving, capable tow vehicle. It’s amazing to me how something so small—a drop of water— can entirely destroy the practical value of the car.

[Nerd Alert]  I’m reminded of that scene in “The Fifth Element” when the evil Zorg chokes on a cherry and Father Vito Cornelius says, “There, you see how all your so-called power counts for absolutely nothing now, how your entire empire can come crashing down because of one little cherry.” [/Nerd Alert]  We live by a tenuous thread all the time, and little moments like this make that thread briefly visible.

I guess there’s nothing for it but to keep on towin’. We’re heading to the California redwoods next.