Tech Q&A

Richard Simonds

A source for fuses?

Q: I need to find a source for the 80-amp metal fuse in my 1987 300SDL. The one in my car has failed, and it is important to have a spare available rather than to be stranded for lack of a simple fix. The dealers want $6 and more per fuse. I have heard that this is a common item. Do you know of any sources?

A: A club member recently did some research on the availability of these fuses and was able to find them at NAPA Auto Parts for $1 each, but later discovered them for sale online for $0.37 through CarPartDiscount.com. The part number is “Mercedes-Benz Pudenz 000 545 03 34/0005450334 Diesel Glow Plug Strip Fuse – 80 Amp.”

Cruise control downhill?

Q: When traveling in the hills near my home, I use my cruise control to maintain a constant speed, avoiding speeding tickets and improving my gas mileage. My question is: How does the cruise control keep my speed from increasing while going down a long hill? I can feel it slowing me down and don’t know if it is applying the brakes, which might create a risk of overheating and warping my brake rotors, or is the car slowed using engine braking? Am I doing the right thing by using the cruise control downhill? 

A: When you have Adaptive Cruise Control engaged, and if the speed change is gradual, the system uses the accelerator to change speeds. However, in traffic conditions when the car needs to slow more quickly to adjust to abrupt changes in traffic flow, it uses the brakes so that the brake lights will alert drivers behind that you are slowing down. In addition, on a long downhill grade, the cruise control will continually apply the brakes, and this can definitely cause overheating and potential rotor warping. The car may begin to sound as if you are driving on a washboard road while making a shuddering vibration. The best way to maintain speed on steeper grades is to turn off the cruise control, select a lower gear if needed, and tap the brakes intermittently to stay within the speed you want to maintain. And you definitely don’t want to use Cruise Control in the rain; if you’re driving through water that slows the car, the system will attempt to maintain your set speed by accelerating, which risks a hydroplaning car and a driver’s loss of control.

Looking for factory-spec tires

Q: I am bringing my 1989 R107 560SL out of storage for the summer and considering buying new tires. I currently have factory-spec Michelin Primacy MXV4 205/65 R15 94V on the car. My research shows that the tire is no longer available and was replaced by the Premier series. The Premier tire seems to be produced in 94H, but not the 94V configuration. I was hoping to keep the factory size. Any recommendations on sources for premium tires I might look at?

A: Because most car manufacturers are increasingly offering larger-diameter wheels with lower aspect-ratio tires, it is becoming difficult for owners of classics to find replacement tires in 13-, 14- and 15-inch sizes. The good news is that there are some resources to help you find what you need for your car: Tire Rack, Coker Tires and Universal Tires all have excellent, and informative websites. Each of these companies will ship your selected tire to you or your installer for mounting and balancing. As an aside, I always look for installers who use the Road Force wheel-balancing system, which seems to have the best success banishing balance, shimmy, and pulling-left-or right-gremlins.

ML shifting-issue update

Q: I am hoping another club member has experienced my problem and possibly resolved the issue. I have a 2012 gasoline-powered ML350 with 74,000 miles. Recently, the transmission developed a hesitation problem with up- and down-shifting at low speeds in lower gears. The transmission will usually finally bang into gear, although sometimes it simply does not kick down and feels like it is lugging. The transmission has been flushed and a new filter installed. The diagnostic computer does not report any fault codes. The transmission works just fine at highway speeds and when kicking down to pass.  My independent service technician has tried everything he knows, including flushing the transmission again, but the problem persists. Is it possible that this problem can be fixed by reprogramming, or should I just live with it?

A: My shop foreman has informed me that M-B Technical Service Bulletins are sent to all dealers when an issue appears in a region; this one has been reported in both Florida and Washington. He has received no TSBs for the shifting problems on the ML’s 722.9 transmissions. Other research indicates that this problem wasn’t corrected until after your car was produced. At that time, Mercedes-Benz updated both software and hardware; it is not possible to install the later software package on the earlier VIN numbers.

MBUSA has no TSBs on the issue and apparently doesn’t think it is a sufficiently widespread problem to justify developing a  fix for it. The short answer is beginning to look like, “Just live with it.” Sorry I cannot give you a more satisfactory answer.