Gary's Garage - The Fuse Confusion
by Gary Edwards, MBCA Peachtree Technical Advisor
Sometimes you do something incredibly dumb, not realizing it at the time. Sometimes it costs a lot and inconveniences your friends and relatives too. Sometimes it even causes you to waste your most precious commodity, time. I did this; I thank the folks that helped me get through the predicament I created, and I will now confess how I screwed it up.
I love R129s and I consider them to be very reliable. Since I haven’t had a lot of problems with them, there are some areas I haven’t dug deeply into. One of those areas bit me here. We were on our Fall Foliage Drive in October and were making our way to Tail of the Dragon; I was driving my 1998 SL500 Sport. We had indications that Route 129 was closed south of Robbinsville, so we were taking an alternate route from our hotel in Murphy NC. Waze routed us through Tatham Gap, on a one-lane dirt road. Wasn’t really what I wanted to do, but it was sort of fun – the few times we got up to 20 MPH it felt like we were really going fast, and I had to back down once to get where I could let oncoming traffic pass. I turned on my lights as we came down the mountain and got a bulb failure warning which I isolated to the parking lights. On this era of Mercedes, you can figure out in general which bulbs might be bad since the bulb failure light only comes on when the errant light is energized.
When we got to Deal’s Gap, the entry to the Tail of the Dragon, Cheryl wanted to stop at the gift shop, so I pulled in and shut down the car. I looked at the lights and noted both right front and rear parking lights were off. This had to be more than a bulb, so I looked in the fuse box and found that the 15A fuse for the right parking lights had blown. Mercedes fuse boxes generally have a place for spare fuses, so I picked a 15A fuse and restored the right parking lights. I then wiped all the dust off the car and got ready to depart.
Surprise – the car wouldn’t start! Great cranking, no firing. I was suspicious of the fuel pump, but I wasn’t carrying a tool kit or a spare fuel pump relay. I did have two scanners, so I hooked up and looked for fault codes – there were none! I was thinking the car would restart after it cooled down, but no luck there either. I was near despair when I noted that there was no cell phone service on either my Verizon phone or Cheryl’s AT&T phone. I asked the gift shop to borrow their phone, and they pointed out their free WIFI. Once I got on that I was able to call Hagerty and ask for a tow. After a couple of hours, Hagerty had not come up with someone to tow me, so I called Diana Quinn and begged her to come pick us up and take us to the hotel, and I called my brother and asked him to pick us up from the hotel. Thanks to their kindness, we were able to get home by about 11:30 pm, though we missed driving Tail of the Dragon and our Saturday evening dinner. Without asking, our hotel, Stonebrook Lodge, did not charge us for Saturday night, even though we checked out at 6 pm.
The next day, I drove back up to Deal’s Gap with a trailer and loaded the car. I carried a good fuel pump relay with me but swapping it didn’t help. I towed the car home on Monday and started troubleshooting. The diagnostics on an R129 of this age are basic, and the live data does not include fuel pressure, plus the activations do not include turning the fuel pump on. I did not have a fuel pressure gage that fits the Schrader fitting on the fuel rail, so I ran over to an auto parts store and bought one. I was surprised to find that fuel pressure was a solid 55 psi and decayed only slowly when the pump was off – so it wasn’t the fuel pump! I had read that these cars are very sensitive to battery voltage, which was getting a little low after all the starting attempts, so I hooked up a jump box, which didn’t help. I had also read that this problem had been rectified by changing key fob batteries, so I tried that (I had also tried the other key fob, when traveling I normally carry both key fobs just in case). Using my STAR clone, I could communicate with the ECU, but I was worried that the factory alarm system might be blocking the starting, and I could not communicate with those components. The lack of a fault code for the crank position sensor (CPS) told me it probably wasn’t that, but I did unplug the CPS to verify it gave a fault code (must crank the engine over to get this fault code). By this time, I had pretty much decided that I had fuel, I had injection (I could smell fuel in the exhaust) and I probably didn’t have spark.
I was getting to the point where I’d have to be taking voltage readings on the electrical connectors for the base module (provides power to other modules) and the ECU, and I decided to review the basics first. I pulled the cover off the front fuse box and again started reading the functions of the various fuses. When I had done this previously, I had been looking for a fuse labeled “fuel pump” and that doesn’t exist. This time I just read all the functions, and the last one, fuse 34, said coils. I didn’t remember seeing this previously, and it seemed to me I didn’t have functioning coils. I looked at the fuse location, and it was empty!
Unlike a 124, where the spare fuses are lined up either in front of or behind the regular fuses, in an R129, the spare fuses are on the side of the fuse box. When I had grabbed a spare fuse for the parking lights, I had pulled fuse 34 and used it. The illusion that this was a spare fuse was enhanced by the fact that this is in a separate area of the fuse box, farther to the rear, and there were 8 fuse slots with only 3 fuses in it. I inserted a 15A fuse in location 34, and the car started.
So what did I learn from this debacle? For one, even with all my experience working on Mercedes and Porsches, I can still do stupid things. A more basic point though, is that if something was working, then suddenly isn’t working, you need to look closely at what work was done in the interim. Had I paid more attention to the restoration of the parking lights, I could have avoided inconveniencing others and the logistical pain of getting the car home. Also, I learned that you don’t get a fault code on a 1998 SL500 when you drop power to the coils – I know more modern cars will give you a low voltage call out. The activations available on a car of that age do not include the fuel pump, so you need to do basic troubleshooting to see if it’s working. Any of course, never say no to a knowledgeable person offering to take a look. I think I’ll probably carry a fuel pump relay and a fuel pressure gage in the future, along with a 15 mm wrench to remove the test plug, and a Volt/Ohm meter, along with the scanners I already carry.
It's a little hard for me to write this and confess this egregious error, but perhaps one of you can use these learning points to avoid a similar problem in the future.
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