Then the gauges just quit. The flashing idiot lights are bad enough. They certainly got your attention. "What the heck is going on?" you wonder as every indicator beacon on the dashboard raps out a short Christmas Holiday Light Show and then goes dark. "Maybe I should slow down and pull over," you think and then you notice, slow down from what? The speedometer is already at zero, so is the tach and the only Christmas Holiday being taken is by all of your instruments that have now bid bon voyage.
Well, not all of them. The temperature gauge is probably still working, and maybe the clock. But the speedo, tach, oil pressure, gas gauge, all gone. Checked out, left town. Nada. Yet they are still basking in the glow of the instrument panel lights. And the engine is still running. The headlights? Check. The stereo? Still working. You try the turn signals, no problem. Heater fan? Present and on duty. What is going on here?
You, my friend, have just encountered the dreaded Mercedes Benz Dash Flash. It happens to your early R129-bodied cars, like the 300SL or 500SL built before 1994 or so. It happens because they start to get old. It doesn't matter if your car was garaged and pampered or driven every day and left outside. It is only a matter of time. Time, and the effects of corrosion or electrolytic seepage or a ham-fisted jumpstart gone awry. Somewhere under the covers, a component of the electrical system's defenses guarding against voltage spikes and drops has surrendered. It may have even deserted and gone over to the other side. The question is: Which component? There are four simple and inexpensive things to try before you throw in the towel and open your wallet wide.
#1. The first defense your electrical system has are fuses. So go under the hood and lift the lid on your fuse box. Check the yellowing card located inside the lid. In the North American version of these roadsters, fuse number five (5) protects the instrument cluster. If it is blown, it can cause some interesting effects in the dashboard. Even if it's not blown, the simple act of turning it or pulling it out and replacing it can sometimes be enough to make a better connection.
Now that you are in the fuse box anyway, take a good look at the fuses there. If they are the original equipment ones, they are
a) made of Aluminum,
b) Very Old by now, and,
c) probably so oxidized as to be about as electrically conductive as peanut M&Ms.
#2. For a fairly low cost (around $15) you can get a bunch of brand new fuses made of copper. Get an assortment that includes plenty of white 8 amp, red 16 amp and even a few blue 25 amp fuses. Go through the fuse panel (with the ignition key switched off) and replace the fuses, one by one, with the same color and rating. Even if this does not fix your Dash Flash, you have now given your electrical system a new lease on life. The copper fuses will not chemically react with their location tabs like the aluminum ones do and the connectors will stay clean and conducting properly for many years to come.
#3. Next up, if you have recently jump-started your car, or helped someone else start their car, you may have damaged another easy-to-reach component. It is called the Overvoltage Protection Relay and it does just that. It has a small 10-amp fuse in the top and it is intended to keep the sensitive parts of your electrical system safe from big voltage surges. The kind of surges you would get from, say, oh, jumping a dead battery that had been drained by storing a car for many months in the off season without starting it up now and then. (Big Hint here: Start you car every couple of weeks if you store it over the winter. Then you won't have to jump start it in the spring and possibly wreak havoc with the electrical system.)
The OVP is on the passenger side of the engine compartment, under a plastic cover held down by five Phillips-head screws. Once you pull the cover off, you will see several relays and control modules. The one we want is just behind the relay marked HGS. It is simply slide-fitted into a plastic bracket. Don't twist it or you can break the plastic, just pull it straight up. The cable connected to the relay is long enough that you can raise it into the sunlight and get to the fuse. It is under the clear plastic cover and you will need either a fuse puller or a pair of pliers to yank it out and look at it.
If the fuse is blown, replace it and try the key in the ignition again. That may fix the Flash. If the fuse fails promptly again, you have a bigger problem. The relay itself may be bad. A new OVP can be found online for under $75. If it is not blown, just return the relay to its bracket and fasten the plastic cover securely back over the compartment.
The last step is the most radical because it involves taking out the instrument cluster from the dashboard, opening it up and replacing soldered-on components. If you have never soldered electrical circuits before, I do not recommend you learn on this project. The stakes are fairly high in that you can ruin your instrument cluster and then be forced to buy a new one at current retail prices.
The guidelines presented here are intended for people who have worked with circuit boards and soldering irons and discrete electronic components. If that sounds like you, then read on. The actual components are under $6 and the repair tends to be effective and durable. Watch the video first to help decide if this is something you feel comfortable doing.
Be aware of the polarity of the capacitor before you remove it and be sure to put the new part in with the same polarity position preserved. Do a clean and neat job, as there is a fair amount of vibration in a car dashboard as you drive. This means having no loose screws that can work their way out and short circuit things. Do not have any errant blobs of solder, either. This is a task where "neatness" really does count.
When you are done replacing the capacitors and have re-assembled the cover to the cluster, lay the assembly across the steering column, plug the connectors back in, but do not insert the unit into the dash just yet. You will want to test the function by turning the ignition key on and starting the car. The lights should behave normally without flashing and, once the engine starts, all of the gauges should register normally as well.
Putting the cluster back into the dash is just the reverse of taking it out. Line it up with the opening and work it back into the dash, pushing gently from side to side. Push only as hard as it takes to seat the unit. The clear plastic face can easily crack or chip otherwise.
If none of these steps repair the problem, you will have to seek out professional technical help. Replacement instrument clusters are available, both new and used, at the dealer or online. There are several mail order repair services that advertise on the web as well.
I would also highly recommend reading the several forum threads on this topic at Benzworld.org, which is an outstanding resource for Mercedes Benz owners. Mad props to the folks that have posted there about this topic!
Best of Luck!