AlanBob

For over six months I've been trying to solve this strange problem with the turn signals on my 1996 SL500, 77K miles: I get the double-time flash rate when signaling a left or a right turn, but the normal flash rate when using the hazard flasher.

Here is what I have observed:

* All turn signal bulbs are working, just flashing fast. All other lamps are working normally, inside and out.
* No other errors or malfunctions (no bulb out indication, hazard working normally)
* Headlights on or off, no difference. Parking lights, brake lights, etc. all working normally.
* Audible clicker inside the car is working with turn signals and with hazard flasher. Indicator lights in instrument cluster work properly. Internal light in the hazard flasher works properly.
* There are two relays inside the combination relay assembly that flash the left and right side signal lamps. I can feel them clicking normally when operating the turn signals or the hazard flasher.

Here is what I have done to attempt a repair:

* Replaced all four lamps with the specified Sylvania/Osram units. Twice. The second time with lamps purchased at a Mercedes dealership, just to be sure.
* Cleaned all four lamp sockets with a wire brush, despite no evidence of any corrosion.
* Verified good grounding (10 milliohms or less) at all four lamp sockets.
* Checked all the fuses (zero Ohms resistance), tapped on all the fuses. Cleaned all the fuse sockets with contact cleaner. Replaced all the fuses with new ones per spec.
* Wired a 12V 4W bulb with 4 ohms of resistance across a rear turn signal bulb to increase the current draw. No difference in the flash rate. I did not try doing this on front and back at the same time.
* Removed and replaced the combination relay, cleaned all connectors with contact cleaner. Opened the combination relay, identified the two relays for the signal lights, drilled holes in them and sprayed contact cleaner inside, verified both relays working (left or right or both clicking in unison with the signal lights). Sealed holes, reassembled and reinstalled, no change.
* Purchased a good used combination relay from eBay; works exactly as my original one did, i.e., turn signals flash fast, hazards flash normally.
* Removed the hazard switch and verified correct operation per the wiring diagram, including the internal resistor. Disassembled switch, cleaned contacts, verified that the resistor was the same value I measured (can't recall exactly right now, perhaps 100 Ohms, but I don't swear by that). Reassembled and reinstalled switch, verified that it still worked as before.
* Removed the bulb failure detection unit. No change. Reinstalled after cleaning all external connector sockets. No change. Removed one tail light, bulb failure warning lamp illuminated. Same when removing stop light and stepping on brakes, normal operation all around.

So, aside from tearing apart the steering column and replacing the turn signal (combination) switch or examining the wiring harness behind the instrument panel, or spending over $400 on a new combination relay, I'm pretty much out of ideas. Somewhere I recall reading something about a diode in the hazard switch, but I'm not certain that it applies to my car. The switch does have a resistor but no diode, and its part number is correct for my application if the whole Internet is to be believed.

Has anyone ever encountered this situation? I've tried every fix I've seen in the various forums, as well as following a few hunches of my own, but this nut refuses to be cracked. I apologize for the long post but despite my efforts the situation remains.

If you can help, I will be forever in your debt. This one really has me stumped.

Thanks in advance.

AlanBob

SRHsl600

the time I had that problem in a 500 the relay, new from OEM corrected the speed of the flash.

AlanBob

Replacing the combo relay was the second thing I tried. Because they are over $400 new and electrical items are generally not returnable, I purchased a used one in very good condition, on the (perhaps flawed) assumption that it probably wouldn't have the same fault, especially since my research didn't reveal a tendency for these units to fail. The fact that it works properly with the hazard flashers indicates to me that the problem probably lies elsewhere.

Thanks for your input!

Barry O

My car had the same problem as well as the turn signal not canceling after turning. I was told the cause of the non cancel issue was the boot or surround over the column was loose, which in fact it was. My independent drilled a small hole in the bottom of the column and repositioned the boot and tightened everything up. Seems to have solved the fast directional as well as the cancel issue.
If you are a DYI person I've seen threads here on doing the drill routine but I just wasn't comfortable drilling a hole when I don't know whats behind it.
Good luck.

AlanBob

Barry O: thanks for the suggestion. I'm not having canceling issues, but I will take a closer look at the area around the turn signal lever.

AlanBob

Well, after 8 months of zero progress, my fast flashing problem is finally solved. In the end it turned out to be a simple load issue.

I purchased a set of the 50 Watt 6 Ohm load resistors sold for LED turn signal conversions. All it took was one resistor wired in parallel to a turn
signal bulb on each side of the car, and the flash rate went right back to normal. I tested the bulb failure function (fast flash rate) by removing a
bulb, and it worked perfectly. The hazard flasher was unaffected and continued to operate properly.

Total cost: $8.00 and less than an hour of work (most of it spent mounting the resistors to a suitable surface). Plus, I have two resistors left over...

The whole thing remains a bit of a mystery, since I replaced all of the turn signal bulbs with the exact specifed types to no avail. Perhaps the combination relay requires more circuit load as it ages (unlikely) or there's some subtle anomaly in the car's electrical system (possible, but where?).

Anyway, it's fixed now and I'm happy to have the problem resolved. Just wanted to share, in case anyone else encounters this on their car. Thanks to all who offered advice.

Russell Ormerod

Possibly there is resistance in the wiring or ground to the bulbs. Causing the load to drop just below the threshold

AlanBob

Absolutely a possibility.

I checked each bulb socket for continuity to ground, and none showed any significant resistance, so the problem would have to be on the positive side of the circuit. Since it affected both sides, the issue would have to be at least back at the combination relay, but since the 4-way hazard flashers worked properly, it didn't seem likely that it was the combo relay either (which contains 2 flasher relays, one for each side of the car).

So it seemed that the problem lay in the circuit ahead of the relay, possibly the combination switch (turn signal switch) or maybe the hazard switch, which has an internal resistor and sometimes a diode. The hazard switch checked out OK, so my next step was to see if there was a problem with the combination switch or the dashboard indicator lights.

I wasn't really looking forward to digging into the under-dash area to reach the switch, so I elected to make one more try to increase the load, this time using load resistors instead of just an extra bulb wired in parallel. It worked, and I'm glad about that, but I don't know if the actual cause of the issue will ever be determined.

Thanks for commenting!

Russell Ormerod

You did well. That's a great fix. Just one thing. I am not sure if the double flash function works when the hazards are operated with one bulb out. Something to test.