1996 R129 Turn Signals Fast Flash

[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