1996 R129 Turn Signals Fast Flash - PeachParts Mercedes-Benz Forum

AlanBob · #1 04-05-2016, 12:12 AM

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

97 SL320 · #2 04-05-2016, 07:35 PM

Wow, you really did your homework here and applied logic to the diagnosis. Too many posts are " My car won't run, what part should I buy? " Adding a bulb load was great thinking.

You have taken away all the easy stuff so I only have a few things to add.

Is there a trailer light converter in the trunk? This would combine turn signals and brake lights into a left and right for the typical trailer.

Your 4 Watt load addition sounds low, I think the typical 1156 is 12 W so I'd try that.

I'd pull a turn signal bulb and see if the hazards speed up, bulb monitoring ( pertaining to flash rate ) might not be used in hazard mode.

MB uses common ground points ( multiple ground wires on a single stud . ) When the bottom lug loses reference to chassis ground, all sorts of strange things can occur so I'd have a look there.

Quote:

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.

Is it correct that the relays operate at normal time but the lights flash 2X? If so, something else is controlling the lights.

I'd tend to think that the bulb out unit talks to the combo relay as something has to tell the combo relay to flash faster. Let me see if I have a wiring diagram.

And, when this is solved please post the result.

AlanBob · #3 04-10-2016, 05:45 PM

Thanks, 97 SL320. I prefer not to ask for help until I've exhausted my own resources. Sometimes, that's right away.

Quote:

Is there a trailer light converter in the trunk?

I thought of this but can't find any indication that the car is so equipped. Would I see a trailer connector behind the rear bumper? There are a couple of fuse boxes and relays in the trunk near the battery, but these seem to be related mostly to the convertible top mechanism.

Quote:

Your 4 Watt load addition sounds low, I think the typical 1156 is 12 W so I'd try that.

Good point. I was actually thinking of trying the load resistors sold to adapt LED lamps to older systems.

Quote:

When the bottom lug loses reference to chassis ground,

Are you referring to the grounding points at each corner of the car, for the lighting assemblies?

Quote:

Is it correct that the relays operate at normal time but the lights flash 2X?

No, the relays operate in time with the lights. 1X with the hazard, 2X with the turn signals.

Quote:

something has to tell the combo relay to flash faster.

My thought exactly. I haven't been able to determine if the flash "trigger" (if you will) is generated inside the combo relay or from somewhere else. I haven't found anything like this on a wiring diagram yet. Still trying.

Thanks for all the great suggestions. I will try them and report back.

97 SL320 · #4 04-11-2016, 08:22 PM

Quote:
When the bottom lug loses reference to chassis ground,

Quote:

Are you referring to the grounding points at each corner of the car, for the lighting assemblies?

I don't know where the ground points are specifically but they might be near the lights. I've sometimes used a jumper wire from the bulb to ground as a test. Still, something has to be globally affecting the system or at least both lights on one end of the car.

While the turn signal lights are flashing ( left or right ) look closely at the other tail light bulbs / front turn signal bulb and see if an unintended filament is dimly glowing. If so this is a sign a ground has lifted.

AlanBob · #5 04-12-2016, 04:42 PM

Quote:

While the turn signal lights are flashing ( left or right ) look closely at the other tail light bulbs / front turn signal bulb and see if an unintended filament is dimly glowing. If so this is a sign a ground has lifted.

I've played with this at night and never noticed anything. I'll look more closely. Thanks!

AlanBob · #6 05-14-2016, 07:03 PM

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.

97 SL320 · #7 05-14-2016, 07:29 PM

Thanks fore the update, there are lots of threads with no resolution.

It's really odd that something apparently native to the car would cause this. Given Euro cars have a side of fender , just ahead of the doors turn signal, I wonder if there is a hidden load resistor or setting in the flasher.