For model years 1978 thru 1981, the turn signal flasher relay and the Hazard light switch were combined into a single, large, expensive unit. If you can find one new (hard to do) they are typically over $160. Used ones command $80+ on fleabay. Obviously MB decided it was a bad idea, and went back to a separate flasher unit, no longer part of the hazard switch.
But if you have a 1978 thru 1981 model, you are stuck. Here's how to repair/renew the switch/flasher assembly. This is especially useful if you have the symptom of the flasher working when cold, and slowing down and stopping as the car get hotter, and similar complaints.
It is also possible to modify your wiring slightly to use the more commonly available "simple" hazard switch and then insert a separate flasher unit. This is possible as the simpler switch uses the same pins as the more complicated combined type. I am not going to cover that mod in depth, but lets look at the "BIG FAT" all in one unit compared to the simpler hazard switch.
THE BIG FAT UNIT and the SIMPLE SWITCH
On the left is the combo switch, which is larger as it contains a relay and the simple transistor flashing circuit. On the right is the simpler hazard switch, used on 1977 and earlier cars. If memory server, 1982 and forward used the "new look" switches. The simpler switch on the right can be had at a junkyard for $5 to $7. The big combo switch is almost impossible to find except on ebay for over $80.
Same pin pattern
If you look at the back, you'd see they both share the same pin pattern. In fact, you can plug the smaller switch directly into 1978-81 cars, with the only problem being that it will not work. To make the smaller switch work in these cars, you need to add a separate flasher unit.
The one extra pin on the smaller switch is circuit 49 (do not confuse it with 49a). Circuit 49 goes direct to the flasher unit, and then from the flasher unit to pin 49a.
The big fat one incorporates the flasher unit, so pin 49 is removed. HOWEVER, you'll see the connector in the car still has a HOLE for pin 49, and you could put a contact in there, and run a wire to a separate flashing unit.
This is the wiring diagram for 78-81 using the big fat one:
And here for the simple switch with separate flasher:
As you can see, Pin 49 from the simple switch just goes to the flasher, and pin 49a returns to the switch.
All you need to do then is add the contact at hole 49, wire that to a pin of a simple 2 pin flasher, and then wire the other pin direct to the existing pin 49a. When you plug in the simple switch, all flashing functions should work - both turn signals and hazard lamps.
Now as I instead repaired my big fat one, I have not tested the mod, but it should work as described (standard disclaimer: no guarantees).
THE REAL WAY: REPAIR YOUR UNIT
I preferred to repair my unit, so that will make up the bulk of this post.
First, here is the exploded view of the big fat one, so you can see the various major components:
From left to right: The upper case, the button, the button lock and spring mechanism, the electronics circuit board, the lower case, and lamp holder. Not shown are the two metal tabs that hold the switch in the console.
OPEN SESAME
First off, we need to carefully disassemble the case. This is old brittle plastic. And the circuit board inside is even more brittle.
1) Remove the lamp holder by pulling it straight out.
2) Using a flat blade screwdriver, press in on the locking tabs on the side of the case. Ease the case open as you unlock each tab, then wiggle the top case free. The button will most likely come with it.
3) Carefully remove the bottom locking piece (easier when you have already removed the lamp holder).
4) Now you must DESOLDER the contact to Pin 49a, and ALSO the center lamp contact (hidden here by the lamp holder which I failed to remove until later, ooops). I find SOLDER WICK best for these situations.
5) Once desoldered, the PC board should come out easily.
HANDI-CAPS
The two components we are interested in are the two electrolytic capacitors. These tend to dry out and fail, or at least become very heat sensitive, failing more as temperature rises. The one on the bottom (the larger one) is a 47 mFD 16Volt. The smaller one is a 1 mFD 63 Volt. I don't know that the 1 mFD needs to be such a high voltage, it may have simply been what was available at manufacture time. (With capacitors, the voltage rating is simply the highest voltage it can withstand, so for instance if you used a 25 volt 47mFD, it would behave the same as a 16 volt 47mFD. Always use the same or higher voltage when replacing a capacitor).
Desolder them both and take them with you to the parts store to make sure you get ones that are about the same physical size.
As it happens, at the local parts store, the only 1 mFD caps they had were 160 volt (more than enough, LOL). The 47mFD I got was a 35 volt, and it was substantially smaller than the one original used. Don't worry about size differences, so long as they FIT, and the mFD is the same and the voltage rating is the same or higher.
With electrolytic caps, the polarity is important. The capacitors are marked with a - symbol for the negative lead, and the positive lead is always LONGER than the negative lead. On the circuit board, you'll see a + (plus) for the positive lead, and the longest lead should go in this hole. As you can see from the picture, the 47 mFD's negative lead is closest to the corner of the board, and the 1 mFD's negative lead is closest to the relay. Note also that in my new parts, the 1 mFD is actually larger than the 47 mFD, due to the different voltages and manufacturing processes.
Also, make sure you buy RADIAL leads not axial, so you can just drop the new caps straight in. Solder them into place and clip the leads.
The capacitors are in charge of the speed of the flashing. Larger capacitors (as in higher mFD) will flash more slowly. I have not analyzed the circuit, so I'm not sure if both capacitors are used for speed, or just one (I suspect the 1 mFD is actually for debouncing contacts). That said, I suspect that if you were to replace the 47 mFD with a 100 mFD, you'd cut the flashing sped about in half.
I say this as I was kinda shocked at how fast the new caps flashed at!!! If you wanted to be really clever (or silly), you could replace the resistor portion of the RC circuit and use a variable resistor, and have an adjustable flash speed.
7) Re-solder the board to the two solder contacts (49a and 31).
8) I find it easier to first mount the button on the lock board, and lock it into the depressed position. Make sure the contact is inside the contact of the lock boar (depress it as shown here)
9) Then place the lock board and button into the lower case, on top of the PC board. Again, make sure the sliding contacts are INSIDE the contacts of the lower case.
10) Now, wiggle the lower assembly into the upper case - NOT SHOWN (oops) don't forget the metal side clips that hold the switch in the console. They fit in the lower case first, just before you finish snapping them all together.
That's about it - please ask any questions on these procedures.
This was great information, however, for some reason the turn signals work now, but the flashers don't. Did I damage something? I checked the assembly twice. All the contacts are on the inside and everything looks good. This job was relative easy, the capacitors were replaced with no visible signs of damage to the board. Any suggestions?
@Digmenow
It looks like we can repair your old one and make it more reliable.
Maybe we can fix it, and have it as a mutual spare in the event the one on your 380sl or my 450slc 5.0 goes out.
OT: I noticed the other day that the hazard switch on my 1985 280sl doesn't work and when turned on causes my blinkers to not work. It's much different, but I wonder if if is similar to the small one above.
@Digmenow
It looks like we can repair your old one and make it more reliable.
Maybe we can fix it, and have it as a mutual spare in the event the one on your 380sl or my 450slc 5.0 goes out.
OT: I noticed the other day that the hazard switch on my 1985 280sl doesn't work and when turned on causes my blinkers to not work. It's much different, but I wonder if if is similar to the small one above.
I saw this thread when mine first started acting futzy but my soldering skills are minimal at best and the loss of near vision ain't making close work any easier. :frown
I meant to leave the old one with you but tossed it in the car instead. If you want it, I'll ship it.
Great post! Very informative! I followed all the steps as you provided with my 1981 380slc. I replaced the capacitors with new ones of the same mFD and voltage. However, when I use my turn signals, the light stays on continuously. Maybe they're flashing so fast that it looks continuous. More worrying is the hazard/flasher unit is making a loud buzzing noise and gets very hot very quickly. Not sure if this is a circuit issue or what but if anyone has any knowledge of these issues I would appreciate the help
This was great information, however, for some reason the turn signals work now, but the flashers don't. Did I damage something? I checked the assembly twice. All the contacts are on the inside and everything looks good. This job was relative easy, the capacitors were replaced with no visible signs of damage to the board. Any suggestions?
@Digmenow
It looks like we can repair your old one and make it more reliable.
Maybe we can fix it, and have it as a mutual spare in the event the one on your 380sl or my 450slc 5.0 goes out.
OT: I noticed the other day that the hazard switch on my 1985 280sl doesn't work and when turned on causes my blinkers to not work. It's much different, but I wonder if if is similar to the small one above.
Just for the record the flasher unit on and 1983 through 198? Sls is behind the instrument cluster. It contains a IC chip instead of the capacitors. This chip can be replaced. First insure the relay is still working. Apply 12V to circled pins shown in the bottom photo. The relay should click. You can also clean the relay contacts with emery cloth. I obtained my replacement chip from a German seller on eBay at a cost of $5 including shipping. It can be removed by cutting the leads with tiny wire cutters, then while heating the remaining lead with a soldering iron pull the lead out with needle nose piers. I couldn't find my small wire cutters at the time so I used a device called a solder sucker then while heating the leads I extracted the old chip with a small screwdriver and needle nose pliers.
TAKE NOTE OF THE ORIENTATION OF THE OLD CHIP ON THE BOARD TO INSURE THE NEW CHIP GOES BACK IN THE SAME WAY. Pay attention to the molded in dip in the top of the chip.
That chip has a feature that senses if a turn signal light is out, by increasing the speed of the flashing to let you know. ... For those interested, here's the data sheet on the U243b IC:
That chip has a feature that senses if a turn signal light is out, by increasing the speed of the flashing to let you know. ... For those interested, here's the data sheet on the U243b IC:
The analog flasher unit in my 54 Chevrolets do the same thing. There is a thermal strip of metal that heats up and cools causing the contacts to open and close. I did find that the Mercedes flasher unit with the chip is not compatible with LED lamps. :smile
The analog flasher unit in my 54 Chevrolets do the same thing. There is a thermal strip of metal that heats up and cools causing the contacts to open and close. I did find that the Mercedes flasher unit with the chip is not compatible with LED lamps. :smile
7) Re-solder the board to the two solder contacts (49a and 31).
8) I find it easier to first mount the button on the lock board, and lock it into the depressed position. Make sure the contact is inside the contact of the lock boar (depress it as shown here)
9) Then place the lock board and button into the lower case, on top of the PC board. Again, make sure the sliding contacts are INSIDE the contacts of the lower case.
10) Now, wiggle the lower assembly into the upper case - NOT SHOWN (oops) don't forget the metal side clips that hold the switch in the console. They fit in the lower case first, just before you finish snapping them all together.
That's about it - please ask any questions on these procedures.
7) Re-solder the board to the two solder contacts (49a and 31).
8) I find it easier to first mount the button on the lock board, and lock it into the depressed position. Make sure the contact is inside the contact of the lock boar (depress it as shown here)
9) Then place the lock board and button into the lower case, on top of the PC board. Again, make sure the sliding contacts are INSIDE the contacts of the lower case.
10) Now, wiggle the lower assembly into the upper case - NOT SHOWN (oops) don't forget the metal side clips that hold the switch in the console. They fit in the lower case first, just before you finish snapping them all together.
That's about it - please ask any questions on these procedures.
For model years 1978 thru 1981, the turn signal flasher relay and the Hazard light switch were combined into a single, large, expensive unit. If you can find one new (hard to do) they are typically over $160. Used ones command $80+ on fleabay. Obviously MB decided it was a bad idea, and went back to a separate flasher unit, no longer part of the hazard switch.
But if you have a 1978 thru 1981 model, you are stuck. Here's how to repair/renew the switch/flasher assembly. This is especially useful if you have the symptom of the flasher working when cold, and slowing down and stopping as the car get hotter, and similar complaints.
It is also possible to modify your wiring slightly to use the more commonly available "simple" hazard switch and then insert a separate flasher unit. This is possible as the simpler switch uses the same pins as the more complicated combined type. I am not going to cover that mod in depth, but lets look at the "BIG FAT" all in one unit compared to the simpler hazard switch.
THE BIG FAT UNIT and the SIMPLE SWITCH
On the left is the combo switch, which is larger as it contains a relay and the simple transistor flashing circuit. On the right is the simpler hazard switch, used on 1977 and earlier cars. If memory server, 1982 and forward used the "new look" switches. The simpler switch on the right can be had at a junkyard for $5 to $7. The big combo switch is almost impossible to find except on ebay for over $80.
Same pin pattern
If you look at the back, you'd see they both share the same pin pattern. In fact, you can plug the smaller switch directly into 1978-81 cars, with the only problem being that it will not work. To make the smaller switch work in these cars, you need to add a separate flasher unit.
The one extra pin on the smaller switch is circuit 49 (do not confuse it with 49a). Circuit 49 goes direct to the flasher unit, and then from the flasher unit to pin 49a.
The big fat one incorporates the flasher unit, so pin 49 is removed. HOWEVER, you'll see the connector in the car still has a HOLE for pin 49, and you could put a contact in there, and run a wire to a separate flashing unit.
This is the wiring diagram for 78-81 using the big fat one:
And here for the simple switch with separate flasher:
As you can see, Pin 49 from the simple switch just goes to the flasher, and pin 49a returns to the switch.
All you need to do then is add the contact at hole 49, wire that to a pin of a simple 2 pin flasher, and then wire the other pin direct to the existing pin 49a. When you plug in the simple switch, all flashing functions should work - both turn signals and hazard lamps.
Now as I instead repaired my big fat one, I have not tested the mod, but it should work as described (standard disclaimer: no guarantees).
THE REAL WAY: REPAIR YOUR UNIT
I preferred to repair my unit, so that will make up the bulk of this post.
First, here is the exploded view of the big fat one, so you can see the various major components:
From left to right: The upper case, the button, the button lock and spring mechanism, the electronics circuit board, the lower case, and lamp holder. Not shown are the two metal tabs that hold the switch in the console.
OPEN SESAME
First off, we need to carefully disassemble the case. This is old brittle plastic. And the circuit board inside is even more brittle.
1) Remove the lamp holder by pulling it straight out.
2) Using a flat blade screwdriver, press in on the locking tabs on the side of the case. Ease the case open as you unlock each tab, then wiggle the top case free. The button will most likely come with it.
3) Carefully remove the bottom locking piece (easier when you have already removed the lamp holder).
4) Now you must DESOLDER the contact to Pin 49a, and ALSO the center lamp contact (hidden here by the lamp holder which I failed to remove until later, ooops). I find SOLDER WICK best for these situations.
5) Once desoldered, the PC board should come out easily.
HANDI-CAPS
The two components we are interested in are the two electrolytic capacitors. These tend to dry out and fail, or at least become very heat sensitive, failing more as temperature rises. The one on the bottom (the larger one) is a 47 mFD 16Volt. The smaller one is a 1 mFD 63 Volt. I don't know that the 1 mFD needs to be such a high voltage, it may have simply been what was available at manufacture time. (With capacitors, the voltage rating is simply the highest voltage it can withstand, so for instance if you used a 25 volt 47mFD, it would behave the same as a 16 volt 47mFD. Always use the same or higher voltage when replacing a capacitor).
Desolder them both and take them with you to the parts store to make sure you get ones that are about the same physical size.
As it happens, at the local parts store, the only 1 mFD caps they had were 160 volt (more than enough, LOL). The 47mFD I got was a 35 volt, and it was substantially smaller than the one original used. Don't worry about size differences, so long as they FIT, and the mFD is the same and the voltage rating is the same or higher.
With electrolytic caps, the polarity is important. The capacitors are marked with a - symbol for the negative lead, and the positive lead is always LONGER than the negative lead. On the circuit board, you'll see a + (plus) for the positive lead, and the longest lead should go in this hole. As you can see from the picture, the 47 mFD's negative lead is closest to the corner of the board, and the 1 mFD's negative lead is closest to the relay. Note also that in my new parts, the 1 mFD is actually larger than the 47 mFD, due to the different voltages and manufacturing processes.
Also, make sure you buy RADIAL leads not axial, so you can just drop the new caps straight in. Solder them into place and clip the leads.
The capacitors are in charge of the speed of the flashing. Larger capacitors (as in higher mFD) will flash more slowly. I have not analyzed the circuit, so I'm not sure if both capacitors are used for speed, or just one (I suspect the 1 mFD is actually for debouncing contacts). That said, I suspect that if you were to replace the 47 mFD with a 100 mFD, you'd cut the flashing sped about in half.
I say this as I was kinda shocked at how fast the new caps flashed at!!! If you wanted to be really clever (or silly), you could replace the resistor portion of the RC circuit and use a variable resistor, and have an adjustable flash speed.
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