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.
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.
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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