Clock Repair

Clock Repair (Sept 1, 2005)

Overview

Apparently there is a common problem with the clocks in our W126 (and probably other models) MB's. It's seems to be a familiar topic on various message boards.. The problem starts with the clock intermittently stopping. Flicking the setting knob usually gets it going again for a little while. The problem gets progressively worse until it stops all together. In most cases the fix involves replacing a pair of electrolytic capacitors (100mf 35v). I found several links to the procedure and they are listed below. Here's a quick little reference, in these links go away..

Links:
Cluster Repair

Tools:
Fig-10

Procedure
The purpose if this procedure is to give you a general idea of what worked for me.

1. Remove cluster according to MB maintenance procedure 54-250.

2. Remove the clock/tach assy. There are about 10 Phillip screws around the perimeter of the back of the cluster assy. Because the speedo & outside temp display assys (center) are mounted on top of the clock/tach assy, they have to be removed first. Fig-1 With the all the remaining screws removed, carefully lift out the Speedo assy, guiding it around the cluster casing. Then repeat process for the clock/tach assy.

3. With cluster on bench, remove the tach and clock hands as shown in Fig-2. Then remove face plate, Fig-3.

4. Remove clock mechanism (Fig-4). Desolder the clock ground post, by heating the solder joint the using the solder sucker to suck the hot solder from the joint.
Note: The solder sucker basically works like a big spring loaded hypodermic needle. You push down the plunger, pressing the button releases the plunger and that draws air and hot solder through the opposite end. Pushing down the plunger again forces out most of the cooled solder. After several times, the solder sucker should be disassembled and cleaned. Proper use of the appropriate soldering iron (never more than 25 watt for normal pc boards and may something a bit higher wattage for beefier connections) and the solder sucker should leave the repair area undamaged and the component neatly unsoldered. (see Fig-8)

5. The original capacitor(s) were rated at 100mf 16v, which according to a couple of articles I read suggests are under rated and prone to failure after time. Most likely, they were used because of the size restriction. Radio Shack sells a 100mf 35v which is the same size as original at over double the voltage rating. Note that caps come in 2 mounting styles (axial "horizontal" and upright "vertical"), these are upright caps. (see Fig-7)

6. Carefully remove old cap using 25 watt soldering iron and solder sucker as explained in step 4. Be sure to note the polarity of the cap on the pc board. (Like a battery, electrolytic caps have +/- polarities and will damage the device if installed backwards.) The pc board may denote the proper polarity as seen in Fig-8. If the pc board does not, look for some indictor on the old cap before removing it (usually a + sign) and make the pc board accordingly.

7. Insert the new cap pressing it securely against the pc board. Spread the cap leads to temporarily hold the cap tightly to the pc board. Then apply heat to one of the leads and the pc board track for a second or two. Touch the solder to the heated area and apply just enough solder the fill the hold and secure the cap lead. It should require more that 1/8" inch of solder. (see Fig-9)
Note very important: Use solder specifically for electronic repairs. This is a very small gauge solder, not the heavy stuff used in plumbing. Go to Radio Shack and ask the guy there for the appropriate stuff.

8. You can test the clock by connecting a couple of jumper leader to the clock and 12v battery. Observe appropriate polarity.

9. Reassemble instrument cluster making sure any electrical connection are clean. I may also be a good time to replace the various cluster light bulbs, since they have a tendency to dim with age.

10. I was going to take this opportunity to repaint my needles, since some have faded. I just could not find the appropriate shade of fluorescent orange.

	Back of Cluster (clock is on left side of this picture) Fig-1	Removing clock hands and needles. Carefully lever them off protecting the faceplate. Fig-2
	Clock/tach face, held on by 3 screws. Fig-3	Clock mechanism is secured to the backplate by 2 screws and the -12v ground lead. Fig-4
	Old 100mf 16v capacitor (my clock in 1989 560 SEC, had only 1 cap) Fig-5	Another view of cap. Fig-6
	Old cap and replacement cap. Fig-7	Before removing cap, note polarity. These caps have a +/- lead. The (+) lead is noted on the back of the pc board. Fig-8
	New cap installed. Fig-9	Required tools: Solder sucker, 25watt iron, solder Parts: 100mf 35v cap from Radio Shack Fig-10