W123 300D Clock Question - PeachParts Mercedes-Benz Forum

Mölyapina · #1 04-18-2013, 06:12 PM

So, I guess the question is twofold:

1) How should I test the clock bit in tach/clock thingie? I thought that the plastic gears were the only things that went, but apparently not.

2) I already tried testing it by running it off the car battery, after seeing this thread from Benzworld. At first I had no action, so then I readjusted the jumper cables and got sparking from the positive cable on the clock (

). I immediately pulled the negative cable, and everything calmed down. Here is the picture from that Benzworld thread, with "a" being the place I hooked up the positive cable. Did I do any harm here?

martureo · #2 04-18-2013, 09:38 PM

You can test the clock with something as small as a 9v battery. If there's not movement then you probably have a repair in your future. I don't think you hurt anything. If you did you can inspect the traces for burn marks, but I really doubt you harmed the clock.

The most common failure is the capacitors. Simply, the material gets old and they no longer function the way they are supposed to.

I know you already know some of this but I'll quote all of my instructions from that benzw()rld thread again for the benefit of others.

Quote:

Pulling a cluster is pretty easy.

You need to remove three screws below the seam of the dash. You might have to pry the little plastic covers off first. And one white plastic screw near the hood release. Then it just lowers.

Reach your hand up and behind the cluster, then push it directly out.

You'll need to disconnect the speedo cable, the oil pressure line, the power lines for the clock and the other lights, and the connector for the oil/coolant/fuel panel.

Lay the cluster down so you can look at the rear. The clock is the far left panel and you remove it by removing four screws, one near each corner of the panel. Then pull it directly out.

To remove it enough to replace the capacitors you need to desolder the ground connection that looks like a bullseye with a stick through it. Also remove almost all the screws holding the rear panel on. Now you need to remove the three screws holding the front of the clock on. BE CAREFUL, I would not try to remove the needles for the clock and tach (if you have one) but try to work without bending them or the shafts on which they sit.

The caps you need to replace can be bought at Radio Shack. 100 uF 100v or 50v. #272-108.

So desolder the connections holding the capacitors on. (If you don't know what they look like they are the only components on the board that look like soda cans. Make note of which connection is negative and which is positive. If you solder the news ones on incorrectly you'll have to do this again.

The new caps have a black stripe down one side, this is the negative contact. Solder the new caps in place and snip the excess wire. Reassemble and make sure you resolder the ground connection on the backing plate.

Done!