Clock fix - '78 300D (trouble with ground) - PeachParts Mercedes-Benz Forum

rgnprof · #1 01-27-2006, 09:17 PM

I followed the procedure as outlined in other places to fix the clock by replacing the capacitors with Radio Shack capacitors (I have just the clock, no tach on this car, but i decided to try it anyway).

I'm having a problem - I'll try to describe what is happening. When I first put the clock back in - nothing, still didn't work. I pulled it out again and looked it over and plugged just the mechanism into the 12v lead from the car and started checking voltage at different spots on the board - seemed alright, but it still didn't work. There is a small post on the clocks circuit board (ground) that sticks through a small hole in the casing that was soldered originally (I had to desolder to get the clock mechanism apart from the back casing). I decided to ground that post using one of my test leads from my voltmeter and PRESTO! - the clock started working. So I tried to resolder that post again to the back when I reassembled (AGAIN) the clock, but it still doesn't work. For some reason I'm not getting a good ground and I can't figure out why??

Can anyone help?

Thanks, ryan

rhodes2010 · #2 01-27-2006, 09:25 PM

I just did this fix last week on my own clock.
Same model clock, no tach.

If you put the ground lead onto the post does the clock work ?
If it does I would think that your solder is not sticking to the back
of the clock and is insulated by flux or something.

rgnprof · #3 01-28-2006, 12:34 PM

I have tried to attach a picture to better explain my problem. This is the back of the clock assembly WITH the clock attached. If you look closely you can see a little post just barely sticking through the casing right in the middle of the copper ring - when I took the clock apart the first time this post was soldered to the brass/copper (?) ring which is attached to the casing back. I have tried to resolder this post to the ring and the casing back, but the clock does not run. I can get the clock to run if I take my voltmeter test lead and touch one end to the post ONLY and the other end to a ground (in my car I am using a bolt that secures the seat track to the floor). Grounding the post to the ring or the casing back does not work. I am confused (not an electrical expert) and I'm wondering if this was what was wrong all along.

Anyone have any ideas? Thanks much.

ryan

Lycoming-8 · #4 01-29-2006, 12:13 AM

Why not just rig up a wire that connects the post to a good ground and forget about the case? Seems to work when you do it artificially, right?

rhodes2010 · #5 01-29-2006, 01:45 AM

I agree with lycoming-8.

But hold on.
Please run your test lead from the case to the seat bolt ground
used for the post. If clock runs this says that post is making good contact
with case.

Then... are you reinstalling the whole instrument cluster with all clock screws
replaced, and the wiring harness plugged onto the back of the cluster ?

I would not expect the clock to kick in until put back in the cluster, and the cluster wiring harness in the dash attached.

My thinking here is that the ground signal comes from the harness to the
clock body through attachment to the cluster, to the post (that you
soldered back).

Not trying to be insulting with my details, just taking into account your non-electrical confession.

I believe you needed the fix. You probably did not waste your time.

rgnprof · #6 01-29-2006, 04:47 PM

I figured it out! As it turns out, one of the leads from the new capacitor that I had soldered in was poking through the rubber gasket and evidently making contact with the metal clock casing. I had trimmed down the leads on the new capacitors after soldering them, but didn't cut them close enough. Resoldered the ground post to the back and problem fixed. I feel better - knowing that I didn't waste my time replacing the capacitors.

Anyway, thanks for all of the help.

Ryan