Actually it is a 1981 300SD; was mistake. Now that I am OK with pulling out the instrument console I am wondering about the tach. While it does not work 95% of the time, the other times it goes crazy - back and forth like a whirligig.

So I am thinking maybe the problem is the electrical connection behind the instrument console. There is a large circular plug with about a dozen pins in it that is unplugged to remove the instrument console. I am wondering if one of those pins runs the tach.

Anyone know?

However upon contemplation, there are two wires on the passenger side of the speedometer. One runs the clock probably and the other runs the tach. One is a single pole push on female connection and the other is actually a plug which fits over two round pins. Not sure which runs which.

Maybe the multi-plug has nothing to do with the tach?

On my 500SEL (I think they are the same for tach and clock) the single wire connector is battery power for the clock and the 2 pin plug is ignition power and tach signal (TD) for the tach. The ground for the tach and clock is through a flat buss bar that goes from the tach/clock circuit board to the speedometer and then to G102 behind the instrument panel.
Hope this helps.

PaulM

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84 500 SEL (307,xxx miles)

Here pics of the back of my instrument cluster. I see a copper ground at the bottom of the speedometer cluster but nothing close to the tach or clock. Do not see a ground there at all. Do you see anything?

In the first picture, see the bar just behind the speedometer face plat with a hole at the end. Now in picture two, see the two bottom screws holding the speedometer in. The screw on the left goes through the speedometer cover, the metal bar, and the tach housing. That is where the tach gets it's ground. The ground wire from the speedometer goes to the ground point behind the instrument panel. The other end of the metal bar in picture one also supplies the same ground to the gauges on the other end of the instrument panel.

I see you are doing the odometer gears. I just did that today. Not a bad job if you get the correct gears. I didn't but it was my fault. I ask for gears for a 1984 126 but the speedometer is from a 1990 126. They must be a little different. I just had to file a small collar off of one of the gears and every thing worked fine.

Another little tip. The plastic light guides (one visible in picture 1) don't do a very good job of reflection of the dash lights. I cleaned mine real good and painted them a bright silver. I did mine many years ago and they are still bright silver and I have to turn the brightness down.

PaulM

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84 500 SEL (307,xxx miles)

pmckechnie - I do not see a ground wire from the speedometer unless it is the one plug with a tab at the bottom which plugs over one pin in the center speedometer. You can just see it in this pic as a black plug resting on the wood of the dash; I had just unplugged it from the instrument cluster. Is this what you are calling a ground wire?

On the driver's side I found three connections - this black plug which slides over a single pole on what I believe is the center of the speedometer; guessing this makes the needle turn on the speedometer. A multi-plug which is rectangular and has eight pins. And a round multi-plug with about a dozen pins and a somewhat triangular plug in the center which helps you figure out how to guide it in.

So I understand that the copper? tab or bar on the bottom of the speedo is probably a ground and maybe touches the tach somehow. But I cannot figure out your comment about a second ground wire unless it comes from the center of the speedo.

I'm sorry but I am not good at explaining things but I will try again to explain how the tach gets its ground.

Internally the tach ground is connected to the metal plate that holds the tach to the instrument panel.

From that metal plate the ground goes through the metal bar on the speedometer circuit board. The actual connection is at the lower left speedometer assembly mounting screw. See photo 2 of post 4 above.

The metal bar is connected to pin 3 of the 8 pin connector on the speedometer circuit board.

The plug that goes into the 8 pin connector has a brown wire that connects to pin 3.

That wire goes to the ground point behind the instrument panel along with many,many other brown wires.

Is this any better?

PaulM

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84 500 SEL (307,xxx miles)

Yes, thanks. Great explanation. Got it. I saw a bundle of wires from the 8 pin plug and they were wrapped with the original factory black tape (I assume) and headed toward the engine compartment. I had thought that a problem with the tach's ground might make it work sporadically and not work most of the time. But I do not see now how that is possible unless the other end of the tach wire (going to the front of the engine) could be found and checked. It is so secure at the instrument cluster that even after 35 years I do not think it could come loose. This is a long shot but do you have any idea how I would find the other end of the tach wire/ground wire?

The instrument cluster was a piece of cake except that I dropped all the gears and had to figure out for myself what went where. Did not take clear enough pictures. Hint to the wise. I tightened down the clear plastic cover (the one with the cutout so you can roll the gears) too much and the new gears would not turn. Took me four times of untightening and drive testing to get it right but the gears work fine now. I will say that $75 is way, way too much for Odometer Gears to charge. Way too much. That said I am set for another 35 years now.

Thanks for your help.