Replacing the Clock on a 1973 280 (W114) With a Tachometer
 

Greetings Again Everyone,

Wanted to take just a few more minutes and document the replacement of the clock on the W114 instrument cluster with a tachometer.

Several other things the PO failed to tell when I bought my car was that most of the light bulbs were burned out in the instrument cluster and the clock didn't work! Good thing I didn't pay too much for this German masterpiece or I'd have whacked him (sorry, my wife is Italian - they really do talk like that!!).

So I couldn't see replacing the clock with another clock, as they seem to fail often anyway and knowing the time wasn't nearly as important as knowing the engine speed. After noodling around, I saw that certain 300 series had a similar cluster with a tach so I figured "why not".

The tach I purchased was a VDO unit in their "Vision Series" which is apparently a throwback to their earlier gauge designs. The unit I bought has a 2 1/8 diameter, which is a standard for this type of gauge. Shopped around and bought my unit from Jeg's for around $90.00.

Once I had the tachometer, I pulled the instrument cluster and took it apart. I won't go into the removal of the cluster as it's well documented in other parts of this forum. With the cluster out and on my bench, I removed the clock and took some measurements. What I found is that if you cut the chrome bezel away from the body of the clock (just cut the tabs between the slots around circumference), the tachometer will actually slide through the bezel and fit perfectly in the cluster!

I wanted to use the same lighting circuit as the rest of the cluster to light up the tach, so I took two pieces of wire and connected them to the same circuit on the cluster circuit board. You'll have to play with the routing of the wire in the cluster, as I ultimately flipped the red wire "up and over" instead of "down and around" as depicted.

I then put the three wires together into a small harness and connected it to the tach through an existing hole in the back of the cluster. It was easier to make a short harness and connect the rest up after re-installing the cluster in the car, rather than trying to mess with 6 or 8ft of wire while reassembling the cluster itself.

Once everything was back together, I ran the red wire to a switched circuit out on a terminal block near the left front inner fender, ran the blue wire to the negative side of the coil and ran black to ground. Fired it up for the first time yesterday and it ran great!

Anyway, that's it. If you want that 300SEL look on your instrument cluster, this may be the way to go.

Hope this info helps!

Here's the Finished Product in the Car
 

Here's what the tach looks like after installation.....

I have been meaning to do this for two years, thanks for the write up!

Cool!!! (my clock always runs late 5 min per week); much rather have a tach.

Anyway you can get a closer pic of the dash (curious if it looks out of place)?