To replace the tach cable (on a 280SL, assume 230 is similar): remove the decorative leather/vinyl covered panel under the instrument area; remove the screws holding the metal box under the panel area; push the box to one side; with your head on the pedals, reach behind the dash area to find the single knurled nut holding the bracket on the tach; remove the nut and the ground wire on the center post; remove the bracket (note orientation); gently push the tach slightly forward into the car.
Note: my car had a blob of tar(?) around the cables where they came thru the firewall so when I pulled the tach into the car, the cable was stuck and the cable center pulled out from the cable covering. Make sure the tach cable is free to pass thru the firewall so the slack will be available.
Remove the tach cable end from the engine. Take the cable out of any brackets or holders along the valve cover. (Read this entire msg before pulling the cable). Now the tach can be pulled into the car far enough to get a few fingers to remove the cable from the tach. There are also some wires going to the two small posts for the bulbs.
Now the tach is free. The cable must be pulled into the car, cannot be pushed out into the engine compartment because of the insulation that is on the cable.
There are two cables (on my car) going thru the same firewall hole. If the tach cable won't pass thru the tiny firewall hole because the end is too big, mighht have to remove the other cable partly. To make room for the tach cable end to go thru the hole, I had to remove the water temp cable from the sender, pull it partly into the car until the thin unprotected part of the temp cable was in the hole. This allowed the tach cable end to pass thru. (No such thing as an easy job on these cars.)
Install is reverse. You might replace the bulbs, and get a new gray intrument seal while the tach is out. A new firewall rubber grommet for the two cables (odd pyramid shape) is available. The grommet has to be cut to install on existing cables.
Suggestions I've read: a fast turn of the tiny exposed center post on the tach should move the needle. Try dropping some oil down the cable; attach a slow drill to the engine end of the tach cable to see if the tach needle moves. Needle flutter can be fixed with a new cable, bad screaming might require a tach rebuild.
Hope this helps,
Richard M
