Jim Saylor says its easy, maybe for Jim and maybe compared to brain surgery but for me it was a tough job.
I replaced the speedometer cable on a late 280SL a year or so ago. Here’s a step by step as far as I remember. The description is mine. The risk of error or accident is yours. Suggest reading the whole step by step before beginning. I made this step-by-step in baby talk so anyone even as as inexperienced as I was can (I hope) do the job.
Speedo cables are available from re-sellers, on Ebay, or from MB. Reseller or Ebay cable is about $20 to $30. MB cable is about $60 to $80. I ordered both to compare, then I used the MB cable even tho it cost four times as much. It looked better finished, had better sheathing, and since I did not want to ever replace it again, I went with the more expensive (better?) cable. There are two size cables: one for auto tranny, one for manual. Be sure to get the correct size.
Accessing the speedo cable behind the dash is the fun part. The tach unit has to come out first so you can get a hand in to reach the rear of the speedo unit.
To get the tach out, you need to remove the terminal junction box under the dash below the instrument area. It's about three inches square with a thick cable going into it. Unscrew the box and push it aside. The cable does not have to be removed. Then take off the protective/decorative under dash vinyl covered panel. Remove the few screws holding the panel.
Now get into a comfortable (ha) position on your back looking up under the dash maybe with your head on a pedal. Now reach into the maze of wires in the back of the dash on the side toward the driver door until you see (with a flashlight [torch]) or at least feel the bracket holding the tach. The bracket is a square “C” shape; a central pin or shaft on the tach holds the bracket by a fingertight round knurled nut. Note the orientation of the bracket.
Remove the nut and take off the ground wire that is also on the pin. The bracket should come off (or fall off). It has sharp points on the ends to better grip the back of the dash to hold the tach in place when it goes back on.
Now push the wires around a bit carefully so you can begin to push the tach unit into the car. The tach unit has wires and a cable attached so don’t push more than needed to get a grip around it from the front of the dash. The tach unit has a rubber seal around it to mate with the dash body. On many (most) cars, this is dried out and should be changed. A new one is available from MB for few dollars. (The speedo also has one so get two.)
Work the tach unit out of the hole but don’t pull too hard since the wires and the cable are still attached. The tach cable is held by a fitting at the end of the cable that mates with the connector on the back of the tach. Turn the end then pull the cable out of the unit.
[Caution: when I tried to pull the tach into the car to access the cable end, the cable was caught in the grommet where it enters through the firewall. It had been glued to the firewall grommet with some tar.
When I gently pulled the tach unit slightly into the car, the end of the cable pulled out of the cable and came off with the tach unit. Since I was replacing the tach cable anyway, it was not a disaster. Make sure the tach cable and speedo cable can pass freely thru the firewall.]
There are also wires for the two light bulbs. These are connected to bulbs and there is also a cross connection between the two bulb units. Observe the wire placement carefully and maybe make drawing to be able to put them back correctly.
Might want to think about changing the bulbs since they are now conveniently exposed.
After the wires and the cable are off, the tach is free to come off.
Now that the tach is out, you have a nice “big” hole to allow you to put your skinny (I hope) arm into the hole to reach behind central instrument unit. DO NOT disconnect the central unit connectors. One of these as is an oil tube that will leak into the car if disturbed. The central unit can be pushed SLIGHTLY into the car if needed for access but don’t strain the connectors.
Reach behind and past the central unit to feel the speedo back. It is held on the same way as the tach unit. Loosen the central nut, catch the “C” bracket and begin to push the speedo into the car, not too much as the light wires and the cable are still attached. Remove the speedo cable end and the light wires to free the speedo unit.
Now you begin the cable phase of the operation. The speedo cable comes into the dash thru the firewall grommet. There are two grommet holes in the firewall for four cables on a late 280 SL, not sure what is on earlier cars. Look at the holes carefully and note which cables pass thru what holes.
The speedo cable can be pulled out from the dash through the firewall. The cable must go out toward the engine side, cannot be done by bringing the cable into the car interior. Maybe a good idea to tie a string to the end of the cable to use as puller to get it back. I don’t recall the details of how to get the new cable back into the rear of the dash.
The firewall hole has a rubber grommet. If dried out, should be replaced...available from MB. I remember for some reason you have to cut a slit in the grommet to get it around the cable. This is also little hazy. I remember that I had to unhook the water temp tube (also passes thru the grommet hole) and push it partially into the firewall to allow the speedo (or tach) cable to pass thru the hole. It was a very tight fit. Pull or push cable thru the firewall.
Now you can remove the tranny end of the speedo cable. On the late 280SL manual, the cable is held to the car at the tranny by a single bolt that must come ALL THE WAY out. Just loosening the bolt at the tranny will not allow the cable to come off as the bolt passes thru a channel on the cable end. The bolt must come out all the way.
It was challenge (ugh) to get access to the bolt as it is well under the car and half hidden by a plate that sits almost over the bolt. When installing the new cable, make sure the new cable is firmly seated in the tranny hole. The bolt will not go into the channel unless the cable is all the way in.
Re-installing is the reverse.
Hope others will chime in with corrections and more hints.
Richard M
