tachometer?

[jjb-w116-hu]

hi again guys - is there any way to add a tachometer or rev counter ? i find the clock far less useful than i would a rev counter... i have seen combo rev/clocks on some models .... is it possible to alter?
appreciate any advice....

[WGB]

It is easy enough to buy a tach that will fit the housing but you must get the correct tacho for your motor both for the redline to be correct and also because 6 cylinder calibration is different to the V8.

The tacho also has to be wired up and I am not sure whether your harness will have the correct cables in it.

From memory you will have to run a signal cable from the ignition module to the back of the tacho.

This topic was covered quite extensively in this forum about four years ago so I would start doing some searches as I remember the wiring modifications being clearly illustrated in photos.

Bill

[oscar]

I fitted a tacho to my 280S but the problem was it doesn't have an ignition module and it only seems to read correct up to 4000 revs then drops off even though the engine revs higher.  I ran the pickup straight from the coil and I'm guessing either there's too much current or something running through it or there's some issue with the tacho.  The power for the tacho I took from the clock 12V connection as others have done too.  I read in an earlier post your 280S is later 80 model? If so it may have the ignition module and in that case you route a signal wire from the terminal block behind the ignition coil as per the pic below.

On a different note, I noticed in another thread your in NZ.   Don't know where you usually get your parts from but before Xmas we were travelling south of Hamilton and drove past a place called Mercworx.  Looked like a merc lovers mecca for used spares and would've loved to have stopped but we were in a hurry to get to Taupo.  The Mrs was driving and I kept saying "turn around", "nah, keep going", "turn around" etc for about 10km.  I've regretted not stopping ever since.  Anyhow, not sure if you're familiar with the business but I reckon they'd be worth a call to source a working tacho.  
http://www.mercworxs.co.nz/index.html

[jjb-w116-hu]

hi guys thanks for the feedback and good to know it can be done : oscar yes I am well known to Steve at Mercworx and it is indeed a great place to get a tan and get dirty forraging for parts like a bear in the woods!

[jjb-w116-hu]

many of the other pics in the installing a tacho thread from 2006 are vanished - does anyone have the originals? i am pretty sure i have an ignition module but i'll check with my mechanic chap.
or is there somewhere one can look under the hood? yes Oscar, mine is a 1980 280S 

[koan]

many of the other pics in the installing a tacho thread from 2006 are vanished -

That's why encourage people to put their pics in the galley here rather than free hosting sites that move pictures around or even lose/delete them.

koan

[oscar]

Here's the post by 116.025 with his pics inserted.
When you pull the cluster out and see the clock connection the second pic will make more sense.  But from the first pic, signal wire attached to right hand side of terminal block behind the coil when viewed standing in front of the radiator. Then the power is piggybacked from the 12V terminal to the clock.  

But below this quote is a pic of the back of my cluster.  One thing I forgot to mention was the need for a VDO two pronged female socket.  These sockets are found in a few places on our cars so grab one including the wires that go to it.  The plugs only fit one way and ensures you wont short the tacho by putting the plug on the wrong way.  Just remember, regardless of colour, top wire from the plug is the signal wire, bottom wire is the 12V supply.

Ok, so first is the under the hood shot...



The signal that the tachometer uses comes from the terminal on the right, as noted in the picture.  Since yours is a non-tach car, there will currently be 2 wires attached under the screw on that terminal, which are green and yellow striped.  All you need to do here is attach a third wire(with one of those ends with a hole in it), and route it appropriately through the fire wall.  When I did the 280SE (the one I did second) I routed it through the same grommet as a heater hose, which was a bit more accesible from inside the car than the way I routed it when I did the 280SEL.  Also, a bit of experience on the 280SEL quickly taught me it is far better to route the tach signal wire underneath all of the other stuff than over, so you don't bump it when you do other things.  



This is the shot of my rig inside the car.  To make the connections to the tach, I cut some ends off of a 116 I use for parts, and used a wire nut to fasten it to the dinky wire I used for the tach signal.  There can't be much current in the tach signal, since I used a single strand out of some phone wire lying around...For the 12V power, I borrowed another end and stripped some of the insulation back and after shoving the wire into the terminal that powers the clock, I pushed the whole thing back onto the clock.  There's enough room since the female end has a circular space.  

A fairly simple job all in all, but one tip from experience...either disconnect the battery or the fuse that has the clock on it...I popped a fuse when I put the tach in the 280SEL...the 12V wire for the clock flopped around, and since I had the key in the ignition(so I could listen to some tunes while I worked  ), I saw a little arc as the current jumped into the key, knocking the fuse out of the slot.  Being stubborn as I am sometimes, I popped the fuse back into place and kept working...a few minutes later, the clock wire hit a metal cross member that is normally behind the instrument cluster, this time actually blowing the fuse...so, having finally learned my lesson, I removed the negative terminal from the battery...