Author Topic: 250SE (W108) Tachometer Install  (Read 526 times)


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250SE (W108) Tachometer Install
« on: March 27, 2019, 05:47:08 PM »
Hello Fintail Land,

Does anyone have any experience with tachometer install in a W108?  I want to put one where the clock is (as in a 6.3), but not sure if it's a standard size or what else may be involved.  Anyone done it?
-1959 190 Ponton
-1961 190SL
-1967 250SE Sedan, 4 Speed


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Re: 250SE (W108) Tachometer Install
« Reply #1 on: June 06, 2019, 08:32:32 PM »
The cluster removes fairly easy since it only stays in place with a rubber band around it. The tricky part is disconnecting the  coolant line I would connect the coolant line on the block.

Best way use some hooks on the left/right side and wickle it out. The job is much easier to do if you remove the steering wheel than just turn the cluster over and remove the screws to get the clock out.

See the ebay link it got good pics of the cluster.

Aaron H

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Re: 250SE (W108) Tachometer Install
« Reply #2 on: February 20, 2020, 10:37:36 AM »
Ok, so installation is time consuming, but straight forward.  However, you must first know that there were tachometers made for 4, 6, and 8 cylinder engines.  While all versions will fit in the clock opening, they are not interchangeable.  For example, if you install a 4 cylinder tach in a 3.5, 4.5, or 6.3 car the reading on the tach will be off by half.  Since the 4 and 6 cylinder version tachs are either hard to come by or stupid expensive most people are ok with using a V-8 tach in their 6 cylinder.  Also, bear in mind that the v-8 tachs will have a different red line for the different v-8 engines.  The same for 4 and  cylinder engines. 

Installation involves the use of a bipolar plug to connect the tach to wires coming from the coil to the tach.  Wires are to be run through a rubber grommet in located in the firewall.  See picture below for wiring explanation. 

EVkarl, the W108/W109 instrument clusters are not held in by the rubber pad around the outside of it.  The rubber band is only there for insulating and anit vibration purposes.  They're held in with a plastic knurled nut on the back side of the cluster.  In your picture do you see the metal rod that runs across the back of the cluster?  See the threaded rod protruding out away from the cluster?  This is what the plastic knurled nut screws on to. The plastic knurled nut looks much like the plastic nuts that hold the tail light bulb holders on to the tail light housing.  Access to the rear of the instrument cluster to remove the plastic knurled nut is difficult, and requires small hands and arms. 


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Re: 250SE (W108) Tachometer Install
« Reply #3 on: February 21, 2020, 05:44:40 PM »
If you're lucky, the last guy took off that nut and tossed it. Given the size of my frame, I used to lay on my back in the driver's seat, half hanging off and reach up feeling for knob with my finger tips. The first few times that it wasn't there, I was surprised, but had to confirm it by standing on my head and looking up with a flashlight.

There was a straight 6 tach in the 250ce. I almost had one once when I found a 250ce engine, but the guy who pulled it tossed the instrument cluster thinking it just another clock like all the rest.

There are circuits that can adapt between the 6 and 8 cylinder wave forms. It should be something fun to build with a modern raspberry pi 0.  Not that finding the v8 version of the tach is that simple.