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#1
12-19-2009, 02:40 PM
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Mechanical to electrical speedometer conversion anyone?
I have done the research on this site and others, and have not found anyone that has accomplished this task, yet.....but maybe you have.
Replaced the automatic transmission in a 1985 300D (123 chassis) car (mechanical speedometer with cable), with a automatic transmission from a 1985 300D (126 chassis) electrical speedometer. There is no oiling capability for the lubrication of the speedometer gears and rear tailshaft support bearing if you were to change out the tailshaft. Maybe this is what I need to examine closer and get some oiling capability on those gears and bearing... I don't have a schematic (yet) of the electrical operations to place a operational electrical speedometer in the instrument panel. Maybe someone has a schematic that would help me out, I have all other parts. If anyone has new information not listed previously on this site, it would be appreciated. Thank you for looking, and Merry Christmas.
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'87 300E | 2.6 engine Artic White - Navy blue interior, chrome rims, very clean and sharp! 91 300E 89 300E The rest of my collection are just cars and trucks...no more "automobiles"...   
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#2
12-19-2009, 03:36 PM
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There is probably some sort of amp like the tach amp in the system. The twisted pair from the speed sensor turns into a single wire that plugs into the rear on the speedo on the 126 cars where the cable would connect on mechanical cars. I'll know more as I tear apart my donor car.
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#3
12-19-2009, 09:14 PM
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re:
Well thanks, at least someone read my post. I know I'm asking too much but thought there maybe someone that has been through this.
I am aware of two wires at the speed sensor at the trans and that a single wire plugs into the instrument cluster, but I don't know if the signal also runs through the wheel speed sensor. I still have the donor car available and in the process of tracing the wiring down (not an easy task). 
__________________
'87 300E | 2.6 engine Artic White - Navy blue interior, chrome rims, very clean and sharp! 91 300E 89 300E The rest of my collection are just cars and trucks...no more "automobiles"...
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#4
01-14-2010, 08:22 PM
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Since you already have the transmission with the electronic sensor, this swap should be relatively easy, I think. I'm not that familiar with the 123/126 instrument clusters but all you really need is to get a 126 electronic speedo installed in the 123 cluster. The wiring is simple. It's not one wire, it's three wires. The single green/yellow wire in the center (if present) is an output to the cruise control system. The three (or four? I forget) wires are the input to the speedo.
The schematics should be in the ETM (Electrical Troubleshooting Manual) for the W126. That will show you how to wire it up. You will almost certainly need a converter box though, like the one from Yellr.com (for 2-wire inductive sensor!), or from Dakota Digital. This will let you adjust the signal for your specific gear ratio and tire size. 
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#5
03-08-2011, 02:05 PM
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anyone know the P/N of the spedometer sensor on the 126? my spedomoeter randomly flies all over the speed range and im out of ideas as to what it is. i cant find the part listed in fastlane or Allpartsexpress, if i have a P/N perhaps google or a dealership can help me easier.
__________________
1987 300SDL "Rudolf" 239k 8/10 Gone but not forgotten -1980 F250 4X4 351C Snowplow "The Beast" -1976 Caddy Eldo Convertible, in triple red. "Red headed Slut" -2002 Honda Accord EX-V6 loaded 107k, My first brand new car! *IM A HORRIBLE SPELLER*
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