123 speedo into 126 cluster - PeachParts Mercedes-Benz Forum

t walgamuth · #1 09-12-2010, 03:50 PM

I am thinking of putting my five speed manual setup in my 85 sd. I seem to remember that the 85s have some sort of electronic speedo. I am wondering if I can put in a 123 speedo into the cluster and simply hook up my cable from the tranny to the speedo?

lutzTD · #2 09-12-2010, 04:19 PM

Quote:

Originally Posted by t walgamuth

I am thinking of putting my five speed manual setup in my 85 sd. I seem to remember that the 85s have some sort of electronic speedo. I am wondering if I can put in a 123 speedo into the cluster and simply hook up my cable from the tranny to the speedo?

I would check the sizes, I think the 126 speedo is flattened off at the bottom to fit in the cluster. Not sure if the 126 is larger diameter or the cluster itself is shorter. If you chage the face it appears the rest is the same, or maybe you can slice the bottom of the w123 speedo

t walgamuth · #3 09-12-2010, 04:26 PM

Thanks!

I looked at fastlane for parts and it seems that all 126 speedos are electronic. How do those work?

JamesDean · #4 09-12-2010, 04:30 PM

There is a hall effect sensor on the side of the transmission, this sends signal to the electronics at the speedo. There is a 6-pin interconnect in the drivers footwell on the right side near the gas pedal (on the center console).

You might want to look into what equipment the 124 manual cars had. I believe they had electronic speedos... 201s had mechanical/cable driven.

Alternatively, if you CAN get the 123 gauge into the 126 cluster, just run the a cable and call it a day. But as other have said not sure on fitment.

You probably wont have cruise control, unless you do some finagling. I know on my 201 there is a device that converts the mechanical signal into electrical impulses for the cruise control. I'd imagine the 126 doesnt have this contraption as its all electronic.

lutzTD · #5 09-13-2010, 08:40 PM

Quote:

Originally Posted by JamesDean

There is a hall effect sensor on the side of the transmission, this sends signal to the electronics at the speedo. There is a 6-pin interconnect in the drivers footwell on the right side near the gas pedal (on the center console).

You might want to look into what equipment the 124 manual cars had. I believe they had electronic speedos... 201s had mechanical/cable driven.

Alternatively, if you CAN get the 123 gauge into the 126 cluster, just run the a cable and call it a day. But as other have said not sure on fitment.

You probably wont have cruise control, unless you do some finagling. I know on my 201 there is a device that converts the mechanical signal into electrical impulses for the cruise control. I'd imagine the 126 doesnt have this contraption as its all electronic.

perfect opportunity to ditch the troublesome MB cruise control and replace it with an aftermarket one. Thats next for me after I fix the AC

compu_85 · #6 09-12-2010, 06:15 PM

IIRC W124s have mechanical speedos too.

Finding a place to route the cable might be troublesome.

-J

Ryan Z · #7 09-12-2010, 07:48 PM

At one time I had replaced the speedo with a unit from another year SD.
The speedo worked, but the gauges went nutty. I thought there was a short in the system. I bought another '84 SD speedo and everything works great now. Might be something to look out for.

SirNik84 · #8 09-12-2010, 07:50 PM

I have a 124 mechanical speedo and I'm going to make it fit into my 126 cluster. Its next up on my "to do" list for the SD. I'm going to Japan for 2 weeks, but if no one has a conclusive answer by the time I get back I'll post pics of my transplant.

t walgamuth · #9 09-12-2010, 08:13 PM

Thanks guys!

Is there a way to retrofit the sensor to my five speed rom my 84 280e euro?

I was hoping it screwed into the place a cable went previously.

compu_85 · #10 09-12-2010, 11:15 PM

As I understand it the whole tail section of the trans is different on the automatic on an electronic vs. mechanical speedo.

-J

t walgamuth · #11 09-13-2010, 02:30 AM

So I would have to find a stick tranny tail piece set up for an electronic speedo?

Does one exist?

Billybob · #12 09-13-2010, 10:09 PM

Quote:

Originally Posted by compu_85

As I understand it the whole tail section of the trans is different on the automatic on an electronic vs. mechanical speedo.

-J

I have owned both versions of GETRAG 5 speed manual transmissions 717.400 is the mechanical speedo version and 717.402 is the electronic version (1985 280SE M110 engine)

It looks as though the differences are slight and may allow modification of the .400 to the .402 configuration. The .402 uses the same sensor as the 723.2/3XX auto transmissions.

The electronic speedo sensor is attached to the tranny’s tail section on the passenger side with a single screw after sliding into a boss designed for it there. The furthest inside portion of that boss has a white plastic sleeve/cup into which the forward most end of the sensor fits. The sensor picks up the teeth on a gear spinning inside the tranny tail section.

On the mechanical speedo .400 tranny the speedo cable attaches on the driver’s side where the cable is inserted into a gear driven receptacle which is driven by a gear turning inside the tranny.

The .400 tranny also has as part of its tail piece casting the unfinished boss designed for the electronic sensor! The difference from the outside is that the boss has not been finish machined thru into the tail pieces interior and the cast area for the securing screw has not been drilled or tapped for it.

It may very well be possible to remove the tail piece and have the sensor boss machined out to allow the sensor to be installed along with the plastic sleeve cup that seals that opening. That way that tranny could be set up for either or both mechanical and electronic speedos.

The primary concern would be that the internal gear which is read by the electronic sensor exists within the .400 mechanical speedo configuration. That should be fairly easy to determine using the EPC to compare the respective .400 against the .402 configuration. Even if the gear is absent or different it may be possible to add it.

The tail piece on these 717 5 speeds can be removed without disturbing the entire transmission internals as it houses the 5th speed/Reverse function separate from the rest of the tranny. Removing it would only require a gasket and seals to get it back together.

This might not be the simplest route to adapting a manual speedo tranny to an electronic speedo chassis but if the tranny was going to have it’s rear seal replaced or the tail piece off for some other reason it may be worth checking out. Adapting speedos from other chassis vehicles might work but might create other problems also.

t walgamuth · #13 09-13-2010, 10:42 PM

Wow, that is an impressive write up.

Unfortunately my tranny is not a getrag, at least I don't think it it.

The 123 speedo appears to be the exact same diameter as the 126 but the 126 has the flat spot on the bottom. I wonder if the 123 can be put in the 126 cluster?

t walgamuth · #14 09-27-2010, 10:37 PM

My five speed is the later version with the bell housing cast into the tranny body, from my 84 280e euro. Do you think this version would be compatible to adding the electronic sensor for the speedo too?

compu_85 · #15 09-13-2010, 02:37 AM

I'd assume so, they sold W126s 280S/SE cars with a 5 speed.

-J