500 SEL 2.47 swap into 300CD

[lutzTD]

I worked on this all day yesterday. the diff is different than the one I pulled. Has anyone done this? It was all smooth until I tried to bolt up the 4 bolts in the back. they missed by an inch, luckily as was trying to figure out how to fabricate an extension I put the mount on backwards and it lined right up.

so I have it all together and I am getting a rubbing sound. I did many things at the same time so I cant account for it with any one change and it was too dark to finish last night. my list of changes are:

1. replaced differential with 500SEL donor
2. replaced axles with new Cardone rebuilds
3. replaced rotors, calipers and shoes
4. installed lowering springs

My question is: since the 126 housing seemed to be a little wider, does anyone know if maybe the combination of the 126 housing and the lowering springs would make the axles too long to fit?


The sound is coming from both sides the best I can tell, but it has a harmonic tone which sounds like the disk. I am hopeful it is just the disk rubbing the caliper. My old calipers and disk were junk so I can put them back on. I will raise it tonight and see if I can find the noise, but if anyone has put in a 126 diff and can give me some confort level that the axles fit ok it would make my day go easier until I have time to check it tonight


the disk on the driver side was cut in two, it seems like maybe it was not releasing completely. Is there something at the system level that would hold pressure on the rear brakes, like a bad proportion valave or something? I did not replace the lines but Im thinking the one in the back looked kind of bad, would that make any difference to the sticking issue?

Originally Posted by lutzTD

It was all smooth until I tried to bolt up the 4 bolts in the back. they missed by an inch, luckily as was trying to figure out how to fabricate an extension I put the mount on backwards and it lined right up.

You probably needed to use the cover plate from your original diff.

[lutzTD]

Originally Posted by ForcedInduction

You probably needed to use the cover plate from your original diff.

that was my first thought, unfortunately the bolt pattern is different. in the diff I got from the yard the cover was 1" wider and had fins

[lutzTD]

I checked the shafts when I arrived home, no problem there. so I removed a wheel and removed the bolt holding the axle stub and backed it away from the hub, OK there too. so I narrowed it down to the brakes. At first I thought it was the rattle spring hitting the rotor, I pushed the edges back with an extension, no difference. I looked for rub spot everywhere on the rotor, caliper and backplate and can see nothing. I put it together and still it is screetching. I took it all apart again and still cant see anything so I put it all back together and went for a drive. it gets worse and better as I turn. I pulled the e-brake to see if that made a difference, maybe it was dragging or something, no difference. I think now it is the Advance rotor rubbing on the backing plate. there is a lip on the backing plate and a groove on the rotor where it fits. I will drive it to work tomorrow, I know its not the axle or the calipers or rotors, its not the diff or drive shaft so I will let the backing plate rub and get shiny, then grind off the shiny spots.

as far as the 2.47, I love it so far. no issue taking off and though there is a drop in power at 60, the turbo boosts to 15psi and it pulls pretty good, best thing is its running around 2200 rpm as best I can tell since the speedo is way off. Its also nice on the surface roads, I used to have to jump from 3rd to fourth and back all the time, now third is perfect.

[rudolfgreen]

nice troubleshoot. the 126 axles are a little bit longer, and wore the cv joints prematurely.
since putting in proper 123 shafts, no problems (yet) gps works well as a corrected speedo.

[lutzTD]

Originally Posted by rudolfgreen

nice troubleshoot. the 126 axles are a little bit longer, and wore the cv joints prematurely.
since putting in proper 123 shafts, no problems (yet) gps works well as a corrected speedo.

thanx, it made a ton of noise coming in this morning, but I know its safe even if the looks I get say it sounds like it going to explode.

I am probably going to invest in a hot rod speedo adapter. they use a motor to turn the speedo and an electrical pickup on the driveshaft to get signal. It is tunable to whatever you need to be accurate. I think the smooth action will help my cruise control be more stable too. they are not cheap, but everything else for the swap was resonable so I will seriously consider it. I dont like carrying a GPS. I always forget things in the car and I doubt it would last long sitting in my unlocked car.

[macdoe]

Hello, I have a 1981 300TD wagon I use to commute on the highway every day.It has a 3:46 ratio diff in it. I was wondering if I could get rid of the 3:46 diff and swap in a diff from a 1988 420 sel. I believe the newer diff has the fins. What do I need to do? I have a new set of 123 axles.

Will this work?

[UriahT]

Originally Posted by macdoe

Hello, I have a 1981 300TD wagon I use to commute on the highway every day.It has a 3:46 ratio diff in it. I was wondering if I could get rid of the 3:46 diff and swap in a diff from a 1988 420 sel. I believe the newer diff has the fins. What do I need to do? I have a new set of 123 axles.

Will this work?

If you have an 81 wagon, it came with a 3.07, not 3.46.
A 2.88 from an '85 W123 is your best, and easiest, bet.

[Brian Carlton]

Originally Posted by macdoe

Hello, I have a 1981 300TD wagon I use to commute on the highway every day.It has a 3:46 ratio diff in it. I was wondering if I could get rid of the 3:46 diff and swap in a diff from a 1988 420 sel. I believe the newer diff has the fins. What do I need to do? I have a new set of 123 axles.

Will this work?

No, it will not work.

You need the diff from a 1985 W123 vehicle for a perfect fit.