300 SDL Compatible Differentials
 

The differential on my 87 300 SDL has been on its way out for a long time now. I was looking for a suitable replacement in the local junkyard. To my dismay, I found no 300SDL's, but I did find quite a few second-gen w126 chassis long wheelbase cars. Specifically, I found a few 420 SEL's, a 500 SEL and a 560 SEL. I understand some, if not all of them have different gear ratios than my car's. My question is would a differential from one of these cars will be a drop in replacement for my car and which one would have the most beneficial gear ratio?

Hf:

Two points to consider:

1) As you noted, ratio. Yours is 2.88:1; the W126 gassers that you found have 2.47:1.

2) Physical size. Yours has a 185mm ring gear, and is often referred to as a "small case" differential. The 2.47 has a 210mm ring gear, and is referred to as a "large case" diff. Although they share common mounting points, the flex disc diameter is different, and the axle lengths are different.

There would also be speedometer error, and the ABS will object to a change in driveshaft speed.

Are there any gassers with small case differentials? Also, where is the speedometer pickup and can it be changed to correctly report speed with a different diff?

The 300SE/SEL 126s of 1988-91 used a small case diff. There are two versions of the small case however, one has bolt-on side covers which carry the side bearings; the other is without side covers, and the bearings are retained by snap rings. The ratios for the six-cylinder gassers are 3.07 and shorter.

The speedo pickup is in the tail housing of the transmission. The trigger wheel (of varying tooth count) can be changed by removing the tail housing.

I put a complete rear subframe from a 1989 560SEL in my 1987 SDL. Used the SDL drive shaft. Everything bolted up. Still have to change the gear in
the speedometer to correct reading.

Thank you for the information! I'll probably keep hunting for a 300sdl diff for the ease of replacement.