[W126 Coupe] Replacement diffs -- SECs

[Gerry Van Zandt]

Everyone,

It's coming up on a year since I completed the replacement of my  
560SEC 2.47 rear end with a 3.07 unit.  Here's a report on the  
replacement and my experience.

With the help of list member Gary Dempsey last year, I was able to  
obtain a 3.07 unit, along with a spare stock 560 2.47 unit.  The  
spare 2.47 unit would be used for its exterior housing, into which  
the 3.07 gearset would be placed.  The stock 2.47 on the car would be  
retained in case the desire was to go back to a stock configuration  
in the future.

Gary had lined up replacement swaps for his and my cars with a local  
rear-end / transmission guy here in Portland, and Gary did all of the  
logistics as far as sourcing the replacement and new rear ends from a  
junk yard.  He also obtained same units for his own 560SEC.

As Barry Corno mentioned, for the early-series 500 and 380SEC cars,  
the 450SL/SEL rear end is a direct bolt-in, and he was able to do  
this.  However for the post 1985 cars, the rear-end carrier is  
different and thus requires the retrofitment of the 450 gears INTO a  
late-style housing.  This is technically not too complicated of a  
swap, but one has to know what they are doing.

Gary can talk about prices, but I will say that I was able to obtain  
the two rear ends, including shipping, and get the labor for the swap  
done for SIGNIFICANTLY (thousands of $$$) LESS than the prices that  
I've seen quoted here for said work.

After the work was done, I had a dilemma of the speedometer being  
"off" by about 10-12 MPH at freeway speeds.  There are a couple of  
solutions to this:  you can either modify the transmission to send  
the proper signal to the speedo, or you can alter the electric signal  
coming to the speedo so that it reflects the real speed.

I chose the latter.  Steve Lemberg of Bergwerks helped me out with a  
radio adapter box, which is also commonly available on line from a  
number of vendors for around $100-200.  The unit I got has four wires  
and is about the size of a cigarette pack.  First I did a detailed  
check of the difference in indicated vs. real-life MPH, by comparing  
the odo reading to a measured 10-mile distance on the freeway.  Next  
you turn this into a fraction and do the math, and come up with a  
ratio number.  Once you have this ratio number, you look in the  
extensive table of numbers supplied with the ratio adapter box and  
set the appropriate DIP switches inside the ratio adapter box.

The four wires are connected to switched +12V power, ground, signal  
in from transmission and signal out to speedo.  The most difficult  
thing is determining which wire coming out of the back of the speedo  
is the correct wire to cut to splice into the box.  The power and  
ground wires are simple -- there is a large chassis ground point  
right behind the cluster, and (with Talbir's help...thanks) I found a  
rather unadulterated switched power lead in the power to the digital  
temperature gauge LCD readout.

After connecting things, and some troubleshooting (a problem was  
caused by the fact that I made a three-wire setting of a DIP switch  
on the ratio adapter rather than a two-wire setup) I tucked the ratio  
adapter well up and behind the dash cluster (sort of at the top  
behind the center vents) and buttoned everything up.

I've been driving the car approx 400 miles per week round trip up to  
Seattle from Portland since April, so I've gotten some good mileage  
under the wheels.  Everything is working very well with no problems.   
The car is getting approx 1-2 MPG less than with the stock rear end  
(I would say it's getting 14-15 highway MPG though I've seen as low  
as 12-13 on some tankfuls -- normal variance).  Acceleration is  
noticeably quicker -- I haven't had it to the track yet this season  
for a quarter-mile run, but I'd estimate at least 0.5-0.75 second  
faster in the 1/4 mile.  The car tested out stock at 15.1 seconds  
quarter mile with the 2.47 rear end, a couple of seasons back.

On driving freeway speeds, I find that at 60 MPH the car is running  
2,500 RPM.  At 80 MPH the car is running just over 3,500 RPM.  Above  
80 things are really spun up and it gets pretty noisy if you are on  
the freeway for a length of time at that speed.  For some people it  
might get tiring.  But I don't spend too much time at 90 MPH+ in the  
car, so it doesn't bother me much.  Also above 80 and you are REALLY  
sucking down the gas, but we all know that even with the 2.47s.

All in all it's a replacement that I would heartily recommend if you  
have thought about it. You need to find someone who is competent and  
careful and who can do the gear swap if you have a later car -- if  
you have an early car you are home free.  Other than that, you need  
to get a ratio adapter to change your speedo signal appropriately,  
and then you're good to go.  The SECs were among the first-generation  
of MB cars to move to an electrical speedo signal -- no speedo cable  
is used.

Cheers,
Gerry