1990 300sel rpm vs mph??? - PeachParts Mercedes-Benz Forum

robaincltnc · #1 05-13-2001, 08:21 PM

What is the expected RPM in a 90 300sel @ 65mph with overdrive engaged? I have just had the tranny rebuilt and the rpm's seem to be hi. The rpm at 65 is 3500. I don't believe that the overdrive is working.
Any advice?
Thanks from Charlotte

Mike Murrell · #2 05-14-2001, 12:00 AM

According to the tach, my '91 300-SEL turns 3400rpm at 70mph. These straight-six 103 motors do rev a bit higher than some.

sixto · #3 05-14-2001, 04:53 PM

Rob,

My 91 300SE revs something like 3300rpm at 65mph. If your overdrive wasn't working or if you were in third gear, you'd be closer to 4500rpm when doing 65mph. In fact, drop the shifter into third while doing 65mph and you'll feel the difference and see it on the tach.

I also notice a pronounced transition state. If I accelerate birskly, the tach will lead the speedo by something like 500rpm. I mean that during brisk acceleration, the tach will read 500rpm higher than it does when cruising at the same speed. I thought my torque converter or tranny was about to quit, but I was assured that the behavior was normal. That's what I get for growing up on 5 speeds and big V8s with no tachs.

I don't think that anything's wrong with your car, but you might see if fresh tranny fluid brings the revs down a couple hundred rpm at 65mph.

Good luck,
Sixto
91 300SE

robaincltnc · #4 05-16-2001, 07:48 AM

Sixto,
I see your point but what if the car is in 4th but the overdrive isn't kicking in what would the rpm's be?
Thanks,

stevebfl · #5 05-16-2001, 09:25 AM

There is no "overdrive" in that trans. All MB transmissions till the 722.5 were straight one to one out the back hi high gear. The 722.5 came first in the 129 body 300SL (I think). It then also appeared in the 140 chassis 6 cyl cars.

If you are in high gear then you are doing the right rpm. You might suspect either gauge if things aren't in order. The trans is going to have one turn on the drivesahft for every turn of the motor in high gear. Anything less would be slippage and that won't last long!

jcyuhn · #6 05-16-2001, 09:31 AM

These cars don't have any type of overdrive. The transmission
is a four speed, with fourth gear being direct drive, 1:1
ratio.

As well, they don't have a lockup torque converter. All
engine power is always transferred via fluid coupling,
never mechanical means. That's why engine RPM can rise
ahead of speed when you toe into the throttle.

This discussion comes around on a regular basis. Folks are
so accustomed to modern Amurican/Japanese iron with steep
overdrive transmissions which turn only 2000RPM on the
highway. Ya gotta remember, MB built the S class to deliver
the best possible performance out of a given engine, fuel
economy was not really a consideration when the 126 was
designed.

If it makes you feel better, 3500RPM is just loafing for
this engine. I've read the stress on an engine is roughly
proportional to the square of the RPM. Hence if redline -
which is what, 6500RPM - is the maximum stress level, think
how much less 3500RPM squared represents - it's only a
fraction.

robaincltnc · #7 05-16-2001, 09:37 AM

Thanks for all of your reply's..
I feel better.
Thanks,
rob

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