Odd situation to explain........
 

As you all know I have two Benzes a 1979 300SD.....and a 1983 300D.

I have almost a 1,000 RPM difference between the two cars on the road at 70 mph.

Both are the 4-speed automatics....both have differentials stamped with 3.07. Both run tires nearly the same diameter.......This baffles me. The 300SD cranks way more rpms on the road and it IS shifting through all 4 gears. with minimal slippage (as all automatics have some).

You can audibly hear the differences, its very noticible...so its not a tach inaccuracy thing.

Maybe the ratios comming out of the trans are different? Its a bigger heavier car with a towing capacity, that would be my GUESS.
Wodnek

odd
 

That is very odd indeed. My only guess would be the speedo is off, but by that much though???? I'd like to see what other people have to say about it.

No, high gear should have a 1:1 ratio, aka "direct drive" on both trany's well, as direct as you're gonna get with an auto-trans

Well as far as speedo accuracy.....morning traffic averages about the same...70 mph is normal in light traffic areas.....plus I have driven by stationary radar at limit and thats what others were doing...( and we are talking 30% difference here) ....and since these things are nighmares to rebuild I am assuming they have the original gearsets in the housings.

No I havent jacked the car up to turn the driveshaft and count the turns on both....which is hard since the availible space in my driveway is not flat and car would roll on the jack.

Bonehead, Cannot really see wrong rear end gears but anything is posssible. Suspect one transmission by malfunction in passing gear mode perhaps rather than final drive. Do not know much about automatics unfortunatley. Just trying to generate ideals. Could be something as simple as a linkage problem or kickdown switch if lucky.:) At least I am relatively certain it's not your glow plugs. :D just re read your description. Possibly torque converter is slipping far more than normal? They can go bad of course. Have a look at that transmission fluid as well. Hope someone with more torque converter experience chimes in.

I think Barry has a good idea there, the trans on the SD might not be going into high gear. What RPM's do you see in each at 70? 3000 and 4000? Also, it's possible the torque converter on the SD is on the way out. Hope not.

The tach speed should be about 3125 rpm at 70 mph with 195/70/14's.

It would be almost impossible to remain in third at this speed. You certainly would notice rpm's approaching 4500.

So, which one is not near this rpm?

It might be time to do a differential check.

with a 3.07 diff you should see approx:
SPEED RPM
60 2750
65 3000
75 3400
80 3650
85 3850
90 4100

I read this with my 300D and 3.07 gears before I installed a 2.88 diff. Rounded as close as I can read.
If your tach is correct (and I would think it hard to be off by 1000 RPM!) then you just may either be stuck in 3rd gear or the diff gears are not what is stamped on the case. Like Brian says, its time to manually count the revs (up on a hoist I presume)

IIRC:

Jack up one wheel.

Mark the tire and the driveshaft.

Turn the tire 10 rotations and watch the drive shaft. Count the rotations.

Divide the drive shaft rotations by 20.

A 3.07 should give you 61.4 rotations of the driveshaft for 10 turns of the tire.

..............but, this is from memory.............and.............it's been a long time. :D

I am going to try that on both cars..(manually confirm gear ratio)..torque converter does not appear to be slipping as that should be most noticible at low speeds and the car trys to drive away in gear as it is which is normal with these...

I know the SD isn't hanging in passing gear...it is going into fourth...at 60 if I drop gear shift down one notch the RPMS do pick up markedly.....and thats all the kickdown switch does. SO it is going into hig gear.....

I does act like one of these does not have the gears that are stampped on the housing. Both cars should operate at the same rpms....and the difference......is pretty large. More than can be attributed to slight tire size differences between them. And both are really very clearly stamped 3.07 on the bottom corner of the differential housing......I love the off the line pickup the 300SD has....but I love the high end power the 300D has....above 60mph the W123 will eat my W116 alive.....but below 60 the W116 will eat the W123 alive. Well...no rain is expected till tomorrow so might get to do that before dark tonight. Swinging by the stealership to see if they have injector line return hose handy.....mine is seeping pretty heavy now and I think it is breaking down rapidly the the return line hose under the car.....my 2 week old filter has a LOT of flecks in it.......better take car of this before it starts pumping fuel directly onto the ground......

I would not trust 25+ year old guages to be anything but close. On our boat the engines will be in synch but the tachs will read 1k rpm difference. Never trust old electronic guages.

"A 3.07 should give you 61.4 rotations of the driveshaft for 10 turns of the tire."

I can't figure this out...

If you turn the wheel one turn the driveshaft should turn 3.07 times....

So if you turn the wheel 10 turns the driveshaft should turn about 30 times....

Or have the POR15 fumes gotten to me today ?

With one wheel on the ground and not turning (assuming it is not limited slip :D ) the carrier will turn at wheel speed, and the drive shaft will turn at at twice the carrier speed.

I don't think POR15 was involved...

Best Regards,
Jim

If both wheels were off the ground its 3.07 times with one wheel on the ground you double that due to the differential action. THats why it can be dangerous to be spinning wheels on a open differential in the snow...becasue that spinning wheel is agtually spinning at twice the speed indicated on the speedometer.

Don't even need to rely on the tach.......1,000 rpm difference is very noticible....

Its not just a little different...its a lot different at the same road speed. And its noticible without even looking at the speedo either if you drove both.

Wonder where this is going to go if the rear is a 3.07? Only parellel that I can think of is my two 240ds both identical. One at 70 sounds like a threshing machine out of control while the other at that speed sounds normal. Or normal for a 240 as comparison with other cars is hard if not impossible. :D This should get interesting. Now not so sure about your glo plugs. :D

If you try to make a test with both wheels off the ground, the drive shaft has a tendency to remain stationary and the wheel that you are not turning will attempt to spin in the opposite direction, so, it's not usually a valid test.

I would expect the 300SD to have lower gears because it weighs more then the W123. That might make a difference, I bet the trans are geared different.

How???
 

How would high gear on both trans be different? They are both 1:1 ratio

For example I know a 420SEL has lower gear ratios then my 300SDL. For example the 420 can carry third to 120mph and mine can only carry it to 83. The V8 has a lot more power so it doesn't need the rpms to pull a heavy car.

I am thinking the same thing, the W116 must have a solid 500lbs on a W123 so MB wants it to spin higher so the 617 can pull it along.

Point is the differentials on BOTH are stamped 3.07 and the RPMS are dramaticly different on both..has nothing to do with smoothness as both are fairly smooth..didn't get a chance to check that tonight (check revolutions of wheel VS driveshaft...had lot of yardwork to do as we are getting rain tomorrow.

Problem with both wheels of the ground is the one with less drag is going to turn more.....works better with a limited slip differential............well it the rain holds off tomorrow I will check it then or if it stops for a while.

The difference you mention is in the spread of the gear ratios from 1st, 2nd, 3rd etc to Top gear (4th, 5th, 6th, whatever) which is a 1:1 ratio UNLESS the transmission is equipped with an OVERDRIVE.

To my knowledge, manual transmissions have not had an OVERdrive for a lot of years, and automatics never had them. 'Overdrive' in an automatic is usually another word for locking torque converter, for a 1:1 ratio without slip.

Best Regards,
Jim