High RPM at cruising speed - PeachParts Mercedes-Benz Forum

fredyarvin · #1 05-08-2006, 01:05 PM

Hi,

I have an 84 300D Turbo Diesel. On other posts, I see that I should be about 2600 RPM at 60 MPH. I'm at or a little over 3000 at 60. At 70, it's about 3600. Is that too high? The car sounds like it should shift into a higher gear. The final gear shift is a little before 40, but I can't say for sure it that is the highest gear.

My shifting is pretty good, a little hard going into second, smooth for the next shift, often an immediate shift into the next gear. I've adjusted the Bowden cable to remove slack, verified that I have a steady 12" to the Vacuum Modulator at idle, down to 1-2" at full throttle. I also turned the T in the modulator CW two turns after I bought it to reduce flaring. A couple weeks ago I disconnected the switch beneath the gas pedal (it shifted the same), but I haven't done this recently.

Possibly unrelated issues are that the fluid level is higher than Max when checking a warm engine at idle, and that I have an intermittant fluid leak that I'm currently diagnosing.

Thanks for your help, Fred

Brian Carlton · #2 05-08-2006, 01:20 PM

Quote:

Originally Posted by fredyarvin

Hi,

I have an 84 300D Turbo Diesel. On other posts, I see that I should be about 2600 RPM at 60 MPH. I'm at or a little over 3000 at 60. At 70, it's about 3600. Is that too high? The car sounds like it should shift into a higher gear. The final gear shift is a little before 40, but I can't say for sure it that is the highest gear.

The speeds are a bit off for the stock tires (195-70-14) but, I'd bet that you have a 3.69 rear axle in that vehicle. It was built with a 3.07 and someone must have changed it to the shorter axle for more power around town.

Naturally, you suffer with the noise and reduced fuel economy on the highway.

3600 is definitely faster than you need. I've put a 2.88 in the SD and it runs about 3000 rpm at 70.

If you are ambitious, and don't mind the loss of torque around town, you can consider a 2.88 or a 2.47 differential for that vehicle. Personally, after driving the 2.88 for six months, I wouldn't suggest the 2.47 unless you have no hills to climb on the highway.

fredyarvin · #3 05-08-2006, 05:29 PM

Hi Brian, thanks for the response.

I searched around a little and found I could measure the ratio by testing the number of axle rotations per wheel rotation. With one wheel jacked up, I got approx. 1.52. I assume that I double this as the other wheel was stationary. That would give me 3.04, so it may be possible that I still have the 3.07 axle. Assuming that I've measure correctly, any other ideas? Is there a way for me to verify that it is in 4th gear?

Thanks, Fred

Brian Carlton · #4 05-08-2006, 05:41 PM

Yep, that's a 3.07, for sure with those numbers.

But, the tach speed makes no sense. Additionally, it can't be still in third gear, because the speed increase would be nearly 50%, so that's not possible.

Have you verified the speedometer with the roadside markers? The only option, other than the tach is inaccurate, is that you're travelling about 10 mph faster than the speedometer indicates.

#5 05-08-2006, 11:51 PM

Don't forget about the torque converter. It's not a lock-up type so there is always ~10% slip.

3000rpm should be between 60-65mph in an auto.

#6 05-09-2006, 12:02 AM

Quote:

Originally Posted by ForcedInduction

Don't forget about the torque converter. It's not a lock-up type so there is always ~10% slip.

3000rpm should be between 60-65mph in an auto.

10% slip seems a little high to me. I get about 67-68 mph at 3000 rpm with 3.07 and 205/70 tires. Assuming 185/70 tires, I still would expect about 65 mph at 3000.