Tach hunting at highway speed...no surging at idle - Mercedes-Benz Forum
 
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#1 (permalink) Old 12-01-2011, 05:58 AM
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Tach hunting at highway speed...no surging at idle

Guys, I have been surfing the forum trying to find someone who has had a similar problem and I just don't see it. So...here goes. A '97 E300D with 208k miles. Sometimes when I am cruising down the highway around 55 60 mph the tach start to fluctuate back and forth a very slight amount and I can feel the engine surge slightly. I have never observed it doing this at idle. Is this a symptom of the throttle position sensor or what? I am stumped!

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#2 (permalink) Old 12-01-2011, 08:54 AM
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I have no knowledge about actual wiring on those cars, but driving diesels for long time I learned that in older cars tachometer pickup is taken from alternator. On this car it could come from CPS.
Start with the basics.
How old are your alternator brushes?
Display voltage on your clima LCD and observe it during driving. Than you can display rpm sensor to check if the fluctuation is in sensor, or the gauge.

Last edited by Kajtek1; 12-01-2011 at 09:18 AM.
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#3 (permalink) Old 12-01-2011, 10:17 AM
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Since the engine is fluctuating, I suspect the tach is working as you would expect. The question is why is the engine fluctuating? Turn the ASR (or do you have ESP) off, and see if this changes things.

For the avoidance of doubt, when answering posts I am referring to the W210 Mercedes...not the Renault12ts.
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#4 (permalink) Old 12-01-2011, 10:23 AM
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Thinking about it again, this is non-turbo diesel and engine surge is pretty common on it. We do have couple of topics covering it, but per my memory no solid fix.
This engine has no throttle BTW.
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#5 (permalink) Old 12-01-2011, 11:08 AM
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The lockup torque converter on W210's is different than most non Mercedes vehicles. It is not "locked" or "unlocked" it is a progressive lockup, Don't know the diesel spec's but on the 420 it isn't fully locked until 3000rpm in 5th gear, which is about 90MPH IIRC. If the diesel is similar in operation, what you could be seeing is the amount of "lockup" varying slightly depending on engine load, and that also giving the "surging" as well.


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#6 (permalink) Old 12-02-2011, 04:37 AM
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I have a '99 E430 with that like surge problem--Mercedes dealer says that it is the torque converter just like the previous post. Anyone know if the converter is electronically actuated or by fluid preasure?? Wonder if it is the Trans ECM or the torque converter?
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#7 (permalink) Old 12-02-2011, 10:19 AM
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Quote:
Originally Posted by johneb48 View Post
I have a '99 E430 with that like surge problem--Mercedes dealer says that it is the torque converter just like the previous post. Anyone know if the converter is electronically actuated or by fluid preasure?? Wonder if it is the Trans ECM or the torque converter?
not sure how it is controlled. What I do know from looking at a graph is that on my 420 the lockup starts at about 1600rpm in high (5th) gear, but it is not fully locked until 3000 rpm in 5th gear. What that means is that in the U.S. your probably never really locked up. Cruising down the road with CC on, you can see the tach needle moving around slightly even on flat ground.


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#8 (permalink) Old 12-02-2011, 10:27 AM
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Per my impression, the converter lock is not working on my E300DT.
Even at 80 mph/3000 rpm when I take the foot off gas pedal, the rpm drops about 200 rpm.
On W211 the converter lock is so firm, that for some time I was thinking I have 6 speed transmission.
Than again, the OP engine is known for surging.
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