RPM holding at 2k+ 80kph

1979-1991 280SE, 300SE, 300SEL, 380SE, 380SEL, 380SEC, 420SE, 420SEL, 420SEC, 500SE, 500SEL, 500SEC, 560SEL, 560SEC
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420.SELect
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RPM holding at 2k+ 80kph

Post by 420.SELect » Sat 23 Dec, 2017 7:08 pm

Hi guys,

I have a few questions,

I was driving down the freeway today (first time since I purchased the car) and above 80kph the RPM seems to stay at or above 2k, even once at speed and with no acceleration. It sort of feels like it isn’t shifting up to the next gear and is stuck.
When driving 70 and below nothing seems out of the ordinary; the revs usually Are below 1500, cruising on a flat road.
I’m not really familiar with with the normal levels so Just wondering what the normal RPM would be cruising at 100.

The ABS light seems to be coming on and off randomly. A lot of the posts I have read about the ABS warning light talk about a few main things; the OVP, fault fuses or the tone rings in the seals (if they had been damage during removal)
Just seems weird that it sporadically illuminates and then clears
I am going to look into this but I need to drive it prior; Is it dangerous/detrimental to the vehicle if I drive with this occurring??

Last thing, the oil reading on the instrument panel stays at 3 (top reading) when driving; usually sits around 2 when idling. Is this an indication the oil pressure is high or is this normal?


Chris
1988 420SEL - Agnes

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T-Modell
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Re: RPM holding at 2k+ 80kph

Post by T-Modell » Sat 23 Dec, 2017 7:35 pm

Hi Chris,

I don't know exactly the ratio of the 420, but imho for the Oz market it's shorter than here, so 2,000 should be alright.
420.SELect wrote:
Sat 23 Dec, 2017 7:08 pm
....Last thing, the oil reading on the instrument panel stays at 3 (top reading) when driving; usually sits around 2 when idling. Is this an indication the oil pressure is high or is this normal?
That's how it's supposed to be; if falls towards 1 or below when idling, you should worry.

Regards
Thomas
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Re: RPM holding at 2k+ 80kph

Post by Ivanerrol » Sat 23 Dec, 2017 7:46 pm

420's should rev around the 3K mark at 100KPH.

We need bit more in your description.
Will the car go faster than 80kph?
Any misfiring above 80kph?

Could be a Fuel pump relay problem associated with a crook OVP
Could be a faulty fuel pump.

ABS light coming up is certainly indicative of a failing OVP
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Re: RPM holding at 2k+ 80kph

Post by hents99 » Sat 23 Dec, 2017 7:56 pm

Diff Ratios:
2.24 - 500
2.42 - 420
2.65 - 560

ABS is very sensitive to voltage drop also. Measure alternator charging voltage.
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Re: RPM holding at 2k+ 80kph

Post by CraigB » Sat 23 Dec, 2017 8:14 pm

I'm not sure 420 ratio either, but i know the 380 is a lot lower in ratio than my 500 and it does feel a bit like it should change up by comparison. Mind you once i got used to it i kinda liked it giving it a more responsive feel, ready to rocket past passing cars etc. Its similar with earlier W108 and w113 cars and some people fit V8 diffs trying to have it cruise at lower revs, but in the process cutting the performance that Mercedes designed it as. I love using the revs in my W113 and for power in the right places i think the engineers got it spot on and it is just a matter of mind adjustment that this is normal. All these comments assuming a 420 has a shorter diff. Just seen Hents post - 380 is a lot shorter - 3.something, 2.42 is a better ratio but sounds like 2000 at 80 and 3000 at 100 it might not be far out. Count your gears - hold it in low gear and it will start in 1 and then count through 4 gears. Otherwise will start in second.

Do you feel anything through your pedal before the light comes on? I had that and was a sensor but once light came on it went, but ABS not working then although brakes otherwise worked fine - so not dangerous to drive. But agree with other comments.

And i agree your oil pressure is normal.
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420.SELect
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Re: RPM holding at 2k+ 80kph

Post by 420.SELect » Sat 23 Dec, 2017 9:58 pm

Thomas,
that’s reassuring.
I’m no Benz expert and havent spent time driving one so i thought I would just get advice on the matter.

Ivan,
Okay I’m glad Itz not out of the ordinary for it to be in that range. I was a bit worried.
Yeah it gets up to 100 without any issues; I don’t hear or feel any misfiring.
I’m going to replace the OVP anyway so hopefully that helps but I am looking to replace a lot of the parts that commonly fail so hopefully that should mitigate the amount of issues in the future

Chris
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Re: RPM holding at 2k+ 80kph

Post by Ivanerrol » Sat 23 Dec, 2017 10:21 pm

Benz's of the era were built for highway cruising. not dragoffs from the traffic lights.
You will find that the torque curves for pre modern Benz's get going at 2.8 - 3.0K revs and above.
Cars were made to drive at 160Kph all day long.
Having said that the W126 560's were king of the dragoffs. Putting your foot down in a 560 is guaranteed to make you smile.

You do realise that the famed Falcon XY GTHO 0-100 time was 8.9 seconds. A modern commuter car does better than that.

Because of modern fuel economy standards and emission regulations cars nowadays get to cruising speeds at relatively low revs.
The engines have their torque curves tuned for 0-100kph times of less than 8 seconds.

It's a remote possibility that a faulty OVP may have an effect on your rev counter
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420.SELect
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Re: RPM holding at 2k+ 80kph

Post by 420.SELect » Sun 24 Dec, 2017 12:00 am

Thanks for the information,
While I love cars - pre-modern in particular - I have never really been too savvy with the mechanical or technical side of things; something I am slowly working on.
Even before I bought Agnus I knew that this style of Benz - with exception to the 560 as you have just informed me - was intended to be a cruiser; not a rocket off the lights.

Reading though the posts I think that it’s just my mind set of what is ‘normal’. As I have only really had relatively modern cars I’m not overly familiar with the how the older models were tuned.

Thanks for your time and knowledge. I have found this very informative and can put my mind at ease knowing that is was just my inexperience rather than any cause for concern.

As for the OVP, I will replace it and update on the result.

Chris
1988 420SEL - Agnes

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Re: RPM holding at 2k+ 80kph

Post by ngruzevs » Sun 24 Dec, 2017 1:17 pm

Revs at 100 sound normal, my 420 ticks over at ~2100 @100KPH.

In regards to the ABS, I recommend cleaning the three sensor (two front, one rear). When I pulled mine out, couldn't believe how grubby they were.
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Re: RPM holding at 2k+ 80kph

Post by hents99 » Sun 24 Dec, 2017 6:57 pm

380 diff ratio is 3.46 as I remember.
Recently made 380SEC engine overhauling and after when driving I thought that transmission holding 3-gear in.... but yes final ratio is quite slow.
People used to make diesel conversions for 380 because diff ratio is suitable for diesels.
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