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Old 03-08-2003, 09:31 PM
Registered User
Join Date: Jan 2003
Posts: 140
Turbo Engagement???

I am confused as to how a Turbo operates in my 1985 300TD. The car is quite slow up to about 2500 RPM when it suddenly starts pulling. Is this turbo unit pedal actuated or does it engage only after certain RPM? I drove an Audi recently where turbo engaged as you pressed the gas pedal but my car does that only after it passes 2500 RPM.

Igor Zeljic, Minneapolis.
1984 MB 300D Turbo Diesel W123 Sedan 132K
1987 MB 300D Turbo W124 Sedan 295K
1983 Porsche 944 104K
1989 Audi 80 Quattro 180K
2002 Audi A6 3.0 Quattro Avant 49K (Wife's car with warranty )

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Old 03-09-2003, 01:01 AM
Larry Delor's Avatar
What, Me Worry?
Join Date: Jun 1999
Location: Sarasota, Fl.
Posts: 3,026
That sounds about right to me.... It maybe should be a little closer to 2000 rpm instead of 2500 rpm.
"...rationality of thought imposes a limit on a person's concept of his relation to the cosmos." -Nash

04/71 280SE 195k
09 Jetta TDI
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Old 03-09-2003, 06:32 AM
Senior Member
Join Date: Sep 2002
Location: New Jersey, U.S.A.
Posts: 2,398
Hi Igor, good question. The turbo-charger is always "engaged". It's driven by exhaust gas and it's turning whenever the engine is running. It does however need to be spinning fairly fast to provide an increase in manifold pressure(boost). The boost pressure increases with RPM. When you feel it start to "pull" it means the injection pump is providing fuel enrichment. Without fuel, all the boost in the world won't procuce any power. On your car, at the rear of the intake manifold, you'll find a clear plastic tube attached with a banjo fitting. The other end of this tube goes to the aneroid compensator on the injection pump. This lets the pump know the boost pressure so it can provide fuel enrichment. This tube is prone to clogging over time, and when it does there's a serious lack of power. The thing to remember about diesel engines is that the more fuel you add, the more power you make. The turbo-charger force feeds the air charge to the engine. The injection pump adds the fuel. By the way, the 617 turbodiesel may be the best powerplant MB has ever produced, in my opinion.
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Old 03-09-2003, 09:06 AM
Registered User
Join Date: Jan 2003
Location: Okanogan County, WA
Posts: 108
Turbo engagement


I have an '82 and '84 123, both of which pull strongly from low rpm, except when very cold. In 1985, MB made some changes to the transmission and rear end which resulted in lower engine speed at highway speeds. My 1985 126 (all '85 diesel cars had the same revisions) drives exactly as yours, gutless to 2500 rpm and then pulls like a freight train from there on out.

On the plus side, 3000 rpm translates to about 67 mph, versus 58-60 on the earlier 123's. Quieter at speed and better mileage are also good.

On the other hand, maybe it's not normal and we both need adjustments!

I have bypassed the boost cut-out aneroid with no change to the performance, and disconnected the boost feed to the ALDA with major change in a downward direction!! Which leads me again to the conclusion that poor low speed performance is a trade off for the other items I mentioned.

Happy Motoring

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