Do These Engines Have Rev Limiters?
 

I was checking my tach circuit the other night by injecting a signal into the amplifier and bypassing the tach pickup to get the tach to read somethingeek: . As I increased the signal (Hertz), of course the tach did not work:( , but I heard the shutdown solenoid kicking in and out. Do these engines have rev limiters? I also obtained a schematic of the circuit from M.B and it shows a wire going to the overload protection control unit. Is this a rev limiter? If it is my circuit appears to be working, but my tach is not. FYI If you want to check the tach pickup, I measured it with an ohmmeter and found the outsdie, or yellow and black wires should read about 70 ohms. The center pin appears to be a shield.


PeterG

79 300SD 75K miles

It seems to me there is a safety switch on one this old, it's a switch located on top of the air distribution housing, near where the air is sucked into the engine by the fuel distributor. I've never studied how it operates, I believe it cuts out the fuel pump relay or something if the air flow sensor plate deflects down too far, just a guess though. It seems to me it was phased out in favor of the fuel pump relay with RPM limiter in the very early 80's.
Gilly

Gillybenztech,

Sounds like you are describing a CIS injection system on a gasoline engine :)


PeterG,

Perhaps this is some sort of supplemental protection system. I'm under the impression that all Benz diesels (excl the newer ones) use a vacuum operated fuel shutoff and that there is no fuel shutoff solenoid.

Perhaps you heard the overboost solenoid clicking... perhaps this cancels fuel enrichment if the engine exceeds a certain speed. There is a mechanical centrifugal "rev-limiter" in the injection pump. It is possible that this electronic circuit is a backup system in case the mechanical system fails. Does anyone know for sure?

-Joe

Oh crap, I sure was!
On the Diesel engine, the rev limiter is a device inside the fuel injection pump called the governor, simple.
Gilly

It was the shutdown solenoid that was clicking as I increased the frequency. so what is a overload protection control unit for and where is it located?.