What is this thing on my 1985 300SD?

[oilslick]

I can't post a picture at the moment but I can give the part number and a brief discription.

Part Number: 001 540 7197

You can hear this thing ticking when you accelerate. It has three vacuum connections and one electrical connection. It mounts just in front of and below the coolant recovery tank. It sounds like a lifter ticking away. It is annoying!

Also, My tachometer is going haywire. It is working backwards at times. It seems to stick around 1,000 RPM's and then after it sits there for a while it works but I don't believe it's accurate. I have no idea where to begin on it. I have searched the archives but I am still unsure of where to begin. I HATE auto electrical stuff!

Thanks for any help you can give.

Kevin