1985 300TD Tachometer (federal)
 

Hi... New member, new to Mercedes diesel, first post,

I have just spent about 16 hours (half the time on the internet) trying to get the tachometer to work.

What I've done:

Checked rpm sensor... receiving 5.67 volts at idle

Checked OVP relay... its receiving voltage and the fuse is good ...also, have 2 matching OVP relays from another 85 that definitely works in the other 85.

Checked EGR inside kickpanel... no voltage ...It has two wires going into it at the center pole and the pole directly above it. All other posts/poles are blank. The cap has the number 3 on it.

I read this from a 1999 and 2002 post: "the 85 has an electronic EGR control unit. The tach signal runs through it. There is a wire at the positive battery terminal that supplies power to the EGR" Is this true? If so, where is the wire... I looked at the battery and there is not a second wire attached to the + positive post. I then took the battery out and could not find any loose wires that would attach to it.

any ideas or is it time to tear into the dashboard?

Thanks... dazed and confused,

dennis

1985 was a funny year for tach's at Benz. If the Sensor on the block is there and good and the pin is on the harmonic Balancer and the wiring goes up to the Tach Module and it then goes to the Tach (Make Sure The Tach Wire is Plugged in the back of the Cluster ) and she still no work then the Tach Module is prob bad or the Tach itself which is Rare. Follow the gray wire from the tach amp down to the sensor. Spacing should be about 1/8". A thrown belt at one time in the cars life can bend the bracket the sensor sits on. I had a decent picture of this on the Schuman (sic) forum but I cant access it - guess they banned me or something:rolleyes:

Thanks

Checked the pin on the harmonic Balancer and its putting out voltage. I guess its time to pull the cluster out and start tracing.

thanks

dennis

Odd I swore I just replied to this... Does your overboost solenoid open up at >2000rpm <6psi?

I am going to check the pressure when my daughter gets back with the car.
I'll have to guess the rpm as the tach does not work... my ear is pretty good though.

Sorry... you are correct, I am a newbie and posted twice.

THANKS!

dennis

I'll agree with TX... the '85's are a bit different. On the one I had, it worked, so I didn't get into it. On my '84, the sensor from the crank ends at the plug where previous years amp would be, there is just a cap. Good luck.