PeachParts Mercedes-Benz Forum - W126 Tachometer Problem

PeachParts Mercedes-Benz Forum (http://www.peachparts.com/shopforum/index.php)

- Diesel Discussion (http://www.peachparts.com/shopforum/forumdisplay.php?f=15)

- - W126 Tachometer Problem (http://www.peachparts.com/shopforum/showthread.php?t=222834)

W126 Tachometer Problem

VEHICLE: 1982 300SD

PROBLEM: The Tachometer does not work at all.

THE INFO AND SUCH:

My Tach does not work. I tried cleaning all of the connections with contact cleaner and the speed control sensor to no avail.

Below is a VERY crude drawing of the speed control sensor pin layout. From my manual I know :

Pin 2 is my ground.
Pin 6 is my power (12v)
Pins 7,8 are for the tach sensor that is mounted on the block and takes the rotation reading from the crank pulley.
Pin 9 is the ground for the tach sensor.

I turned the car on and pulled off the cap of the speed sensor.

I have voltage at Pin 6. My ground and power have continuity with the car off. I am getting continuity on pins 7&9 and 8&9 as well. I pulled the gauge pod and pulled off the tach cable. I am also getting voltage here.

I can only assume at this point that I either need a new SCS (aka "trash can" ) or that perhaps the gear that drives the tachometer inside the gauge is broken.

Any thoughts from the diesel faithfull???:D

It's very likely a broken solder trace in the tach amplifier. If you're good with dental picks, you can clean the potting goop out yourself and clean up the soldering. Otherwise, you'll have to buy a new or used one (with a guarantee!).

Here's a link for more info on the tach amp:

How the Tach Amp Works

Quote:

Originally Posted by BoostJunkie (Post 1860789)

Any thoughts from the diesel faithfull???:D

I made a diagnostic tool that plugs into the amp socket and tests the gauge and the sensor. It's also a fully functional tach amp. With it, you can diagnose faults in your system. You can borrow it if you like.

So, is the Tach amp inside the speed control sensor or inside the gauge pod?

There are three main components:

1) Sensor. Located at the harmonic balancer
2) Amp. Located on the fender, it's the "trash can" that plugs into the socket you have diagrammed.
3) Gauge. Located in the cluster

Quote:

Originally Posted by techguy512 (Post 1860807)

That would be really awesome! How do I do that?

Quote:

Originally Posted by BoostJunkie (Post 1860814)

That would be really awesome! How do I do that?

PM me with your email address (for me to send the instructions) and your mailing address.

Simple, no?

Problem fixed with the help of TechGuy512 and his amazing (and very reasonably priced) Tachometer Amplifier!!!! It's almost erie watching that needle move now when I drive... :)