Herr Doktor, it should be in the same location as is the W126s', on the inner left front fender. It looks like this when unscrewed from the socket...

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Bob
'82 300D Petrol B-G Metallic

You should be able to see it in this photo in the lower portion. It's a 1980, but it should be in the same position that the 1978's is.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Yes, thanks to all for the photos and information....

We always called that an 'adapter' so I thought the 'amp' was somwhere else...

How in the devil do your get it apart without detonating cord????

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Found a cool thing today....

I placed a 3/4" OD x 1/8" thick nylon washer on the top of the silicon potting and re-installed the tach amp cover.

No more bouncing tach.....at leat for the time being.

Who makes a new tach amp????

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Hi, i know it's been a while, but has anyone got the larger version of the schematic posted in the first post ?
it seems it's now gone and the smaller one is a bit difficult to read.

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Copy and Paste the the original image into Paint.
Then you can use the Zoom In function under the View tab (enlarges 100%).
The enlarged view can be additonally enlarged if need be.

In Paint, you can also use the Resize function to enlarge the image the percentage you choose.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

it's not so much the lines, it's the missing pixels/information when looking at the values for the resisters and capacitors ect: :'(

Some of the values are in the description of operation. Also, if you reverse colors in paint to make out values. Pull the white paper on the LM1815 and more values could be had.

Clean schematic

tach amp wire colors for wires 2,3,6?

New design amp

Designing a Better Tach Amp

The "Clean schematic" may have one or more errors. The inductor shown across pins 2 & 3 should be a capacitor.

I have repaired many of these tach amps. It was always problems with poor solder joints, especially where 12 V comes in on pin 6 to the 37 ohm resistor. Due to heat and poor solder joints, that connection becomes intermittent, or that resistor is burned out.

Scrape away the potting compound, check and replace the 37 ohm resistor if n.g. (use a 1/2 watt instead of 1/4), re-solder the entire board and you may be in luck.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Agreed, many pixels missing.

If you enlarge the image, scoot back from the screen, and use your visual closure skills (aka imagination) then you might be able to make out the numbers and letters.

I got:

***************************************************************
Note: 'Unless indicated otherwise'
* All resistor are 1/4W 5%
* All capacitors in microfarads

R1 32 1/2W, R8 360
R2 360 2%, D1 IN914 , C1 100 15V Electrolytic, Z1 5.5V 0.5W, R9 470k
G1 BC327, C5 0.001 110V
R3 830
C3 0.0033 16V Polyester
C2 2 2 25V Tantalum
R4 1.9k
R6 4.7k
R5 1.8k, R10 27k
R7 1.2 M, C4 4.7 10V Tantalum

*************************************************************

I don't know if my guesstimates are accurate, but that's my read of the values.

Edit: Of course a clean schematic is infinitely better.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

I'm not an electrical engineer and truthfully have very little knowledge about circuits but I managed to make this tach tester out of really cheap common parts. It will test the wiring from the socket and the tach itself only. On my 1982 300sd it moves the tach from 0 to 1000 rpm with key on and engine not running. And No, i can't explain how it works. LOL

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I got the tach working again. Found a crack on the circuit board in the tach amp. I repaired the solder trace and it's good to go.

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The 555 timer circuit you built creates a square wave signal that simulates the output of the tach amp. It goes from zero volts to 5 volts and back to zero at a speed that simulates the single pulse from the pickup on the crank being amplified and cleaned up by the tach amp. If this is truly your first go at electronics, well done! And even if it's not, still well done.

Also, good find on the broken trace. Did you span the break with small piece of wire? Just bridging it with solder will work fine but a stronger fix when the PCB cracked is to bridge the crack with solder and a bit of wire. It can be tricky as the wire tends to want to stick to the soldering iron.

I consider solder joints like the one near the top of your picture as being "bad". You can see a ring or circle around the component lead. Some would call it a cold joint, some call it broken. Either way, the component lead isn't making the best contact with the circuit board trace. It wouldn't hurt to remelt that joint and any others that look like it, adding just a touch of solder which will put fresh flux on the joint. That will let the solder flow into the joint, solidifying the connection.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

Next project....My electronic speedo also has problems....it does this: when i go 20mph, the needle fluctuates between 20 and 0....constantly, not randomly.....when i go 30mph, it goes from 30 to 0 constantly, not randomly...Any ideas why it constantly bounce back to 0?