Mercedes 300D 1984 W123 Tachometer amplifier diagnosis 

VERY nicely-done video, John! And it sure can be tough fixing problems on an older car when someone else has improperly attempted repairs or modifications!

Another complexity with the 84/85 years, the cruise signal comes from the ECU behind the kickpanel (you showed)! There is a way to bypass totally this ECU! The tach gauge plugin is also different on the 84/85 model 300D/300SD!

I bought the digital one; works great!!

Good Job John, The 1985 W123 Federal cars, with black sticker on top of radiator core support did not get the cats but the 1985 W123 California cars did, I think the sticker for California is yellow. Turbo diesels that is.

Just a couple of points FYI. Note that I am more famiar with the W124 systems as they are far more common where I am. Have rarely seen a W123 with a tach: - The 14000 Hz marking on the tach would be a clue that you have the later system. This is the frequency of pulses at which the tach would indicate full scale (6000 rpm/60 s = 100 rev/s. 100 r/s * 140 (or 144 in W124) teeth on the flywheel equals 14000 (14400). Early tachs are only 1 impulse per rev so 100 Hz. - Yes the later tach does not need an amplifier as the signal from the crank sensor is much stronger. The W124 does condition and amplify it nevertheless. I don't know about the W123 but I guess the same because if it was pass through the tach would work even with the EGR computer not getting power. In fact the circuitry is very similar in all of them, the only difference is the pulses per rev which is set by a Resistor and Capacitor on the board. They are not picky about the signal shape or voltage, could be square, sine etc as long as it's above a threshold voltage (I think it's about 2-3 V). You can change the number of pulses per rev by changing the capacitor on the board. You can also sucessfully use the alternator W terminal if you adjust the tach for the proper number of pulses (around 15-20 with the alternator). To test the tach you need a signal generator. A cheap way to this is to use a PC audio output plus an audio amp. Connect the audio amp to the PC, connect the audio output to the signal input of the tach, and use one of those online tone generators to output the correct frequency. Another way is to use an arduino using the tone function

Thank you!

Very informative. 84 is an interesting model years since it has some stuff that is compatible with earlier models and some other things that are compatible only with 85 models. Mine is an 82 and has the “trash can” amplifier, and it’s a trash can indeed. When I got the car the tach was on its last legs, so I had to do some research about. When I took the amplifier apart I found a business card from a European car workshop from Virginia stuffed between the amplifier and the cap, so I immediately knew the amplifier was in need of resoldering. After redoing all the solders it worked well for a while and started to work intermittently again. This time the sender unit failed because at some point someone damaged the insulation and the exposed wires began to disintegrate. Had to replace the sender unit and it worked for a few years, until earlier this year when it began misbehaving again! Now is the amplifier that is failing again. Since I couldn’t take it apart this time to see what went wrong I ordered one of those modern replacement units from Ebay and I am waiting for it to arrive. It drives me nuts to see the needle either dead or jumping! By the way, as far as I know tachs on these cars and the 300SDs from 78 to 85 were always troublesome. They barely outlasted the warranty and began exhibiting all sorts of issues, is like Mercedes didn’t have a clue on how to make a decent tach. I guess that explains why the first NA models and the 81 300TDs Turbo didn’t come with a tach: it was useless! I don’t want to imagine the disappointment of the first owners that had to see dead tachs or jumpy needles after just a few years having paid an arm and leg for these cars.

Thanks for this video. I own a 84 and 82. The 84 has same set up but my 82 despite replacing every component from the sensor on engine to the tack still doesn't work. The amp is good. Every once in a while tach needle will move. I thought bad ground but I just gave up

Thank you so much for this video

You are very good mechanic

really nice video!

I have an 84 wagon, I am guessing that it is a simple fuse behind the glove box for the tac not working

Amazing video - thank you! Need to do my cluster lights as well - do you have a video on taking out the cluster? Thanks!

Thank you for the excellent video. Especially the time lapse parts. My tach, in an 84 300D, just works when the car is cold. Can't be the fuse. My trash can is just a test port (unless somebody removed the PC board). It's starting to sound like it's the instrument itself. Any ideas?

men, you was running without computer assistance, only on the mechanical inyection sistem. the red rele is the OVP, the mechanical rele inside use to fail because welds (leed free welds), just need to weld again and is good to go

What kind of LED did you use? The turn signals seem to be pretty bright...

I have an '82 wagon where the tach and speedometer isn't working. It's not clear to me from the video whether or not the fuse behind the glove box compartment is ONLY on later model w123 or if all w123 have this...Does anyone here know?

Hi i have W201 and when i start the engine, tachometer neadle just jump and stey like that no reading....fuse in OVP rellay is OK...any guests what is problem?

Hello my name is Pierre i am living in France and i wave a w 123 300 D year 1984 without tachometer . Is it possible you help me to put it my car Best regards

ok good

I dont have an ovp, yet pin 3 does not exist on my car