Permanently fix a broken jumpy Mercedes 300D W123 turbodiesel tachometer

In this short video I present possible fixes for a non functioning or jumpy tachometer on a W123 W126 or W116 turbodiesel made by Mercedes during the 1980s. The electronics in these cars was designed in the 1980s with then current tech that has aged out after forty years.
I give some background on how the system works and the different components that typically fail in these cars.
I am the designer of a updated digital tachometer amplifier that I designed in Spring 2021. Replacing the amplifier with a rebuilt unit or my new unit will fix 95% of the tachometer failures out there. Check out my other video for details about the all digital tach amp technology.
Finally I present some information about late W123 turbo cars made from 84-85. I do not sell a solution for these cars. Fortunately 100% of the problems with these models are broken wires and bad connections between the sensor and dash gauge. These are easy to fix things that can be done at home for free.

[@TheLuisg92]

I recently got one of these new amplifiers and it works a charm. No more jumpy, lazy needle. I also got a plastic cap to go along with it. Certainly worth the investment!

[@pacodrake3766]

Glad it worked out.

[@kuldipdhak7972]

Good idea to see relying and fuse how I can open egr box passenger side kick panel

[@pacodrake3766]

I recall the panel unscrews. Right hand side next to passengers right foot

[@dixiemae5042]

Placed one in my cart 👍🏻

[@Eli-kr5bm]

I think newer mercedes suffer from this too, x204 w204 are known for this.

[@alkhashtee]

Thank you very much for your valuable information

[@caracine]

I own one of your digital tach amps for my '83 300D. I'm just getting a solid light with the engine running. I've checked impedance on the two sensor wires (7 and 9) to the stock amp mount and see ~92ohms which I think is ok and I only get resistance on these two wires (not 8). I also get the tach sweep when starting every time. I've read conflicting info about the 7,8,9 connections, some say 8/9 should give you ~80ohms and 7 is the shield. Others (my W123.133 manual) say 7/9 are the sensor and 8 is the shield. Lastly, does the clock power on the cluster need to be plugged in for this setup to work? I'm pretty sure these connections to the amp base may have changed over model years. On your digital amp, what is the pin-out for those three connections from the crank sensor? Does it matter at all? Thanks for your time! Thanks for your great product. I've bought two of your amps before with great success on other W123 300D's.

[@pacodrake3766]

92 ohms sounds high. I usually see around 80. It might be ok. The fact you get a solid light while running says the board isn’t getting a sensor signal as you’ve deduced. I would try cleaning up the sensor if it is caked with grease. Check to see if your harmonic balancer still has the little pin affixed to it as well. Pin 8 is the shield and it should read open when the amp board is out of the car. I connect it to ground through my board so you may see connectivity from 7-8, 8-9 when the board is in. 7 is connected to ground and 8 is connected to ground (through my board) so it’ll look like 7-9 is zero ohms. If the board is out 8 shouldn’t be connected to anything. The clock is unrelated to the tach circuit. It can be unplugged. The pinouts on the plug have not changed over the years. They are set up to connector to MB diagnostic tools that sense TDC. So they’re the same as on the gas cars of many years. The difference is some years add and drop extra connections. So my guess is you have a bad sensor, sensor cable or sensor plug. Try taking to two screws off the sides of the plug and pulling the 7,8,9 connector out to check its condition. The urethane is rotten on many of the older cars and prevents a good connection to the amp.

[@twofishesreclamation565]

Thank you so very much. Well done!

[@user-pm1mz2qc9z]

Hi ive got w123 1985 petro 2lt the tacho is missing can you help me regards alex

[@pacodrake3766]

This depends on how you want to set up your car. If you have a magnetic pickup on your crank you will be able to install my amplifier in the service port. Then you’ll need to wire in a two pin diesel tachometer from a turbodiesel. That is a roundabout way of getting a tach. Alternatively you could seek out a tach off an I4 engine that wires to your existing coil. I’m not sure if there is one for the M115 equipped W123 cars but if not you may be able to modify something from a 190E or start over with an aftermarket tach from Autometer. This may not be aesthetically pleasing if that bothers you.

[@MrRjnr]

great amp, I love it!!

[@w124mercedes7]

Any idea what will cause the tach not to work on a 1987 300d w124 Any help would be greatly appreciated. Thank you 😊

[@pacodrake3766]

Try checking the fuse on your overvoltage protection relay. Then check all of the connections coming off of your tach sensor (on the trans bellhousing) back to the brain box wherever that is on a w124. Good luck.

[@oldsoulsound]

do you need to remove the old electronics from the cap? Or do you supply an empty cap with the new board?

[@pacodrake3766]

Yes, you pull the old electronics out of the cap. Best done with long nosed pliers. It is in there tightly but it can be wiggled and pulled out by hand. I also make a 3D printed cap. On eBay check “seller’s other items”.

[@woodsandbarclay]

What is your website and how do we order this digital amplifier?

[@pacodrake3766]

I don’t have a website. I sell it on eBay. It’s listed under 300d tachometer amplifier

[@92093572]

We have a PistenBulley with a 300D 917.952 engine inside. On this the rpm reading stops at 2500. What do you think is the error on this? does it seems like it is the amplifier? It reads som rps, but it is wrong reading after 2500. You can max the rpm but reading stops art 2500

[@pacodrake3766]

Not sure. Does it have a 300D gauge? Apparently you are getting a signal from your crank sensor. I assume you have a 300D amplifier under the hood? Tell me 1) what is the sensor, 2) what amplifier 3) what gauge 4) did it work earlier and fail or did it never work? I’m not sure why the old VDO system would stop at 2500.

[@92093572]

@@pacodrake3766 Hi, thansk for the quick reply. Yes, all of the instrunents seems like MB type. I did manage to find the electrical diagram for this machine and it seems like the tach is connected to the w output on the alternator. If the reading is off/bad could it just be as simple that the alternstor belt need to be tighten?

[@pacodrake3766]

Ok. The Mercedes 300D system usually reads off the balancer on the crankshaft. Not the alternator. I show it in this video. It reads one time per revolution of the engine. The alternator pickup will be more than one pulse per revolution. Do you have a tachometer amplifier? It looks like a little black garbage can. Check this video again.

[@dagobertovieyra6597]

Ok so how do l order one

[@pacodrake3766]

Go to eBay. It’s under 300d tachometer amplifier

[@user-lk2yj2jk5v]

Looked on ebay, didnt see the item, do you still make them?

[@malalamalala6769]

How d I get one of your circuit boards.

[@pacodrake3766]

eBay or message me directly.