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New tach amp found!

Started by Engineerd3d, 07 July 2021, 03:52 PM


So I found and bought a new reproduction microcontroller based tach amp. It does the sweep and also gives led troubleshooting codes. I am quite excited to try it, however my crank sensor has a slot going though it so it's toast.

Once I get a new sensor I will update on this thread and give you my experiences.


The sensor is a bit tough to get out if you don't know the trick. I've removed them on my w123 and w126. I come in with long (like 12" or 16") of 1/4" drive extensions to the 8mm nut from the power steering pump side and under the vacuum pump. A wobble head or u joint can help depending on the car - somehow my 85 w123 is different than my 82 w126.

I am the designer of this device.  My email is printed on the bottom of the board if you ever need any support.  And of course you can reach me here.


How cool is that?!

What engine is this for? M110?
'75 280S
'77 6.9
'82 300TD


It's compatible with the OM617A turbodiesel. Uses the crank sensor on the front of the engine.  I developed it on my W126 and W123 but it will work on a 1979-80 W116 300SD.

The gassers have different electronics in the gauge cluster.  Luckily they don't have the same age related issues as the turbodiesel.


W123wrench, question for you...

The tach in m 84 300TD works, but past a certain RPM it seems to work backwards.  Idle RPM seems accurate, then it will go up till about 2000RPM, but then it will start going back down.  THen when the transmission upshifts, it actually jumps up to higher RPM (backwards operation).  I also notice that my front crankshaft sensor is disconnected. 
Does the W123 300TD have a different input for the tach?  My 1980 300SD has that sensor coming straight up into the tach amp.
1980 300SD


Unfortunately for me I have not had a chance to replace the sensor. I bought it and it is sitting in a box. I am preparing for a 700 mile road trip and want to replace some gaskets, including the vacuum pump gasket before the trip. I will do the sensor at that time.


If it's the one developed earlier this year on another forum, it works great in my '83 300SD. It seems to be a bit smoother than the original, the start up sweep is cool and it was plug and play.

'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8


Michael yes it is the same one. You were one of the early adopters if I recall correctly. 

Inshan your 84 TD is likely running the new system that does not use a tach amp. If it has the short cap instead of the tall cap like on your 300SD it is the new system.

The 84-85 have two sensors. The front crank sensor is used for the maintenance/service port to get a TDC signal. The tach is actually reading off a Hall sensor located at the bell housing right behind the oil filter.  It passes through the EGR computer and gets routed to the tach gauge.  There are a few connectors to check. One is on the firewall. Check for frayed wires there too.

Then check the big connector on your egr computer. It is located behind the passenger kick panel in the car. The connector often gets corroded and needs to be cleaned up. If all that doesn't improve things you may need a new sensor. It unscrews out of the bell housing and can be replaced.  Good luck.



thank you for the detailed explanation.  Yes, mine is the later system you described.  Will follow your instructions and report back, thanks!
1980 300SD


I finally installed a new hall sensor as well as an air pump gasket in the process. The tach is now functioning very well. I like the sweep like a modern gauge. This was well worth the money.


I followed W123wrench advice and found my egr computer plug full of water and corrosion.  I removed the EGR computer and jumped the tach signal pins and now I have a working tach! 
1980 300SD



I love a free fix.  Except when I don't get to sell a tach amp  ;) .

Glad I could help. Now you need to find the water leak or this will happen again.

Oh and I forgot to mention, if anyone doesn't want a needle sweep I created a custom version of the code that doesn't do it.  Good for those who want to maintain the original look and feel of the instrument cluster.


Hello. I would like to purchase a tach amp from you. Thanks
Some are born great, some achieve greatness and some have greatness thrust upon them.



I had Carl make me one of his tach amps without the sweep, and it works great! Thank you!
1979 300SD #6680 - Astralsilber - 153,750km
1998 E300 Turbodiesel - Rauchsilber - 165,000km