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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.

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

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.

It won’t work on a short cap system. Only 83 and older 300SD. Your problem is likely in your OVP relay, your wiring from the sensor on the bell housing near the oil filter or corrosion in your EGR computer connector.

I love the machine. But that being said, my clone was bottom of the bucket. $239 shipped. It was rough. Inside it was poorly lubricated. I needed to hone and file parts to get them working smoothly. It was a tinkerers dream. If you’re into that good. I am an engineer and I’m more interested in tinkering with sewing machines than actual sewing. I have bought and restored a bunch of old Vikings and Elnas just because I love the mechanical challenge. So keep that in mind. If you have that inclination then these machines can be made to really tick like a Swiss watch. I needed the bigger flywheel to slow things down and I got a better electronic foot control. Then I designed a stitch length guide. I recently sewed a carrying case for a piece of electronic test equipment and it now works perfectly. I’m very happy but it was like buying a kit that needed a lot of fiddling to get right.

@@pacodrake3766 thanks for the reply. i'd def want to sew with it and i am "tinkerer", actually in the process getting a Necci Lelia 513 running smoothly (a bunch of oiling to get the zigzag to work but now motor isn't functioning properly). but, even though i like to tinker, i'm afraid of throwing off the timing since this would be the first time going further than taking off covers. i'm going to to do more research, but thanks for the informative reply. just out of curiosity, would you be willing to sell the STL file for your stitch length control plate?

I have a Rex 607 and also would pay for the stl and specifications for metal hardware.

​@@pacodrake3766 hi, I would like to do the same. Which parts needs filing for it to work smoothly? Is that a way to increase the stitches to 6mm like they did in sailrite?

You are a prince amongst men! Thank you for this 🎉

Thank you for your support

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.

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

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.

@@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?

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.

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

Yes I’m still making them. They’re on eBay under 300D tachometer amplifier.

@@pacodrake3766 Thanks!

Glad it worked out.

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”.

eBay or message me directly.

eBay

Thanks for the feedback. Glad it worked out.

Thanks! It’s good to know people still love these cars and work hard to keep them on the road. I was thinking the sun is setting for the w123 with parts going NLA but given all the people collecting and restoring parts we probably have a ways to go. Last weekend I hung out at a GTG with some w108 owners and realize it never really ends. People will find ways to keep the parts available if they care enough.

@@johnbrooks4154 yes, you caught me loafing around. 😄

No, I just designed this in March 2021. There is no older one. You probably have one made by MB Electronics Upgrades?

@@pacodrake3766 Ah, i have this one i got from a guy who made them off the peachparts MB forum :) thought it was you as they look so similair.

Yes, I was inspired by a photo of the MB Electronics Upgrades design. He used an Atmel ATTINY12 and I used a Microchip PIC16F18313. They’re both eight pin microcontrollers.

www.ebay.com/itm/284262109055?hash=item422f578f7f:g:tTEAAOSwHtxgQ~Us