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I'm hoping someone has any insight on this specific model or even on electronics in general. The car is a late 90´s 190D 2.5
The only thing on the instruments cluster not working are the speedo and the RPM tach. Speedo is easy it's just a cable replacement at most.
My main issue is with the Tachometer. I've read numerous forum threads about this and many service manuals and diagrams as I could find.
Many tachs seem to fail due to bad resistors or capacitors on the circuit boards, simple blown fuses or wires and sensor.
I've removed the dashboard and the tachometer then inspected for burnt resistors and even tested them with a multimeter, all good. However I don't have proper equipment to actually bench test it to be 100% sure but as far as I could test all was good even the coil is OK.
I also found a couple of reports of issues caused by a bad Over-voltage protection Relay that causes both the tach to die and a constant ABS light on. Also tested resistor, diodes and the relay coil, re soldered all points. OVR seems good.
Tested the signal that's generated by the sensor near the starter according to the manual I found for the various w201 it should read at idle speed above 4V (AC), I got above 5V~. Resistance should be around 1.9kΩ an I had 1.8kΩ, so the sensor looks good.
Here's a diagram of what I have and tested. I got some doubts about the reading on the W signal wire from the tachometer connector that was always 13V I just don't know for sure if that is expected or not.
What am I missing? Anyone have any ideas, suggestions or advice?
I think your car must be a late 1980s as that's when the turbo diesel versions were built. The W201 ended production in 1993. Having said that, my W201 is a 91 gas powered car and I don't have a tachometer. It's possible the wires to the tachometer and speedo could need to be replaced given the age of your car and that the rest of the dash is working properly. Make sure the wires are tightly connected and that the connections are clean. I'm not sure what the correct voltage needs to be for the tachometer. I think your best bet might be to contact the MB Classic Center in California and they'll be able to tell you the correct voltage for the tachometer and also where to access the online MB diagnostic information is that you'd be able to subscribe to for a fee as the name of that service has just gone right out of my head as I type this. The MB Classic Center is very knowledgeable about your car and is an excellent resource. I use them when I get stuck with an issue or need parts. Hope this helps you some.
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