BenzWorld Junior Member
Date registered: Nov 2006
Vehicle: 1981 300TD Turbo, 1984 300D, 1992 500E
Location: Madrid, Spain
Mentioned: 0 Post(s)
Quoted: 3 Post(s)
Help. W123 Tach not working
My 1981 300TD Turbo tach started acting funny (needle jumping up and down) until it finally died.
I know there's a lot of information on this here but I'd still appreciate some extra help before I start spending money.
First thing I did was cleaning the sensor at the crank with no results.
I've done some measurements on the amp connector and this is what I got:
Pin 6: 12-13V
Pin 2: 0V
Pin 3: 0V
Pins 7&9: 0V
Pin 8: 0V
This was with the amp disconnected. When I connect the amp I get 4,5-5,5V at the output to tach. I've tried with a spare amp and I get 0,5V there so I guess it's not ok.
I've also tried plugging a W124 tach but nothing happens.
I don't know what voltage I should get at the output to tach wire but my guess (given my lack of electrical knowledge) is that I'm not getting enough voltage.
As I'm getting 0V at pins 7&9 (sensor input) I think the problem is in the sensor or the sensor wiring.
What do you think?
Thanks in advance for your help
Last edited by W123Spain; 02-14-2016 at 03:37 AM.