1969 Mercedes 280se Tachometer Issue

[Xandromega]

Hi I'm having trouble trying to troubleshoot my tachometer wiring I cannot find any tachometer wiring diagram if anyone has one it would be greatly appreciated. The tachometer it's self is working great but when I have it hooked up in and the engine on the needle jumps around so I tested the connector pins to see if I could find ground,power and signal. It's a 6 pin connector but only 4 of them have pins. So I tested for power and continuity on the pins. I found a black/blue wire that has 12-13 volts which goes to pin 15/54 on the rpm gauge I'm pretty sure that's the ignition wire it doesn't fluctuate when I presume the gas. Then I traced two brown wires that go into one pin together to a cluster of brown wires that are grounded to the frame. After that there are two more pins one has two grey/blue wires together that have no voltage but I believe to go to the lights and the last one is what I think to be the signal wire but I could be wrong it's a green/black wire that when I tested for continuity it was blank. So I popped the hood and noticed a green/black wire disconnected from something laying next to the ignition coil I attached it to a ground and tested again for continuity and it beeped. So I'm not receiving any signal voltage and this green/black wire is just laying next to the ignition coil negative side so I then tested negative side of ignitioncoil while I pressed on the gas when it was idlenit was at about 8 volts then when I press gas it jumped way high. So I got really ballsy and attached the loose green /black wire to the negative of the ignition coil hoping it would be the signal. But the car wouldn't start and sounded like craps when I did. So I disconnected it. No damage occurred. Still stumped though. And its obvious someone disconnected this green/black wire for a reason.

[Xandromega]

I know this isn't a transistorized ignition system. I dont have the module under the battery. So I'm pretty sure that the green/black wire is the signal wire that's suppose to connect to the negative side of the coil (But why would it rough start?). The negative side of the coil does already have a green wire that connects to the distributor and condensor. Also I did test the resistor under the coil that feeds the wire to the positive side of coil and it's only reading 1 ohm which supossively is supposed to be 1.4 and apparently is supposed to reduce the voltage on the positive side to 6 volts but it says it's still at 12. Any help is greatly appreciated! Sorry for any confusion

[rolfeschroeder]

I don't know your car, but I do have a 1971 Volvo with what should be a similar setup. The green wire you have from the coil - to the distributor has been added to replace the wire that goes from coil - through the tach and then to the distributor.

Try to find the wire that comes out of the tach and to the dizzy. You can then disconnect your green wire and use the circuit through the tach.

Rolfe
'71 volvo 1800
'98 sl500