Tach likes to read less than 0

[olddman]

Just got through making new wiring harness for engine and forward harness. Took my time. followed wiring diagram and surprise, surprise, everything works. Everything except the tachometer. When I turn ignition run position or start the engine, the tach shows reading less than zero. Removed the Green wire that goes from negative side of coil to tach when I was redoing harness and reattached to negative side of coil when connecting harness wires to temp sending unit, oil pressure sending unit, ballast resistor, and alternator.
Green wire from tach to negative side of coil is getting current when ignition switch is in run position. Red wire from tach going to ignition switch getting current. White wire from tach connected to dash lighting getting current. Black wire grounded. Have checked all connections and all is tight.
Beginning to think that I did something that may have caused diode or something else inside tach to burn out.
Any ideas as what I should try or do? Should I just get a new tach? Tach I have now is an Autometer.

 

[Dana67Dart]

Pictures pictures pictures

 

[my68barracuda]

Just got through making new wiring harness for engine and forward harness. Took my time. followed wiring diagram and surprise, surprise, everything works. Everything except the tachometer. When I turn ignition run position or start the engine, the tach shows reading less than zero. Removed the Green wire that goes from negative side of coil to tach when I was redoing harness and reattached to negative side of coil when connecting harness wires to temp sending unit, oil pressure sending unit, ballast resistor, and alternator.
Green wire from tach to negative side of coil is getting current when ignition switch is in run position. Red wire from tach going to ignition switch getting current. White wire from tach connected to dash lighting getting current. Black wire grounded. Have checked all connections and all is tight.
Beginning to think that I did something that may have caused diode or something else inside tach to burn out.
Any ideas as what I should try or do? Should I just get a new tach? Tach I have now is an Autometer.

Do you have access to another vehicle that you could temporarily wire your existing tach into?
If so I would do that to test the Autometer tach.

 

[67Dart273]

Try disconnecting green trigger wire, power up tach. If it remains normal, then goes neg when you connect the green there is likely something wrong in the trigger circuit.
What kind of tach is this and does it come apart? AKA the Sun "one piece" are crimped together. last I knew, and not very accessible

 

[Bewy]

Did you mix up the + & - terminals on the coil?

 

[olddman]

Car starts and runs but tach isn't measuring rpm's. Needle is pointing to less than zero. This happened to me several years ago and sent tach to Autometer to be repaired. May end up buying new tach this time. Will take picture today and post.

 

[67Dart273]

I forgot to ask, "what type" of ignition system do you have? IE MSD, or any other CDI you cannot hook the tach to the coil

 

[Demonracer]

What ignition system are you running, i.e. distributor & coil?

 

[Mattax]

Green wire from tach to negative side of coil is getting current when ignition switch is in run position. Red wire from tach going to ignition switch getting current. White wire from tach connected to dash lighting getting current. Black wire grounded. Have checked all connections and all is tight.

Just to get this out of the way, some tachs are voltage driven and some are current driven.
IIRC Autogage uses one and Autometer the other. I think this only mnalkes a difference when converting a pulse like one would get from an MSD 6 tach output. But maybe I'm missing or forgetting something.

One test you can make is with a hand held tune up type tach. See if that works.

Green wire is the pickup signal, and yes normally that goes to the negative side of the coil.
The negative side of the coil should not be continously seeing current. In fact I don't know how you are measuring current.
Much easier to measure presence of power (voltage). Voltage on the negative terminal should fluctuate with points opening and closing.

 

[olddman]

I am using electronic ignition (MOPAR) with MSDS coil. Before I disconnected wires to make new harness the tach was working fine. Green wire has always been bolted to negative side of coil and only has voltage to tach when engine is running. When ignition is in run position and engine not running Green wire has no voltage.
Enclosing some pictures. First one is with ignition off. Red hand is lining up with RPM. Second picture is with engine running. Red had is pointing towards shift indicator knob.

Just can not figure out what I did to cause problem. As I said before, tach worked fine before I disconnected it. Hooked it up the same way as before and won't work. Scratching my head!!