Tach not working and cut wires

[jjz28]

My tach hasnt been working so i thought i would check into it and found that the PO had cut the wires going into the headlight relay and wired the blue and red/white wires together and left the black and yellow one taped up not connected to anything. SO the realy has nothing going to it. I noticed the tach has a yellow wire, is it the same wire that is cut and probably why its not working? I have a switch on my headlight to turn it on and off so i guess the relay went bad and he put in the switch. My charging works fine. How could i get my tach to work. I am going to try and post a pic of the cut wires. The blue and red/white are taped together and the black and yellow are just cut.

[DGXSER]

If you have the correct wirign harness there, those should be a blue/black wire and a red/yellow wire hooked together. You should then have a black (ground) wire, and the yellow wire. Per the wirign diagram, that yellow wire is a direct feed form the generator/alternator. I would want ot be really certain that wire is covered and not going to short to ground.

The relay used the yellow wire to prove the engine was running by getting power form the generator and flowing to the ground to pull the relay in, the relay connected the blue/black and red/yellow wires.

[jjz28]

maybe it is a red/yellow and blue black, i didnt look real close at them but does that make the tach not work? It is the original harness, everything mechanical is original, only the seat has been changed.

[DGXSER]

From what I see on the wiring diagram, it would not. However, interestingly enough the tachometer does not appear on the wiring diagrams I have for the 78-79 models.

[jjz28]

I read a post that the tach and headlight relay are together somehow and the yellow wire goes to the tach plugin i think.

[DGXSER]

Well, you could pull the wiring connector at the tach loose and check for continuity between the cut yellow wire and the connector if there is a yallow wire going into it.

[jjz28]

does anyone know for sure if this yellow wire at the relay is the same yellow wire at the tach and will keep it from working. I need to get a new ampmeter or whatever its called. mine has been in the toolbox and doesnt work. I would say its rusted inside, i put a new battery in. I dont know much about those meters either, so i dont even know if i could measure contunuity. Just hoping someone knows about this tach situation and has been there.

[crazy steve]

does anyone know for sure if this yellow wire at the relay is the same yellow wire at the tach and will keep it from working...

No, it is not. That's one of the wiring differences between the '78-79 and '80-81 bikes. In both cases that wire comes from the alternator, but it's not the same wire on both.

There's four wires coming out of the stator; three whites and a yellow. On the late bikes, that yellow wire stops at the first plug. The stator is a three phase 'wye' type, and the yellow wire is the 'center' point. On the '78-79 bikes, this wire is what powers up the headlight relay. Because of the various counter-emfs involved, it's output is 'close enough' to DC to run the relay without a diode. The only place it connects is to the relay.

The late bikes use one of the white output wires to both power the relay (through a diode) and send the signal to the tach. The '78-79 tach also gets it's signal from a white wire, same as the later bikes.

[jjz28]

thanks steve. I saw 4 wires going to the tach and i dont think any were white. I think it was yellow, green, and maybe 2 colors of blue I cant remember but i will look in the morning.

[crazy steve]

On the '79 Special, there should be two plugs going to the tach. One will be the low fuel, high beam, and neutral lights. The other plug is the tach with three wires; brown (power), white (signal), and black (ground).

[DGXSER]

Thanks Steve, for some reason the wiring diagram in the Clymers manual does not show the tach in the E-F and SF diagrams.

[crazy steve]

I don't look at the Clymer much, I prefer the FSM, at least for electrical info. The Clymer manual does show the tach wiring, they just don't identify it or show the tach...

[jjz28]

Thanks steve, are all those in the headlight bucket? I will have to look for those other wires.

[jjz28]

i saw those wires, how could i test them to see if they are getting signal. Would i use my ohm meter ( i got it working, its a cheap one)? What setting should i set it at? Would i test the brown wire or the white signal wire or both. thanks guys.

[jjz28]

ok, i tested on the ac stting and the brown and white wires are getting power. I just put the lead on them and to the frame for ground and turned the key on and they get power.