Topic: Electric tachometer wiring issue

[Canada]

I bought one the the small Sunpro tachs to give the electrical tachometer option a whirl and everything was going fine until something odd started happening. I'm not so sure what is exactly going on but it was initially running very well until  one day I revved her up a little to get someone's attention and shortly after it started to sputter and then eventually stall. I let it sit for approximately 8 hours and it fired up and drove fine on the way home from work. The next day the same thing happened as I came to the end of my ride only this time it sat for 8 hours and then would not start at all. It would start but sputter on 2 cylinders and die. In a desperate attempt to get the machine running I disconnected the ground wire to the tach and it started right up. The tach has 4 wires. One for the internal light, one to ground, one to 12V and the other to the coils. Initially I had the 12V and the internal light wire hooked up to the same terminal so I separated them but that had no effect. The only thing that I can think may be the problem is the way it is hooked to the coil. I attached the wire directly to the front of the Dyna 5 ohm coil at the same spot where the blue wires comes out to go to the dyna ignition module. Could this be the problem? I am confused how this could cause such an intermittent problem.

[FunJimmy]

I attached the wire directly to the front of the Dyna 5 ohm coil at the same spot where the blue wires comes out to go to the dyna ignition module.

Have you tried connecting the tach to the other side of the coil?

[Canada]

I just hooked it to the negative side of the coil and it will not operate. Do you mean to the other coil? Not sure how that would change this problem.

[FunJimmy]

Can't say I know either.
I'm currently installing a Dyna ignition in my CB550F and was planing on using an '83 VF750 electronic tach.
The VF750 factory wiring diagram shows the tach wire going to the possative side of the coils too.

[mystic_1]

Sounds like a loose connection or an intermittant short to me.  The tach hookup may not be the actual problem.  Search the surrounding area and verify all connections.

mystic_1

[tinyrobot]

I just finished wiring up an autometer and it has almost the same leads.  ground, 12V, power, coils, and then this brown one for 2 or 1 phase ignition.  Anyway the coils wire should connect to the yellow wire on your coils.

[Canada]

Switched her over to the yellow wire and the same symptoms. Tinyrobot, what circuit are you using for the grounds, light and 12V. I wonder if my random choices are causing issues. It appears that once the engine warms up it will start to fire on only two cylinders. As soon as the tach is disconnected then she will run fine.

[BobbyR]

Switched her over to the yellow wire and the same symptoms. Tinyrobot, what circuit are you using for the grounds, light and 12V. I wonder if my random choices are causing issues. It appears that once the engine warms up it will start to fire on only two cylinders. As soon as the tach is disconnected then she will run fine.

Losing two cylinders says that somehow you are interfering with that coil. Check for possible shorts. It gets tight under the tank.

[Canada]

It's all good under the tank. I am thinking now that it has to do with my choice of 12V source. I am going to trace that circuit tomorrow and see if it could be looping around to cause a short but it's strange that it is not happening until it is warmed up. What's up with that little treat? It confuses me.

[FunJimmy]

You could try running the 12v directly from the battery to see if a clean source solves the problem.

[Cannibal]

I've seen numerous of those Sun tachs short internally. I had one in my Mustang short out on the freeway. Had to pull off and unhook it. It fired right back up afterwards. I'd bet the vibration on a motorcycle could make the problem worse.

[78CB750CAFE]

is the coil you have the tach wired to the one firing the 2 cylinders that drop out at high revs?

[Canada]

I tried all of those things and yes it was cutting out the cylinders that the coils were feeding. I have resided to the fact that the "tach is wack". I am agreeing with Cannibal on this one! I have a 5:1 tach that looks and works better. I can't find 4:1 mini tach with a white face for the life of me. I'll just do the calculations in my head until I sort this out.

[rbmgf7]

i too say it's the tach. i had a 2" sunpro that worked for a while but then shorted itself out.

i was wanting to gut an electronic tach and transplant the internals into a standard CB tach. therefore, it wouldn't be neccessary to buy tach cables and the rev readings would be crisper

[Canada]

I hear ya rbmgf7. The idea of electric appeals more to me but not at the expense of fouled plugs and side of the road fixes. I like the idea of the transplant but I have grown privy to my mini gauges but I tend not to look at them all that often. If I ever get puled over I'll use the excuse that my gauges were too small to tell how fast I was going. When I look at the stock ones now they look too big but I understand why everyone still love that set up.