RPM tach problems

[74 DART SWINGER]

Today I got a new autogage 3 3/4 8k rpm tach from the parts store and when I hooked it up it wouldne work. I had the 12V(to battery),grounded, and to - on coil hooked up. When I turn the key to on, the tach moves to 0 like it should. When I start it at first the dial flickers real fast then goes down to 0 rpm and doesnt pick the rpm up when its idleing or when I rev it. If anyone has any suggestions please let me know!! thanks!!!

[daredevil]

gotta have a pos feed. the coil grounds it out and this is how it registers. (green coil neg)black groung and white?positive

[74 DART SWINGER]

yes, the BLACK is ground(iv tried it to body and - battery), GREEN(- coil) and RED(12V source,iv tried battery and and other 12v locations). The WHITE is for the light in the tach which i didnt hookup.
With the combo of all these they all had the same result. I have the FBO ignition,should i switch out coils?
thanks

[Shaun]

Have you tried another spot for the ground?

[74 DART SWINGER]

The ground is working. What i did was plug the 12v in then leave the ground off and the tach didnt do anything but once i grounded it,the tach moved to 0 thats how i know its workin. It must not be picking up on the signal that makes it move eventhough I have tried every possible way. Thanks

[Shaun]

Have you tried hooking it to your distributor? It sounds like you may have it crossed somehow. When you start it,it moves,then when you have the key in the on position it falls back down.It almost sounds like you have your ignition wires backwards,or maybe the ECU to ballast wires spliced backwards.
Do you have an ECU,or MSD?

[74 DART SWINGER]

I have an ECU. I dont beleive I have any wires crossed or anything,ill check around tomarrow. How would you connect it to the distributor?

[ehostler]

You can't connect it to an electronic distributor. If you've ruled out all wire connections, then the tach must be faulty.

[dwc43]

Hook the tach to he pos side of the coil. If it works, you have the wires hooked up backwards at the coil. IF it does not work, like E said, it's just a bad tach, send it back for a new one.

[oldguyalso]

If I remember correctly there are some tachs that won't work with electronic ignition with out an adapter. My mind won't come up with the right stuff right now! LOL If I'm wrong someone will tell us! It seems I vaguely remember having to use one with an MSD setup? God, I love getting older!

[72Challenger]

Those adapters are necessary for multi-spark ignition systems like the MSD, but not for a single-spark one like the FBO.

[74 DART SWINGER]

Does the ECU and ballest have to do anything with making a rpm tach work? Tomarrow ill get on this and see what I can do.

[dwc43]

Neither of them will effect your tach. It gets the signal from the neg. side of the coil. If the engine is running and you wired the coil right the tach will work.

As for the tach adapter for the MSD box mentioned above, it's needed cause the tach is wired to the MSD box and not the coil and some need an adapter. Yours will not require it.

Your tach is either bad or the coil is wired wrong with the pos coil wire on the neg. side of the coil. If that's the case and you wired the tach up correctly then it would never work. The coil will fire and the engine will run if it's wired backwards, but you will loose some performance from it since it's not designed to run that way.

[74 DART SWINGER]

Today I tried the tach on our chevy truck( lol) and the tach worked flawlessy. So I went to hook it up to the dart and nothing. I had 2 other coils,I tried them and still nothing. This is really starting to get agrivating when you spend 2 mins to hook it up on the truck and it works but spend 1 day and still nothing for the dart. Anyone know what ignition demon maybe be lurking in there?

[daredevil]

Was this a bad tach or was it something else? It worked on the chevy so what was the outcome. Couldnt you use a test light connected to positive and turn the motor over and see if the coil neg grounds intermittently like it should. Would like to know the answer to this one.

[74 DART SWINGER]

Yes I got it to work What I did was I got a wiring diagram and found out that the wires that were on the coil were switched. So once I got them in their correct place I hooked the tach where it suposed to go and bingo it worked. I guess it was my dumb mistake at first when i rewired the engine compartment a year ago and forgot to put those 2 wires in their places.

[dwc43]

I thought that was going to be your problem. Glad you got it figured out and fixed. IT should run much better and make a little more power now that the coil is wired correctly.