weird ignition issue after installing tach

[95teetee]

ok, put my Sun supertach 2 back into the Duster. Wired everything up, then the car wouldn't start. Turned over fine but not firing. I went out and tripped the solenoid to check for spark, and this time it fired right up. Then I realized (after it had warmed up for a couple minutes, so was more agreeable) that it was starting as I released the key, but not until then.
Not a ballast resistor issue ( i have one of those self-contained electronic distributors that don't use the BR or an exterior control module), but here's what happened. I unhooked the ground from the tach to see if it solved the problem (and it did).
But the tach still works-then it dawned on me that because I had screwed the tach to the column, it already had it's ground. Could the second ground somehow cause this issue? Wouldn't think so, but can't figure out why it happened otherwise.
The car starts normally with the tach's ground wire unhooked...nothing is shorting out anywhere, I double-checked all of that...odd.

 

[abodyjoe]

I don't see how screwing the tach to the column could complete a ground. Isn't the tack plastic? Maybe the tack is bad? Could you have kicked a width in the column when you screwed the tach to it??

 

[67Dart273]

You need to start at "ground zero."

Disconnect the tach wire from the ignition, and I assume you hooked it to coil negative?

Or does this "ready to run" thing have a separate tach lead?

What this SOUNDS like is that when you rassled around in there hooking up the tach, you bumped loose the BROWN wire coming from the key, which used to go to the ballast resistor on the coil side.

When you twist the key to "start," the dark blue ign wire goes cold (IGN1, dark blue)

When you twist the key to "start", ign voltage is / was supplied by the brown (ign2) coming from the ign switch, through the bulkhead, to the + side of the coil.

Normally when you remove the ballast resistor, you must tie the dark blue/ brown together.

 

[95teetee]

joe- the tach has a metal body, so I'm guessing the screw is grounding the system to make the tach work (it's an old hole- the tach was mounted there years ago, but I just put it back in)- I started wondering myself if maybe the screw hit a wire, but it's only about a quarter-inch long machine screw- still, it's possible I guess.
67Dart273- yep, I hooked the tach to the neg. side of the coil. Everything is working fine as long as the tach's ground wire is disconnected, so I'm pretty sure everything else is still connected normally (it still starts and runs fine as along as that ground wire is disconnected, oddly enough...

thanks guys.

 

[BillGrissom]

You need a multi-meter to figure this out ($3 at Harbour Freight, sometimes free). Measure resistance from the tach case to its gnd wire. If <5 ohm, they are connected. If the case is isolated, then something is bad in the tach. Measure resistance from tach in to gnd. It should be >1000 ohm. My guess is it is much less and is shorting coil- by hooking it up. Not sure how it can work with 1 wire connected, but it is an AC signal so can jump a gap (capacitive coupling).

 

[67Dart273]

67Dart273- yep, I hooked the tach to the neg. side of the coil.(it still starts and runs fine as along as that ground wire is disconnected, .

THIS SHOULD BE your huge clue

Either you have

the tach wired wrong

the tach is not compatible with that ignition (Tachs don't always hook to the coil neg, as in MSD)

the tach has an electronic fault/ problem