hey, i just hooked up a new autometer tack and the tach jumps all over the place. i have a 4pin hei distributor and the signal wire is hooked up to the tach output at the distributor.....the tach is already hooked up for v8 from factory so its not the setting....do you guys think i have a bad coil?

 

MSD stands for Multiple Spark Discharge. The box gives multiple sparks for each spark plug firing, not just one. Because of that, you don't wire the tach to the distributor, there is a special output terminal on the side of the MSD box that gives the correct signal for the tach.
"When all else fails, read the directions". See pages 2 and 7 in the owners manual pertaining to the tach hook up.

 

i already removed the msd box.....its now just hei....at the hei connector on the distributor, it has a terminal for the tach....thats what i have the signal wire hooked up to....any other suggestions?

 

Oh, that just took all the fun out of my answer! Your jumpy tach would be more of an indication that your ignition module may be on it's way out. Do you have a spare one to pop in and try? Your bad MSD box may have left a nasty present for your module.

 

lol.....yea, that damn box.....but no, i dont have another module....i thought the tach signal runs off the coil wire, thats kinda why i thought it might be a coil problem....i'll see if i can borrow my bros module and see if that helps.....thanks agian.

 

Your tach reads the electrical pulses of the ignition, which is controlled by the internals of the distributor, the module being one of them.

 

Hey jeu510, do you have an original tachometer in your car? I looked at this issue once a few years back, and there is acompatability problem with hei distributors and original GM tachs - the GM unit is analogue based, and requires a pulse from a coil. GM did not have the hei technology and you will need a part termed a 'tach fitter' intalled/spliced in to make the tach work correctly.
I wanted to instal a repro, original type tacho, and my car had a hei distributor already in it. The choices are;
1. reverting back to a coil and standard distributor, and keeping the original tacho
2. find a very good auto electrical shop and get them to produce a 'tach fitter'
3. keep the hei, reinstall a msd box (which will do the job of a 'tach fitter')
4. change the tacho and original wiring out (bad choice, but you keep the hei)

The hei is an improvement, but not worth the price of the original tacho. You have a way to keep them both - msb box or the mysterious 'tach fitter' I got that term from Ricks Camaros. They sell all the parts, but make the point that hei and old tachos (even repros) do not work with hei distributors.

For the whole story, I decided to go with a new binnacle with a 6 gauge set from autometer, and keep the hei. The cost of a new repro tick-tock-tach, plus console gauge set, plus wiring plus the mystery tach fitter, and it really was a no brainer. The cost of all the new items was less than the repro items, more accurate, and better for me. I was not doing a restoration.

 

no, i dont have a original tach...i wanted to have a new modern look so went with a auto guage from autometer.....i'm going to replace my ignition module later today like camaro69 had suggested...now that i think about it, he's probrably right.....when it was working with the msd box, the module was removed and the box was acting as the module. now that the msd box has been removed, the old module was put back....i'll replace it later today and update on the problem....thanks for your input....

 

well it is either the signal out, from the module, or a fault with the tach. shortens up the effort to trace the fault. good luck.

 

Ok, I finally got the chance to replace the ignition module and capacitor....its all good now....thanks again for your inputs guys....