Installing an aftermarket tach on an Inline 6

[longorange68]

Got a 1977 250 I-6 that came in my truck when I got it. The only reason I know this is because when I went to the Auto Parts store I brought the rotor and cap with me and they said so. I just swapped rear end gears to 3.73's and I was curious of my RPM's at highway speeds. Went to Schucks and bough an el cheapo Equus tach to mount on my steering column. The connections are as follows, one fused 12V power, one tap off of the gauge illumination circuit, one to chassis ground and the other to either the tach port an an HEI style distributor, the negative leg of the coil, or one to a standard ignition coil. Got it all hooked up except I am not exactly sure where to put the final wire that hooks to the ignition system itself. No tach port that I can see on the distributor, and I can't tell what wire is negative on the coil. I have looked at my Haynes manual, and been on the internet for an hour trying to find out what I need with no luck.

[bigblockV6]

Your engine mat not necessarily be a 1977, the prior owner may have upgraded to a late model HEI distributor & coil. One way to find out what era the engine came from is look at the intake manifold, is the intake manifold integral with the cylinder head? If it is then it canm be a 1975 on up, if it's a separate piece that bolts to the side of the head then it's 1974 or prior. As far as the tach led one should go to the coil and the 12 volts wire should tie in with the positive lead of the ignition switch.

[longorange68]

Which leg of the coil do I need to tap, AND how do I differentiate between the two (positive vs negative)??

[redhead69]

usually 3 wires red, black, and either white or yellow (depending on year/style). On alot of GM style caps it has a tach male prong outlet which is just an extension of the yellow/white wire. I hope that helps

[Brad]

Yep, red is + black is - and the 3rd wire goes to the tach wire on the coil (yellow or white).