I'm not quite sure where to post this but, here goes. I have a 69 conv. with a 455 (installed by prior owner) and am installing a tach. It has Mallory ignition with the HEI coil. It is not your typical coil with a pos and neg terminal. It appears to have 1 red wire and a 3 brown wire plug. I need to connect to the negative side of the coil and assume the 3 wire plug is the neg side. Should I splice into one of them? If so would it make any difference which one?

Help,
Steve

 

I deleted the other post. Need some additional info. Does it have a control box, what model number? What distributor model/part number?

 

If the cap has this over the coil it will have a tach terminal next to the red lead.

 

But if there is a control box that terminal may be n/a.

 

It is similar to the pic posted, slightly different. I suspect it is an older unit that has been in the car a long time. It is part # Mallory HEI Performance Coil pn 29212. If you look it up the yellow lead is neg. and a socket from the base of the distributor plugs into it. It has 2 brown wires and 1 black. It seems as if all 3 wires would be negative. I just don't understand why they would have 3 instead on 1? If there is a reason how would I determine which is best for a tach connection.




thanks,

Steve

 

Like this? The tach plug should still be there.
The three wire plug goes into the body of the distributor and connects to the module and they have nothing to do with the tach signal. There are two leads outboard from that three wire plug. One is for power and the other is the tach lead.

 

Here are the connections.

 

Well Thank You!!! I had found one similar pic on the internet and saw that the yellow lead was negative but it wasn't labeled tach. I didn't put it together that I plug into it from the underside. I wondered what it's purpose was since I didn't see anything connecting to it. Thanks again for coming up with that pic and clearing up my mystery. I'm looking forward to seeing the tach work!




Steve

 

Hei power/tach pigtails come in handy. Pretty inexpensive and available at your local autoparts store.

 

That would be perfect. I put a clip on the wire but am not confident I am getting a good connection. That would eliminate any question. At the moment the tach works irradically, It says I'm idling at 1500, which it is not, and then I can rev it up to 3500 and it bounces up and down a bit, but doesn't go much further. I'm hoping it is a bad connection. As mentioned I have a 69. I purchased a new old stock tach on ebay but for a 70/71. They are different, the newer one comes in a plastic enclosure and the 69 in a metal one. So I took the guts out of the new one and drilled a few holes in the old metal clock enclosure and mounted it in it. All seemed well. I now wonder if there was anything I might have done to affect the tach. I didn't drop it or bump it around, but it seems strange that I'm not just getting normal readings. so I am hoping the new connector is the fix

thanks,
Steve

 

There are plastic insulators that go over the studs for power and tach, if the newer gauge used a plastic housing I’m guessing there would be no need for insulated studs. And chance the tach terminal is occasionally touching ground?

 

After thinking about my previous comment, I don’t think grounding would cause your issue. If the power stud for the clock touched ground, it would blow a fuse. If the tach wire was grounding, I would think the engine would die.

The factory tach in my 69 worked the same with the points, HEI, then MSD ignition, and then finally the Holley fuel injection. Unless you have a bad connection, or a module that is sending a signal the tach can’t read, I’m leaning towards a gauge problem. Do you have another tach you could temporarily connect to the wiring IN THE CAR, at the back of the in dash tach? Then you could verify your wiring is sound.

 

I'm thinking its in the wiring also. Going to dig into it shortly. The plastic shell had the terminals mounted to it and I had to remove them and plug them directly to the new harness that was included. Possibly I have the reversed. It's frustrating, it was a lot of work removing the dash to get at all that stuff.

thanks,
Steve

 

I got it figured out, thank God, it was really frustrating me. Unfortunately i had to pull the dash back out to get at the wiring for the tach. As mentioned I modified a newer tach to fit my metal enclosed tach. i wanted to make sure the wiring was intact and correct. Sure enough I crossed the wires. I can't believe I did that,, seriously I can't trust myself anymore. I spent quite a bit of time swapping the internal parts and drilling holes in the metal enclosure. it now works beautifully and makes me smile! But I owe it to you guys who helped me figure it out. I never would have found the neg. terminal and would have just sat frustrated and no doubt cursing and throwing stuff. This site has been an incredible help to me in many ways.

thanks,
Steve