[Shiny]

As suspected, my tach connection was reversed.

It was an easy fix to rotate the tabs while mounted in the dash. The insulator between the tabs and the tach housing is keyed both ways, so is easy to reverse them as I had done after repairing the tach.

The 72 harness routing worked out fine, routing it along the main bundle to the firewall connector block. Pink is 12V, brown goes to tach. I had to clip a plastic "key" off the plastic connector housing, as the 70 block does not have a connector spade in the "IGN" location as seen in my earlier photo. With the key gone, I could then plug the connector onto the tab labeled "ACC".

The tach lead was long enough to go through the central grommet where the AC vacuum line passes, avoiding the need to cut the firewall for another grommet. The lead and inline fuse reached the tach negative terminal without issue.


Tach Wiring


Tach Power Connection

[2002Z4CSS]

The factory tach needs a switched 12v power source. Usually a pink wire will be switched 12v power. The tach feed circuit goes to the negative side of the coil as mentioned prior if you are running a points distributor or the "tach" terminal if using a GM H.E.I. distributor. The tach metal housing needs to be grounded. Mine had a metal type strap that would have grounded the clock to the cluster if I remember correctly. The tach lights can be connected to the gray wire that is from the headlight switch to feed the dash lights and can be dimmed with the dimmer. That wire should be in the harness close to the clock was. Sometimes it will be tapped to the harness but since you already had the clock option just reuse it. The orange with black stripe circuit is a constant 12v power source. Don't connect that to the tach.

[Shiny]

Yes, 2002Z4CSS. Finally figured all that out. Thanks for the good summary.

My 70 had no clock, just a block-off plate, but the "unswitched" orange wire for the clock power was there. I just left it hanging. Good warning on the ground strap, which I did install. I bought a repro set from Ames, which did not have the "correct" strap, but had one that worked well enough.

My notes in this thread are about putting a 72 tach into 70 wiring.

Net:

a) use a 72 tach wiring harness, not 70

b) clip the ear off the power connector housing so you can plug it into a different accessory power outlet on the fuse block


As I've highlighted here, the 72 tach power (pink) connector is "keyed" to fit only into the "IGN" slot on the fuse block. But that slot was not used in 70. There is no spade as you can see. I think it's awesome how Pontiac keyed every power slot differently to reduce chance of mis-assembly. Hard to do that in today's world of "standard" everything.

With the ear clipped off the connector housing, it will plug in any slot. I'm still guessing the "ACC" is switched and plugged mine there. The ACC pin is for power windows and power antenna, which my car doesn't have. I never heard if these were switched or not, so I'll verify after I finish the dash, radio, and steering column install.

[Held for Ransom]

The ACC is switched and powers on with key/switch in ACC or ON positions. I don't know if piggybacking off the IGN wire on the ignition switch would have given you the correct 12V to use. All looks like it should give you no problems.

[dv657172]

Why does a 71 dash tach have what appears to be only a single connection for the coil (brown wire) while a 72 dash tach requires an additional wire connection for 12V?

[OG68]

in short, Tach internal circuitry changed in 72 requiring a 12 volt power source.

Others may chime in with a more concise answer.