Just installed dash tach in '72 GTO.....have questions
 

I just installed an original dash tach in place of my non-functioning clock. I have a new tach harness to install as well. Where do I install the harness? Under the hood?; I did not see any connectors under the hood for this. I don't need another engine harness do I? Or, do I just trace the clock harness back to where it plugs in under the dash and plug the tach harness in its place? Do I need a new printed circuit to go with it as well?

Heres a picture of where your tach harness comes thru the firewall. You use the clock harness plug-in for your light harness for your tach. As far as I know that's it! Hope this helps?

Lyn.

Unless you have a 455 HO engine in your car your distributor should have points inside of it. For the stock GM tachometer you run one wire from the back of the plug-in on the back of your tach to the "-" side of the coil. The exterior metal enclosure of the tach MUST be grounded to the dash sheet metal for the tach to work. It's pretty simple.

I believe I found the hole for the wire in the firewall. It has a plastic push pin installed from the interior side. Is this hole accessible from the interior? Where is the approximate entry point? Is it near the fuse panel? Plus is there a wire plug/grommet sold for this?

On the back of the tach there is a metal 'tang', the same that was on the back of the factory clock. Is this the plug you are referring to? Also, I believe that this is a '68 or '69 tach that I have. It was sold to me as a '72 but I think the seller was incorrect. Would the tach year make any difference?

The tang is to ground the tach to the rest of the gauges.By tang you mean a thin metal strap looking item?

I looked at the back of the tach. The 'tang' is held on with a small nut. It might have been left on from the previous Pontiac it was installed in. If I remove the nut the tang will come off. The factory clock in my '72 has the same thing. I'm guessing its the power wire. Does this also need to be re-attached to the tach, or should I just tuck it up under the dash?

I just discovered what it's for, it's to power the illumination lights. I originally thought it powered the tach. I still need help with the other questions though, thanks.

You may want to get that tach tested to make sure that it works first, most used, GM factory tachometers that are over 40 years do not. That or they are way off on calibration.

These old tachometers are not difficult to wire up however just a friendly warning: if you wire it incorrectly there is a good chance that you will damage the circuit board. I would take the car to a place that restores GM musclecars and have those guys test it and wire it up for you.

When I added a factory Rally Gauge cluster and in dash tach into my GT-37 from the other 1971 GT-37 I had, I just routed the tach wire through the same gromet the vacuun line for the A/C used. It came out in the center of the firewall very close to the distributor. :)

Points or HEI?

The 72 tach is grounded through a metal strap from the tach to the gauge cluster. Green square on the picture.

The tach gets its power from either the TACH connector on the HEI coil cover or from the coil if using points.

The 2 holes are for the lighting. The harness is wrong on the picture but original looks the same; a 'Y' from the clock light harness.

Last picture is the wiring diagram from the 72 Service Manual.

http://i22.photobucket.com/albums/b3...ps3f097fdf.jpg
http://i22.photobucket.com/albums/b3...ps64712268.jpg
http://i22.photobucket.com/albums/b3...psae485e7a.jpg

Thanks for the diagram, that helps. I would like to give a description of what the harness looks like. My car has points, btw. There is one long brown wire with an in-line fuse and a prong connector, I assume this goes on the negative terminal screw on the coil? There is a pink wire with a black line that has a clear-ish looking plastic connector, and at the other end of the harness the brown wire and pink wire w/ black line meet in an 'L' shaped black connector. Are the clear looking and 'L' shaped connectors already provided under the dash to plug into? And it looks like I might have to drill a hole for the brown coil wire, there is no hole provided for it.

According to that diagram, it looks like there is a tach lead and a clock lead that goes to that metal 'tang' that I mentioned earlier. Should that tach lead already be provided in the factory harness even though mine came with a clock?

Lighting harness is the same for the clock or tach.
I think you are using a '72 tach harness with a '70-1 tach.
Adding pictures of harnesses with descriptions plus a '70-1 tach back.

Here is a picture of the back of a '72 tach.

http://www.forums.maxperformanceinc....3&d=1244833867

The tach I have is definitely not a '72. Looking at that picture the black 'L' shaped plug goes into the back of the tach. The tach I have installed has a single prong or tang on the back. Should I now just buy an actual '72 tach, or can I still make the one I have work? I also have a '68 style tach harness as well, it's just a long brown wire with an in-line fuse.

I believe the '68-9 tach harness will work the same as the '70-1 harness (single tang in the back of the tach). Length may have been the difference. All you should need to do is drill the hole thru the firewall, fit the C-terminal end thru and get a grommet on the metal edges of the hole. Was there ink stampings on the back of the tach w/a PN to know the year it came from (doesn't really matter, just curious)?

There are no labels or ink stampings on the back of the tach.

Another Tach Wiring Question
 

Revisiting this thread, hoping to get some guidance on wiring a mis-matched dash tach.

I have what I think is a 72 tach, PN 5657922 (based on HFR's great post). It has the two connectors labeled "Coil" and "12V", as shown in the post.

I put the tach in my 70 dash and, without realizing the tach wiring had changed, I ordered a tach harness for a 70, which has only a single wire for the coil.

I have no issue/concern running this single wire from coil to tach.

Question is where to get 12V. Apparently, per HFR's post, a 72 harness had a pink wire that pulled 12V off the fuse block. I assume it needs to be switched.

My 70 dash harness has a conveniently dangling orange/black-stripe wire with a blue connector hanging near the tach:

http://forums.maxperformanceinc.com/...1&d=1508037533

Any chance this is a switched 12V line I can use for the tach? Ohmeter shows the wire connected to the "CLK LTR" fuse on the block, which makes sense for a clock, but I don't think an electric clock would be switched.

Question is whether the "CLK LTR" fuse is switched or not? If it is switched, I think I'm good to go. If not, where do I connect to the fuse block that is switched (remembering this is a 70 fuse block, not 72)? Is there a connector pin on the fuse block that is intended for switched accessories on a 70?


And for anyone reading this far, I'm still hoping someone can tell me where the metal clip on the tach light harness goes. The only place I see is on the plastic wire jacket, and that doesn't seem right. I can leave it hanging, but would feel better if I knew where it went.

From what I can tell, the clock power is not switched, so won't work to power the tach.

I will order a harness for a 72, but still need to know where to plug power on the fuse block.

My 70 fuse block does not have a spade for "IGN":

http://forums.maxperformanceinc.com/...1&d=1508083176

Looking at this diagram for the fuse block, can anyone confirm the spade labeled "ACC" is switched?

Looking in my service manual, it says the ACC spade lug is intended for a window control relay and a power antenna.

Is the ACC spade a switched power terminal?

http://forums.maxperformanceinc.com/...1&d=1508083403

Finally have the dash back in car, putting stuff together.

I did order the 72 harness to match the tach and am trying to route the tach harness so revisited this thread.

I noticed the connector tabs on my tach were oriented differently than those on HFR's post (compare the connectors on mine with those on HFR's). This could have ended badly, or at least kept my tach from working. :o Thanks again to HFR for sharing the photo, as the connections are clearly labeled and I'll go see if I need to flip mine to get the power lead going to the right place.

I have a 72 tach in my 70 and the wire routing diagram below shows the 70 tach harness.

Anyone willing to scan and post this same diagram for a 72?

http://forums.maxperformanceinc.com/...1&d=1514333174

I don't want to drill another hole for a grommet just for the tach harness, as carguy66cars shows. I will try to route the harness through the center grommet used for AC control vacuum as 2002Z4CSS suggested. It may not reach because it has a separate lead for power from the dash fuse block that the 70 did not have, but it's worth trying.