[ray_mcavoy]

Originally Posted by Gregski

I upgraded my 74 no Tach to a cluster from a 76 I think with Tach, here's what I learned

1. Used Tachs don't work, as a matter of fact all old gauges don't work, or work poorly, I would recommend a brand new one, or even a good quality aftermarket one if I was to do it again

From what I've encountered, the electronics in the 73-75 & 76-77 tachs aren't all that great. And the 73-75 circuits are potted / epoxy encapsulated, making the circuit boards non-repairable.

The electronics in the 78+ tachs are based around a National Semiconductor LM1819 air core meter driver and tend to be much more reliable. They do sometimes have issues with the calibration resistor that causes the tach to read too high or peg the needle. But that's an easy fix as hatzie pointed out --> Fix Squarebody GM Truck Tach

Originally Posted by Gregski

2. I also wanted to retain my old speedo with the new Tach and guess again, can't or at least not with some doing, the non Tach Speedo is different believe it or not than the Speedo that comes with the Tach I think the front plate facia won't fit, you would need to do a swaparoo not sure it is worth it, I may be attempting it cause my newer used Speedo stopped working, see #1 above

Originally Posted by hatzie

You can get into trouble is the warning lamp slot in the bottom of the tach and the speedometer. It's possible to have 0, 1, or 2 warning lamps in the bottom of both 5" gauges.

Yes, I believe that is where Gregski probably ran into an issue with his speedometer swap.

The speedometers in the "regular" 73 through early 76 clusters had a solid face plate without any provisions for a warning lamp. When the tach (or clock) option was added in those years, the "brake" warning light was re-located to the bottom of the speedometer. So those clusters used a speedometer with a narrow "window" or "slot" in the face plate to accommodate that light.

Sometime during the 76 model year, GM changed the clusters to include a wide "window" or "slot" at the bottom of the 5" gauges. With this configuration, the speedometers easily swap among the different cluster styles.

Originally Posted by Chaplain

I apologize in advance for the simi-highjacking of your thread. Any of this familiar?

Yup ... that appears to be a 73-74 or 75-77 or so tach harness. I could be more specific if I could see the end that attaches to the coil / HEI TACH terminal.

Those years used brown (instead of white) for the tach signal wire that passes through the firewall & connects to the coil. And a solid pink (instead of pink w/black stripe) wire for the IGN power connection to the fuse box. The one pictured has Packard/Delphi 56-series connectors that plug into the older style fuse boxes that take glass fuses. A Pack-Con connector was used on the newer tach harnesses that plug into the fuse boxes with the plastic blade style fuses.