Hooking up factory tach harness-78 Chevy

[6772fan]

I bought a factory tach from BigJake97 on the truck board. It's from a 79 Blazer and I'm putting it in my 78 Chevy truck. It came with the factory tach harness and here's my questions:
The 3 prong connector plugs into the back of the tach, then there are 2 other wires that I don't know where they connect. One is oblong in shape with a male clip on one end and female on the other end. The other one runs about the same length and it looks like it connects to the fuse block, but can't tell where to connect it to the fuse block. I've already drilled a 3/4 inch hole in the firewall and connected the other end to the distributor. Also, the best I can tell they appear to have the same connections at the back of the guage cluster, since I have factory guages already, is it a plug and play or do I need to re-pin any of the connectors at the back of the gauge cluster? Any help on this is appreciated. Thanks, Lane.

[87Bob]

Originally Posted by 6772fan

I bought a factory tach from BigJake97 on the truck board. It's from a 79 Blazer and I'm putting it in my 78 Chevy truck. It came with the factory tach harness and here's my questions:
The 3 prong connector plugs into the back of the tach, then there are 2 other wires that I don't know where they connect. One is oblong in shape with a male clip on one end and female on the other end. The other one runs about the same length and it looks like it connects to the fuse block, but can't tell where to connect it to the fuse block. I've already drilled a 3/4 inch hole in the firewall and connected the other end to the distributor. Also, the best I can tell they appear to have the same connections at the back of the guage cluster, since I have factory guages already, is it a plug and play or do I need to re-pin any of the connectors at the back of the gauge cluster? Any help on this is appreciated. Thanks, Lane.

As I recall one is ground I connected mine to the group of grounds on the firewall. The red is a switched 12V through the ignition. Tan goes to the distributor. That's it. The circut board on lt lights the dash lights around the tach.

[Jonboy]

In '79, the pins on the cluster are exactly the same. I am pretty sure '78 is similar. I just checked my '79 wiring harness diagram to be sure.

[ToughBowtieTruck]

Yes 78 and 79 is the same. Some '79s switched to the 85 mph speedometer though, which won't affect anything so long as you don't use the 100 mph face plate on an 85 mph speedometer box, as they are calibrated different. There are markings on the back of the dash that designate which is which on the tachometer by the prongs. You have a power, ground, and tach signal wire.

I installed a factory tach cluster in my '77, the tachometer is completely separate from the rest of the cluster, and the pins remain the same for the fuel gauge and warning lights; the printed circuit reroutes the wires accordingly.

As long as you aren't using a '77-lower cluster, as 1977 was the final year for the true mechanical oil pressure gauge.

[BLE 'BURBAN]

You will have to re-pin the lead for the fuel tank sending unit as well since the non tach cluster has the lead on the right side of the harness connector and on vehicles with a tach, it's on the left side. Look at the back-side of both cluaters and trace the circuits from the gauge to the connector on both to see where you will have to move the sending unit wire to.

[ToughBowtieTruck]

Originally Posted by BLE 'BURBAN

You will have to re-pin the lead for the fuel tank sending unit as well since the non tach cluster has the lead on the right side of the harness connector and on vehicles with a tach, it's on the left side. Look at the back-side of both cluaters and trace the circuits from the gauge to the connector on both to see where you will have to move the sending unit wire to.

Mine works as-is. Plug and play. Never moved any pins or anything . . .

1986 tach cluster works also without any modifications, however I wanted the 70's style gauges and mechanical oil gauge.

[6772fan]

Thanks to all of the great advice from the folks on this awesome board, I can tell you first hand that a 79 factory tach cluster makes up with a 78 factory guage wiring harness with NO changes whatsoever. Once I figured out the tach wiring arrangement, it was a straight plug and play for the rest of it. I finished it up today, went for a lengthy test drive and everything works just as it should, even the fuel gauge required no re-pinning at the connector. Thanks again to all who responded and have a great New Year. Lane

[joe231]

My 80 cluster has the circuits in place for both the large fuel gauge and the small fuel gauge that replaces the clock (or the blank). I realize that I have the newer style of gauges, just figured the earlier ones were probably similar.