Tach cluster swapout

[70K10Hunter]

Been thinking about upgrading to a cluster with a factory tach. My question is... Is the wiring harness that plugs into the back of the cluster wired for a tach? will I need to add a wire(s) and plug into my HEI, or replace the harness? The fuel gauge moves location to the left side of the cluster. do I need a different printed circuit. Is there anything I'm missing?
Truck in question is a 77 3/4 ton 4x4 no cruise, no tilt, pretty basic.

[gmachinz]

The tach cluster requires its own harness-I have a few extra ones in really good shape-PM me if you want one. You need to find a tach cluster with a mechanical oil gauge too since 1977 was the last year for it-they went electric in '78.

[70K10Hunter]

is there a separate receptacle in the back of the cluster the harness connects too?

[ray_mcavoy]

Yes, as gmachinz said, the tach uses a separate harness that provides it's power, ground, and connection to the HEI distributor. The tach has it's own 3-terminal connector on the back that this harness plugs into.

The existing rectangular 18-pin connector that plugs into the back of the cluster may or may not be a direct plug-in swap. That all depends on what style of instrument cluster is currently in your truck (gauges vs. warning lights) and on the year of the tach cluster you plan on installing.

If your truck currently has a factory gauge cluster (with a voltmeter, oil pressure gague, and temperature gauge), then a matching year tach cluster will be a direct plug-in swap. Like gmachinz said, 77 was the last year for the mechanical oil gauge so watch for that when you're picking out a tach cluster. And 77 uses a voltmeter so you'll want to avoid the earlier 73-75 clusters that have ammeters. So you're basically looking for a 76 or 77 cluster to be a direct swap. And yes, the tach cluster uses a different printed circuit to take care of the re-located fuel gauge. Some of the early 76 clusters also had a slight variation in the indicator lights at the bottom of the speedometer/tach (they don't have provisions for a 4x4 indicator).

However, if your truck currently has a warning light cluster (lights but no gauges in those 4 smaller openings on the left side) then the 18-pin cluster connector will have to be re-pinned to work with any gauge cluster (with or without a tach). Re-pinning involves carefully extracting the terminals from the connector and re-inserting them into the correct cavities to match up with the gauge/tach cluster pin-out. And if you have to do this, it opens up the possibility of using a later year cluster with an electric oil pressure gauge (the oil warning light wire can be re-purposed for the oil gauge signal).

[70K10Hunter]

Thanks' ray... yes I have the gauge package not the idiot lights.

[2120]

I'm looking to restore my 77 tach cluster. The green back housing on my 77 cluster has several broken tabs. Could a 78 tach housing be used and modified to restore the 77 tach cluster?

It seems that they're only reproducing 78- and up Tach housings.

[ray_mcavoy]

Originally Posted by 2120

I'm looking to restore my 77 tach cluster. The green back housing on my 77 cluster has several broken tabs. Could a 78 tach housing be used and modified to restore the 77 tach cluster?

It seems that they're only reproducing 78- and up Tach housings.

Most 77's had mechanical oil pressure gauges. Although I've read that some built later in the model year had changed to electric oil pressure gauges.

If your 77 happens to have an electric oil gauge, a 78 housing should be a direct replacement.

But if it has a mechanical oil gauge, you would have to:
(A) Find a good used late-76 to early/mid-77 tach cluster housing.
(B) Repair your existing housing by gluing on replacement mounting tabs salvaged off another cluster housing (the outer mounting tabs are all the same so most any 73-87 donor housing should work for this).
(C) Modify a 78+ tach housing to accept a mechanical oil gauge. That would likely involve cutting out & gluing in the section that's behind the oil gauge. The mounting points are different for the electric vs. mechanical oil gauge and the cluster housing for the mechanical gauge has a hole for the pressure tube to pass through.
(D) Convert your truck over to an electric oil pressure gauge and use a 78+ housing as-is. In addition to an electric gauge & sending unit, you would also need the 78+ printed circuit and you would have to re-pin the cluster connector to match.

[2120]

Thanks for all the Info ray. That's what makes this site so great.