Question about factory tachometer

[72blu]

Hey guys! I recently aquired a gauge cluster for my 1978 cheyenne that has a factory tach. The cluster came out of either a 78 or 79 3/4 ton dually. Unfortunately the plastic lens cover as well as the plastic casing that the gauges sit in is in pretty bad shape (cracked and missing chunks). My question to you guys is would it be possible to simply take the tach and small fuel gauge out of the old casing and put it in the casing thats currently sitting in my truck now? My truck isnt a factory tach truck so im not sure if it will have all the wiring required to switch it over. Thanks!! I also included a pic of the gauge cluster with the tach
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[ray_mcavoy]

Welcome!

The tach vs. non-tach cluster cases are different in 2 key areas. Those are the areas behind the tach and the lower left corner by the small fuel gauge.

Here is a pic comparing non-tach (top) and tach (bottom) cases:


These are both for 77 clusters with mechanical oil pressure gauges so that area will be different as compared to your 78/79. But as you can see, the tach case has a rather large cut-out on the right side behind the tach. I think a person could duplicate that by cutting a non-tach case. Modifying the lower left corner to accept the small fuel gauge looks like it would be a little more work though.

As for the wiring, if your 78 is already wired for a gauge cluster, the 78/79 tach cluster will be a direct plug-in swap. If it originally had a warning light cluster, re-pinning of the connector (and a couple other changes) will be necessary. The tach itself uses a separate 3-wire harness that plugs into power (fuse box), ground, and the distributor (HEI TACH terminal).

Finally, if you replace the cluster case, you will have to remove the flexible printed circuit from the back of the tach cluster and transfer it over to the modified/replacement case.

[72blu]

Originally Posted by ray_mcavoy

Welcome!

The tach vs. non-tach cluster cases are different in 2 key areas. Those are the areas behind the tach and the lower left corner by the small fuel gauge.

Here is a pic comparing non-tach (top) and tach (bottom) cases:


These are both for 77 clusters with mechanical oil pressure gauges so that area will be different as compared to your 78/79. But as you can see, the tach case has a rather large cut-out on the right side behind the tach. I think a person could duplicate that by cutting a non-tach case. Modifying the lower left corner to accept the small fuel gauge looks like it would be a little more work though.

As for the wiring, if your 78 is already wired for a gauge cluster, the 78/79 tach cluster will be a direct plug-in swap. If it originally had a warning light cluster, re-pinning of the connector (and a couple other changes) will be necessary. The tach itself uses a separate 3-wire harness that plugs into power (fuse box), ground, and the distributor (HEI TACH terminal).

Finally, if you replace the cluster case, you will have to remove the flexible printed circuit from the back of the tach cluster and transfer it over to the modified/replacement case.

Thanks for the awesome info! Id really rather not take my chances and start fooling around with my stock case so my next question would be where is the best place to buy reproduction gauge cases? Ive looked around myself but cant seem to find any online.
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[TKCR]

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=74

[willem445]

Originally Posted by 72blu

Thanks for the awesome info! Id really rather not take my chances and start fooling around with my stock case so my next question would be where is the best place to buy reproduction gauge cases? Ive looked around myself but cant seem to find any online.
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I am also wondering! Out of three trucks Ive taken apart so far, all three have had cracked gauge shells/cases. Ive been searching for a new replacement one but have no idea what to search or how to find one.

[wilkin250r]

Anybody have experience with the LMC gauge casing? My casing is cracked, and I haven't had very good luck at the junkyards finding a replacement in good condition.

[Ski-me]

I've used gorilla hair fiberglass filler to make repairs to the cluster housing. Once it dries, clean it up with a file or grinder on drill, to make it work.

[TKCR]

Ebay has a few that i have seen. And then these places:

http://www.usa1industries.com/gauges...ts_per_page=50

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=74
http://www.lmctruck.com/icatalog/cc/full.aspx?Page=76

[ray_mcavoy]

Yes, cracked and broken cluster cases can be repaired if they're not too far gone. In fact, I had to make a couple repairs to the one I pictured earlier. It had the typical broken tab on the left side and the mount at the upper right corner was cracked too. I used some Permatex Plastic Welder which is basically a 2-part epoxy with some added solvents to help it bond to plastic. That was followed by painting it with some white Krylon Fusion paint.

Provided the quality is decent, the reproductions / replacements look to be a pretty good deal for folks with 78+ clusters.

[72blu]

UPDATE
Basically, I cut out the plastic piece that has the three holes for the small fuel gauge and grafted it onto one of my other good cases. I also cut out the larger hole for the tach. The case kinda looks hacked up but it works non the less and the ribbon fits perfectly.

The only issue ive run into is my oil pressure gauge has stopped working. Im positive its not the gauge itself but rather a connection issue with the new tach ribbon. All the other gauges work normally so im not sure as to why this one is being stubborn. Anyone with knowledge about cluster ribbons that could offer me some advice it would be greatly appreciated!!

[ray_mcavoy]

Originally Posted by 72blu

The only issue ive run into is my oil pressure gauge has stopped working. Im positive its not the gauge itself but rather a connection issue with the new tach ribbon. All the other gauges work normally so im not sure as to why this one is being stubborn. Anyone with knowledge about cluster ribbons that could offer me some advice it would be greatly appreciated!!

A couple places to check:

(1) Take a close look at the little metal clips that snap into the holes and make the connection between the flexible printed circuit and the studs on the gauge. Those clips are made of steel and sometimes they get a little bit of rust on them that prevents them from making good electrical contact. Also look for any light corrosion or tarnish on the printed circuit where those clips contact.

(2) The flexible printed circuit has contact "fingers" that make contact with the terminals on the cluster plug. Sometimes those fingers get detached from the plastic backing and can crinkle / curl up when the plug is inserted. If this has happened, you can often flatten them back out and glue them back down to the plastic backer. Also check for slightly bent / distorted pins in the cluster plug that might not be making good contact.

[72blu]

Awesome! I had a feeling it might have had something to due with the old clips I used lol

[burban nate]

I just did some similar modifications to a '91 electric speedo housing to add the big truck tach/fuel combo and a vacuum gauge. I'll post up some pics later as I'm in the middle of painting it right now.

Nate

[72blu]

Thanks to you kind folks I was able to get it back together and finally every guage is working as they should

This was a great learning experience for me and I plan on doing another similar gauge swap for my fathers truck so ill make a new thread when I get around to doing it...