Cluster Upgrade - 09/19/2015

Never one to be satisfied until everything works and is to my standards, I set out to finish the gauge cluster work I started earlier this year. I had already upgraded to rally gauges, but wanted an economy gauge to measure engine vacuum. I also wanted a trip odometer to better track my miles between fillups and in a touch of OCD, I wanted to fix the Fasten Seat Belt light in the dash. The buzzer and seat belt interlocks all worked, but the light on the dash didn't and if someone other than me drove it they probably would be really bothered and forget why the buzzer won't go off...

Spare economy gauge cluster. I've got 4 sets of rally gauges for these cars... I'm putting the 120MPH speedometer out of the '74 and putting the '73-74 gauges overlay in because it doesn't have the "Unleaded Fuel Only" lettering. Goofy, but why not? The '74 speedometer is within a 5-6 thousand miles of the existing one anyways. The lens on the economy cluster is beautiful, but cracked and broken in 4 places. The '74 lens has no cracks at all, but is filthy after sitting outside in the elements for over a year at the body shop...

'74 lens is cleaned up using a combination of Windex and Simple Green. Came out pretty nice.

Speedometer is transferred. Dirt and grime inside housing is blown out.

Fully assembled and ready to go. Note the black button. There isn't a cluster lens without a hole in the left side, either you got a clock that had its adjuster coming out the hole or you got an economy cluster or a block-off plate (base Catalina only it would appear) that has this little black button covering the hole.

The back side. The only difference I can see between the 2 printed circuit boards is the clock gets an extra piece reaching out with always on +12V to power it. So the 2 printed circuit boards are not 100% interchangeable, but if you run a +12V feed to the clock it doesn't matter. Little vacuum port on the back of the cluster gives the signal to the economy gauge.

Factory engineering drawing showing the general routing of what needs to be added.

Make up a little nifty vacuum harness.

Old cluster comes out.

Vacuum hose is fed through old speedometer grommet hole. Plastic trim cover has to come off to get the hose.

Snaked through the steering column and ready to install.

Investigation of the seat belt light circuit found that the idiot light cluster shares the +12V feed with the lowest pin, which has 2 pink wires (GM indication of +12V switched). Oddly enough, the rally clusters that I have ALL have a 4th pin that instead of sharing comes from the plug itself. Since all of my clusters are '74-76, maybe something changed between '73 & '74 due to the seat belt starter circuit interlock? No clue. Either way, added a splice and soldered in a new pin 3rd from the bottom that duplicates the functionality. Seat belt light should work now.

Cluster goes in without issue.

Seat belt light and brake warning light work, all indicators work, dash lights work. Doing this at night isn't fun due to the lack of light, but it sure makes testing these circuits easier.

Hose is routed out and up.

And routed behind the main engine harness and to the manifold vacuum feed on the vacuum tree on the back of the carb. Definitely don't want the line touching anything hot and melting.

And it works! Electronic gauges all work correctly too. Out of curiousity I checked the gauge accuracy with a handheld vacuum pump... Each major tick on the gauge corresponds to roughly 2.5inHg, so the red stops around 5-7.5 and the maximum tick is about 17. The engine pulls 19.5inHg at idle so it will be maxed out. When you blip the throttle it goes down just like the vacuum signal does, so it works.

Not bad at all. None of this is life changing, but it is definitely one of those convenient little items that I enjoy. Not to mention I was REALLY tired of looking at a broken clock...

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Last updated September 19th, 2015