HELP! I need pictures of a 77 factory tach circuit board or info!

[1774btcrew]

I am in the process of getting all of my gauges working correctly. I had pulled out my factory 77 tach when I did my custom Speedometer and Tach. The problem is I cut up the wrong year housing and circuit board so none of my gauges work correctly. That isn't the biggest problem. I have checked and it shows a 77 circuit board as one year only. I have several tach clusters and I am not sure which cluster is the one that came out of my truck since it has been so long.
What I am looking for is pictures of a 77 circuit board with tach or information on how to tell the clusters apart. I know the early ones had amp gauges instead of volts and I know the later had electric oil pressure gauges. I am just lost on what makes a 77 unique. Any help would be much appreciated. It is finally down to 100 degrees here and I am motivated to work on my truck after a long hot summer!

[ray_mcavoy]

Originally Posted by 1774btcrew

I know the early ones had amp gauges instead of volts and I know the later had electric oil pressure gauges. I am just lost on what makes a 77 unique.

You're on the right track with the voltmeter vs. ammeter and mechanical vs. electric oil gauge. 1976 and 1977 were the only two years to use a voltmeter along with a mechanical oil gauge. In late 76 there was a slight change made to the warning lights at the bottom of the speedometer/tach. The early 76 clusters have a "Brake" light at the bottom of the speedometer and a "Fasten Belts" light at the bottom of the tach. The late 76 and 77 clusters have a "4 Wheel Lock" and "Oil" light at the bottom of the speedometer (even though they're not always used) and a "Fasten Belts" and "Brake" light at the bottom of the tach.

The early 76 tach cluster printed circuit will be marked with the GM part number of 8917776 and the late 76 - 77 one is #25015093. I've compared a couple of these and they look nearly identical except for the area where the above mentioned warning light bulb sockets go. GM must have been planning ahead because the 76 circuit has room for all 4 bulbs even though only 1 or 2 are actually used. The copper traces on the unused bulb holes are covered with the lamination plastic.

To ID the plastic cluster housings, first look for the mechanical oil gauge hole to know that you're dealing with a 77 or older housing. Then look at the ammeter/voltmeter mounting area ... one for an ammeter will have 2 rectangular spring clip holes while one for a voltmeter will have 3 rectangular holes (only 2 are actually used though). That'll get things narrowed down to 76-77. Finally, take a look at the warning light baffles near the bottom of the speedometer/tach area. Early 76 will only have 1 bulb hole offset to one side of the baffled area. Late 76 - 77 will have 2 bulb holes with a divider baffle cast in the middle.

[1774btcrew]

Thank you SO MUCH for taking the time to do that for me! That is EXACTLY the information I was looking for! I sell parts for these trucks for a living and just never got that in depth on the clusters. Just another great anomaly on the 77's! So many unique things to this year. Figures that I own two of them!
Jeff

[ray_mcavoy]

You're welcome!

I've done quite a bit of research on the factory instrument clusters and have been slowly working on creating an identification guide that I'll eventualy add to my website.

You're right, 77's definitely have a lot of unique one year only parts as well as being the first year for some items/options and the last year for others.

For reference, here is a pic showing a couple of 77 cluster housings. Clean white one in the foreground is for a tach cluster and the dark dingy one in the background is for gauges (without tach or clock)

[1Cobra427]

That is good info - I also own a '77 that as yet is still a driver but will tear into it when time/money is better. I have come across SO many unique issues for this year I am collecting info as I read that I will need in my future. Thanks for the post from someone watchin and learning!