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1977 G20 Instrument cluster guages not working
#760097 May 02nd 2020 12:07 am
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Just picked up a 1977 G20 and the instrument cluster is not working. The guy that I bought it from said he thought the computer on the back of the gauges did not match up with the plug. I am looking for any help I can get. Many of the wires in this van have been cut since it was recently stripped and painted. The instrument cluster did light up when turning on the lights and that is all. Pictures attached.

Attached Files 20200501_211145.jpg20200501_211148.jpg20200501_211317.jpg20200501_211236.jpg20200501_212006.jpg20200501_212028.jpg

1977 Chevy G20 with 350
Re: 1977 G20 Instrument cluster guages not working
jtozier406 #760101 May 02nd 2020 4:26 am
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Looks like the correct gauge set to me, so the "computer" which is actually just the printed circuit board should be correct as well. I would be willing to bet money the previous owner has cut a bunch of wires they shouldn't have. Get a repair manual, a decent multimeter, and start tracing wiring.

The speedometer works off of a cable that has a coupling which will thread onto or push onto the back of the speedometer. It's hanging out of the dash in the 3rd picture.

Welcome to the site by the way. Go do an intro and let us know more about the van.


[Linked Image]
Re: 1977 G20 Instrument cluster guages not working
jtozier406 #760106 May 02nd 2020 10:52 am
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Before you go doing a bunch of tedious grunt work tracing wires, are you sure the plug wasn't just inserted upside-down?


[Linked Image from i.imgur.com]
Re: 1977 G20 Instrument cluster guages not working
Scrojira #760107 May 02nd 2020 10:59 am
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Originally Posted by Scrojira
Before you go doing a bunch of tedious grunt work tracing wires, are you sure the plug wasn't just inserted upside-down?


Nice thought. I will check that tonight! Also on the side with the fuel and temp sensors there is a threaded piece on the back of the cluster. Is something supposed to attach to that?

I will get pictures up soon with an intro.


1977 Chevy G20 with 350
Re: 1977 G20 Instrument cluster guages not working
jtozier406 #760109 May 02nd 2020 11:58 am
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Just tried flipping the plug and if I am not mistaken it can't be done. It looks like the plug has wider black plastic tabs on one side preventing it from installing the wrong way. I will look into tracing wires. I see some wiring diagrams for $20 online, are those any good?


1977 Chevy G20 with 350
Re: 1977 G20 Instrument cluster guages not working
jtozier406 #760111 May 02nd 2020 12:10 pm
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Gotcha. The plug on my '78 can be pushed in either way.


[Linked Image from i.imgur.com]
Re: 1977 G20 Instrument cluster guages not working
kursed #760119 May 02nd 2020 2:50 pm
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Originally Posted by kursed
Looks like the correct gauge set to me, so the "computer" which is actually just the printed circuit board should be correct as well. I would be willing to bet money the previous owner has cut a bunch of wires they shouldn't have. Get a repair manual, a decent multimeter, and start tracing wiring.

The speedometer works off of a cable that has a coupling which will thread onto or push onto the back of the speedometer. It's hanging out of the dash in the 3rd picture.

Welcome to the site by the way. Go do an intro and let us know more about the van.


The speedometer cable was plugged in. Should that have been working even if the other gauges are not since it is its own cable?


1977 Chevy G20 with 350
Re: 1977 G20 Instrument cluster guages not working
kursed #760147 May 03rd 2020 9:58 am
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Originally Posted by kursed
Looks like the correct gauge set to me, so the "computer" which is actually just the printed circuit board should be correct as well. I would be willing to bet money the previous owner has cut a bunch of wires they shouldn't have. Get a repair manual, a decent multimeter, and start tracing wiring.

The speedometer works off of a cable that has a coupling which will thread onto or push onto the back of the speedometer. It's hanging out of the dash in the 3rd picture.

Welcome to the site by the way. Go do an intro and let us know more about the van.


Is the Haynes repair manual the best option or should I try to hunt down a GM service manual?


1977 Chevy G20 with 350
Re: 1977 G20 Instrument cluster guages not working
jtozier406 #760148 May 03rd 2020 10:20 am
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The speedo cable should work on its own if attached properly. Pretty simple set-up, the cable goes from the back of the dash panel and attaches near the transmission. Would the threaded opening on the back be for an oil pressure line attachment. Haynes and Chilton manuals are usually the basics for repair, you can't beat having a factory manual, tons of detailed and comprehensive information along with illustrations, exploded views and wiring schematics.

Re: 1977 G20 Instrument cluster guages not working
Meanmachine #760149 May 03rd 2020 10:34 am
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Originally Posted by Meanmachine
The speedo cable should work on its own if attached properly. Pretty simple set-up, the cable goes from the back of the dash panel and attaches near the transmission. Would the threaded opening on the back be for an oil pressure line attachment. Haynes and Chilton manuals are usually the basics for repair, you can't beat having a factory manual, tons of detailed and comprehensive information along with illustrations, exploded views and wiring schematics.


Thanks, and yeah the Speedo was not working either but it was plugged in. I will try to find a factory repair manual.


1977 Chevy G20 with 350
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