1977 gauge cluster questions

[3x2GTO]

My 77 cluster has gauges, brake warning light, no tach or clock, and it's seen better days. I'm guessing I can't install a cluster that has gauges and a clock? Are the new printed circuits a good quality, maybe some brands better than others? Also LMC sells circuit board clips, I don't remember these being on my gauge cluster, Are they necessary?

[ray_mcavoy]

Yes, you can change over to a cluster that has gauges with a clock (or tachometer if you wanted).

In fact, since your truck is already equipped with a gauge cluster, that makes a gauge/clock cluster (or a gauge/tach cluster) a direct plug-in swap. The gauge, gauge/clock, and gauge/tach clusters all use the same cluster connector pin-out for a given year. And the wiring for the clock (or tach) is separate from the rest of the cluster wiring.

However, the key to making it a direct plug-in swap is getting a replacement cluster from a compatible year donor truck. GM made several changes to these clusters (and therefore the connector pin-outs) over the years so not just any 73-87 cluster is a direct plug-in swap.

73-75 had ammeters instead of voltmeters so avoid those years for a direct plug-in swap in your 77.
78-up had electric oil pressure gauges instead of mechanical so avoid those too for a direct swap in your 77.

That leaves you with finding a 76 or 77 cluster. And even then, there were a few subtle changes. For example, it was in this time frame that they changed over from the small "windows" for warning lights at the bottoms of the speedometer & fuel gauge faces to wider "windows" that each accommodate 2 warning lights. And I think that made for a few minor changes to the connector pin-out with regard to the wiring for some of those lights. I've also read that they may have changed over to electric oil pressure gauges late in the 77 model year.

The 78-up clusters (which are probably easier to find) can be made to work in your 77 but you'd need to do some re-pinning of the cluster connector and add a sending unit for the electric oil pressure gague. Might have to swap out the temperature sending unit too since they went to a different Ohm range sometime around 79 or so.

Yes, you will need the circuit board clips (either new ones or re-use the old ones if they're not rusty or corroded). The clips snap into the square holes in the back of the plastic cluster housing and are what make the electrical connections from the printed circuit to the studs on the backs of the gauges.

[3x2GTO]

WOW, thanks for the info, that's a lot for me to digest, so I think I'm going to try a new printed circuit and replace any defective gauges that I find. I did just notice that my cluster has the big "brake" warning light on the left side, and also smaller brake / seat belt warning lights at the bottom of the fuel gauge pod, not sure if this correct or maybe swapped at some time? The previous owner removed the bulb from the big brake warning lamp, because it will stay lit with key on or off. I'm guessing there may be a problem with the brake control valve or E brake switch, not sure which would throw this light on though.

[ray_mcavoy]

You're welcome!

Yes, the "brake" warning light down in the lower left along with the smaller "brake" light (next to the seatbelt light) in the bottom of the fuel gauge face is factory original. This was another one of those subtle changes they made over the years.

The "brake" light started off down in the lower/left corner on gauge & warning light clusters. But when a clock was added (or a tach which used that lower left for a fuel gauge), the "brake" light had to be re-located to a different spot. For 73-76 they put it in the bottom of the speedometer face. Then for late 76 or 77, moved it to the bottom of the fuel gauge (or tach) face. And finally for 78 they eliminated the "brake" light in the lower/left (for all cluster styles) and made the one at the bottom of the fuel gauge (or tach) the standard location. Since 77 was right in that transition period, those gauge clusters ended up with the 2 "brake" light provisions.

I don't think 77 originally had the "brake" warning light wired up to the parking brake (that came along in later years). Instead, it's just connected to the pressure differential switch in the proportioning valve. So try temporarily unplugging the connector from the proportioning valve. If that allows the "brake" light to go out, the switch is bad (assuming you don't have a brake pressure issue that has tripped the switch).

But if the light stays on, check the wire (tan if I remember correctly) to see if it's shorted to ground. Look for any place where the insulation may be chafed and allowing the wire to touch metal or for any place where the wire might have gotten pinched under a bolt/bracket/etc. The switch is on the ground side of the circuit (completes the circuit to ground when closed) so the wire being shorted to ground would cause the light to stay on.

[3x2GTO]

I just checked the E brake, and your right, no switch there. The "BRAKE" light stays on whether the connector is plugged in or not, so it must be shorted, I'll trace that out. The problem may lie in the firewall connector, my horns weren't getting power and one of the spade terminals inside the firewall connection was bent over and not making contact with
the dash harness. Thanks again for all your help, I'll post results next Week, when I find the short.