Confused about re-pinning 74 Gauge Cluster Connections to a 78

[Gregski]

putting in a 1978 gauge cluster with a tachometer into my 1974 GMC 1/2 ton that had gauges but had no tachometer

found the ORIGNINAL CLUSTER CONNECTIONS chart (thank you HATZIE) but am confused about repinning two things specifically

IGNITION - on my '74 two pink wires go to pin #5 and #12, on the new '78 cluster somehow three pink wires need to go to #4, #6, and #12

however...

RH TURN - on my '74 a Dk Blue wire goes to pin #11 but on the new '78 cluster somehow it also needs to go to pin #12

so there is a conflict on pin #12, who wins, the turn signal or the ignition? LOL

[Titomars]

Might be a dumb question but did you try tracing out the printed circuit for pin 12?

[hatzie]

No conflict at all. You need to remove terminals and repin the cluster plug to match your new gauge cluster. This is why you can't just plug her in and go. The Ammeter wiring could make a nice crispy mess of the 78 printed circuit too.

1978 IGN circuit #39 is on terminals 4, 6, & 16 only. You'll need to find an extra terminal with a 12" chunk of wire attached as a pigtail. I'd suggest one of the two ammeter wires you'll be removing as you will not be using them for anything else.

1978 LH turn circuit #14 is on terminal 11. This is the only thing on that terminal.
1978 RH turn circuit #15 is on terminal 12. This is the only thing on that terminal.

In the 1974 cab... Be very careful to remove and completely isolate the white and black ammeter wires found in terminals 2&3. Be careful to leave enough wire at the Instrument panel plug that you can use both terminals, if needed, for extra IGN, GND etc wires.
Remove both glass Ammeter fuses in the engine compartment and replace with blown fuses. The ammeter is on circuits 105 & 106 in the GM wiring diagrams. The 106 fuse will be close to the junction block I'm not sure where the 105 fuse is.

Instrument panel swap terminal guide from the 73-87 electrical FAQ.


Year specific GM wiring diagrams are in the RTFM link below. Download both 1974 & 1978 GM wiring diagrams.
Note: The cluster plug is shown in the GM wiring diagrams from the wire entry side not the terminal side... The terminal guide page from the FAQ is the opposite... Just to be confusing I guess.

[Gregski]

Originally Posted by Titomars

Might be a dumb question but did you try tracing out the printed circuit for pin 12?

Good idea thanks, I did just that this morning, and took the rocket surgery out of this, I even made a mens men diagram of it, that a dummy like me can read, even color coded

Here's how you would read it

BLUE light - Hi beam
- use Pin #1 and Pin #10 for ground

YELLOW lights - cluster lights
- use Pin #2 and Pin #10 for ground

GREEN lights - turn signals
- use Pin # 11 for left turn signal and Pin # 10 for ground
- use Pin # 12 for right turn signal and Pin #10 for ground

ORANGE lights - 4 Wheel Lock & Choke Lights
- use Pins #14 and #16 for 4 Wheel Lock
- use Pins #15 and #16 for Choke
(* Pin #16 may just be a ground)

RED lights - Belts and Brake Lights
- use Pin #13 for Belts light and Pin #10 for ground
- use Pins #17 and #16 for Brake light
(* Pin #16 may just be a ground)


done, nice and easy and in English

and there aint nothing on there for IGNITION (Pins # 4, 6, 12)

Pin #4 goes to the Volt Meter

Pin #6 goes to Oil Pressure / Water Temp / Fuel (may just be a ground)

Pin #12 goes to Right Turn Signal

as a matter of fact I recall driving my truck with the entire gauge cluster unplugged and sitting on the work bench in my garage so as far as I'm concerned it has no ignition intelligence at all, but I'm learning so open to ideas

[Gregski]

Originally Posted by hatzie

1978 IGN circuit #39 is on terminals 4, 6, & 16 only.

Thanks HATZIE you're giving us a lot of info so I want to break it up and go point by point.

The diagram I found shows it being on 4, 6 and 12, see the last column below

[hatzie]

I think the author of that sheet dropped the ball.
Position 12 is definitely the turn signal and 16 is definitely ignition power in 1978.
Apparently the brake, choke, & 4x4 lamps use a switched ground.

[hatzie]

Originally Posted by Gregski

Good idea thanks, I did just that this morning, and took the rocket surgery out of this, I even made a mens men diagram of it, that a dummy like me can read, even color coded

Here's how you would read it

BLUE light - Hi beam
- use Pin #1 and Pin #10 for ground

YELLOW lights - cluster lights
- use Pin #2 and Pin #10 for ground

GREEN lights - turn signals
- use Pin # 11 for left turn signal and Pin # 10 for ground
- use Pin # 12 for right turn signal and Pin #10 for ground

ORANGE lights - 4 Wheel Lock & Choke Lights
- use Pins #14 and #16 for 4 Wheel Lock
- use Pins #15 and #16 for Choke
(* Pin #16 may just be a ground)

Terminal position 16 is ignition power. These lamps use a switched ground.

Originally Posted by Gregski

RED lights - Belts and Brake Lights
- use Pin #13 for Belts light and Pin #10 for ground
- use Pins #17 and #16 for Brake light
(* Pin #16 may just be a ground)

Terminal position 16 is ignition power. These lamps use a switched ground.

Originally Posted by Gregski

and there aint nothing on there for IGNITION (Pins # 4, 6, 16)

Pin #4 goes to the Volt Meter

Pin #6 goes to Oil Pressure / Water Temp / Fuel (may just be a ground)

Pin #12 goes to Right Turn Signal

If you look at the 1978 GM wiring diagram... cluster plug positions 4, 6, & 16 are ignition power (circuit #39) Usually that's a Pink wire with a Black tracer stripe or just plain Pink.

The way you resolve the difference between my JPG showing the terminal side of the plug and the GM wiring diagrams showing the wire entry side of the plug is to find terminal 1. GM conveniently kept the High Beam Indicator wire on terminal 1 for all of the trucks that used the Delphi 8900371 eighteen terminal plug. The high beam lamp is circuit #11.

Originally Posted by Gregski

I recall driving my truck with the entire gauge cluster unplugged and sitting on the work bench in my garage so as far as I'm concerned it has no ignition intelligence at all, but I'm learning so open to ideas

Analog cluster... Ignition power and Ignition intelligence are 2 very different things. These are not computer controlled clusters so she will run with the cluster unplugged... but the gauges and lamps do need juice when the key is on if you want them to work.

I clipped the pertinent section from the 1974 & 1978 wiring diagrams. 1974 doesn't have a circuit # legend but the 1978 circuit #s are the same with the exception of the ammeter wires #105 & #106 that are not present in 1978.

1974 Gauges

1978 Gauges

[1774btcrew]

Very nice info everyone! Wiring makes me go cross eyed! I just never can seem to wrap my head around it! I have the same problems with home electronics. Mechanical stuff is no problem for me though.

[Gregski]

Originally Posted by hatzie

In the 1974 cab... Be very careful to remove and completely isolate the white and black ammeter wires found in terminals 2&3. Be careful to leave enough wire at the Instrument panel plug that you can use both terminals, if needed, for extra IGN, GND etc wires.

OK, I am about to do this, and in my 1974 the ammeter is on Pin #3 black wire and Pin #4 black wire with white stripe.

Pin #2 is gray wire for cluster lights.

Originally Posted by hatzie

Remove both glass Ammeter fuses in the engine compartment and replace with blown fuses. The ammeter is on circuits 105 & 106 in the GM wiring diagrams. The 106 fuse will be close to the junction block I'm not sure where the 105 fuse is.

Can I just remove that entire black and white wire with the fuse in it that runs from the small black two prong junction box on the firewall to the bulkhead connector on the firewall on the driver side?

[hatzie]

Originally Posted by Gregski

OK, I am about to do this, and in my 1974 the ammeter is on Pin #3 black wire and Pin #4 black wire with white stripe.

Pin #2 is gray wire for cluster lights.



Can I just remove that entire black and white wire with the fuse in it that runs from the small black two prong junction box on the firewall to the bulkhead connector on the firewall on the driver side?

Right. 3&4 not 2&3.

Removing the whole wire works too... That's the black with white tracer wire circuit 106.
The black wire for Circuit 105 goes from the firewall plug, right next to 106, to one of the two fusible links on the 3/8" battery lug on the starter. Not sure where the inline 4A AGA glass fuse is in that wire.