Tore into it today and figured out my problem. Had to add an additional ground for the Fuel gauge. The 1990 used one ground. The 1978 cluster (I believe it's a 1978 now, I just bought another tach cluster from a 1979 and it says UNLEADED FUEL ONLY under the Fuel gauge) has multiple grounds. The 1990 cluster has a better laid out ribbon, and it is also labeled better. Anyway if anyone is curious, here is a list of what pins are what wires after I wired it up in place of a 1990.
Old Cluster with Tachometer (1978 or 1979??? No mechanical gauges)
Pin # - Fuction - Wire Color from 1990 K5 Blazer
1 - High Beam Indicator - Light Green/No Stripe
2 - 12 Volt + Controlled By Headlamp Switch - Light Gray/No Stripe
3 - Ground - Black
4 - 12 Volt + Keyed To Ignition Switch - Pink/Black Stripe
5 - Oil Pressure Gauge From Sender - Light Brown/No Stripe
6 - 12 Volt + Keyed For Fuel Gauge - Pink/Black Stripe
7 - Fuel Gauge From Sender - Pink/No Stripe
8 - Ground (for Fuel Guage) - Yellow (I cut out the seat belt warning lamp for this, tied into the Black ground wire)
9 - Temperature Gauge From Sender - Turquoise/No Stripe
10 - Ground - Light Blue/Black Stripe (tied into Black ground wire)
11 - Left Turn Signal Indicator - Light Blue/No Stripe
12 - Right Turn Signal Indicator - Dark Blue/No Stripe
13 - Seat Belt Reminder Lamp - Yellow/No Stripe (I ended up cutting this out and using the wire/pin for my fuel gauge ground)
14 - 4x4 Indicator (there was a separate wire not part of the 18 pin connector, I have a doubler, so I did not use this)
15 - Choke Indicator - Dark Brown/White Stripe (Is now my Service Engine Soon lamp)
16 - Ground - Dark Brown (tied into Black ground wire)
17 - Park Brake/Unequal Brake Pressure Indicator - Light Brown/White Stripe
18 - Not Used
Here is how the 1990 K5 was originally wired up with the factory electronic speedometer:
Pin # - Function - Wire Color
1 - High Beam Indicator - Light Green/No Stripe
2 - Ground - Black
3 - Oil Pressure Gauge From Sender - Light Brown/No Stripe
4 - 12 Volt + Keyed From Ignition - Pink/Black Stripe
5 - Low Voltage Warning Lamp - Dark Brown/No Stripe
6 - Not Used
7 - Not Used
8 - Coolant Temperature Gauge From Sender - Turquoise/No Stripe
9 - 12 Volt + Controlled By Headlight Switch - Light Gray/No Stripe
10 - Left Turn Signal Indicator - Light Blue/No Stripe
11 - Right Turn Signal Indicator - Dark Blue/No Stripe
12 - 4x4 Indicator Lamp - Separate wire from 18 Pin Connector
13 - Seat Belt Warning Lamp - Yellow/No Stripe
14 - Fuel Gauge From Sender - Pink/No Stripe
15 - Park Brake/Unequal Pressure Warning Lamp - Light Brown/White Stripe
16 - SES (Service Engine Soon) Lamp - Dark Brown/White Stripe
17 - 12 Volt + Keyed From Ignition Switch - Pink/Black Stripe
18 - Speedometer From Sender - Light Blue/Black Stripe
Martin
