Refer to the
1973 Wiring Manual. Verify ignition power (circuit 39) and ground (circuit 150) to the cluster. Inspect and service the cluster-to-cab ground attachment above the park brake pedal assembly.
While you're at it, verify the most fundamental and overlooked aspect of vehicle wiring: Make sure all of the cab, engine, and frame grounds are installed, clean and functioning properly...
...Grounds are subject to harsh environments. They are virtually ignored during routine maintenance. And, they are easily overlooked when troubleshooting. Over the years I've heard variations of, "Nah! They're too simple...." "Grounds don't have any moving parts! There's nothing to wear out!" "A ground couldn't possibly cause all these symptoms!" "Come on, what could go wrong with a ground!?!" Yet, they are a common source of headaches on trucks....
Now, is a great time to inspect and service all of the vehicle grounds: (1) battery-to-engine; (2) battery-to-right frame rail; (3) battery-to-right radiator support; (4) right rear cylinder head-to-firewall; (5) engine-to-left frame rail; (6) dash-to-left kick panel sheetmetal; and (7) rear lamps-to-bed sheetmetal, taped splices, and frame rail....
Which end of the speedo cable are you having trouble reconnecting? Is the speedo making noise or jumping or...?
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