i have a 1990 f250 with manunal transmission. It wont start due to that fact that the ignition wire isnt getting enough voltage. I pulled the wire where it connects to starter solenoid and it was reading 6V. Replaced clutch saftey switch and there was no change. Any ideas of what else could be causing the lack of voltage?

The truck would start up fine after it had been sitting for a few hours but wouldnt start if it was running a few minutes before. wont start at all now

 

6V as measured with a high impedance DVM is an open circuit.

 

Look at the lower left portion of the diagram below. It shows the ignition switch terminal pin out.

The Red/Light Blue wire is terminal ST and goes to the clutch safety switch.
When you turn the ignition switch to Start 12 volts from the two BATT terminals which are hot at all times passes 12 volts to the ST terminal Red/Light Blue wire.

If you measure battery voltage at the BATT terminals and 6 volts at the ST terminal when the key is turned to start probably the contacts in the ignition switch are bad or there is something wrong with the wiring at the ignition switch.