GMT400 1990 Chevy K1500 383" crate motor, installed 12 years ago, maybe 40,000 miles on it. Runs great. starts EASY but sometimes takes 5 or 6 or 27 attempts at turning the ignition switch ( KEY) . hot, cold, doesn't matter. Might start first try, might take 27 tries.

Fresh starter rebuild, LAST year with NEW ZEN ( made in Brazil, decent quality ) starter solenoid, NEW ground cable from battery to engine block.
Battery fully charged, tests at 770 Cold Cranking Amps. Normal charging voltage 14.4 volts with headlights and heater ON high.
Everything is clean and tight. Good voltage to starter solenoid. normal resting volts 12.6 at battery. sag to 12.4 at starter when cranking.
The Question: DOES the 1990 K1500 have a starter RELAY, and if so; WHERE is the start relay mounted ?
I see that the actual ignition switch is a STANDARD Motor US105.
Does the full starter solenoid trigger current go THROUGH the ignition switch, or is there a starter RELAY hidden somewhere ?
I have looked pretty deep into the under dash AND underhood wiring, the only Relays I have found are the HORN relay and fuel pump relay.

Is it cranking those time but not firing?

Is there 12V on the starter solenoid terminal WHEN IT'S IN THE ACT OF SCREWING UP? Not, when it works right; butt, WHEN IT'S IN THE ACT OF SCREWING UP?

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. I will put a test lead on there, check it, and report back.... Thanks for the tip !
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. Strutaeng: "Is it cranking those time but not firing?" .....
Starts instantly when engine rotates. will roll start at one MPH.

So nothing happens at all when you turn the key and it malfunctions? No clicking just dead?

Audible CLUNK at starter (solenoid). It's Trying. Acts a lot like insufficient amps to fully energize the PULL-IN coil in solenoid.
I have just today obtained a new Standard Motor parts US105 switch, plan to install soon, under steering column.
Been meaning to try the switch for 5 or 6 years.
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Wiring schematic I have found shows NO start relay, just a 12-ish gauge purple wire to Clutch switch, then Yellow wire to START contact on IGN switch.
I will report back after new IGN switch install.

I think your starter is bad. There is only enough amperage going through the ignition switch to activate the solenoid, and you are doing that. Only if the contacts are bad in the solenoid would it not be the starter.

90 do not use a starter relay. Purple leaves the ignition switch goes through the park neutral switch and then straight to the starter from there. The only place there is a connection is at the bulkhead at the firewall.

either starter or battery cable most starters are garbage from parts houses nowday's they give you a lifetime warranty because you'll wear the bolts out changing them. Just the world we live in now QC and long life are things of the past.

Time is up. This needs a resolution.

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. Installed NEW starter a few years back. Last winter when I had NO START, even with repeated key operation, I was able to warm starter with electric heat gun, get a start, so I removed starter, replaced brush set and new HIGH-Grade Solenoid ( Brazil manufacture). Starter is pretty clean, probably not more than 200 starts on the fresh brushes and new solenoid.
Battery load tests at 800 Cold Cranking Amps.
New Negative lead from battery to front of engine block, crimped, soldered heat shrink 2 gauge welding lead, 24" long.
6 gauge ground from frame to block.
10 gauge from battery to body.
The newly installed ignition switch: (Mexican) Standard Motor US105 install has so far given reliable first turn starts, for about a week.