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1990 F150 XLT Lariat Extended Cab Short Bed
5.8L EFI V8 E4OD Auto 2WD Dual Tanks
I was having occasional issue every few months (it's done it 5 or 6 times that the starter would stay engaged after the engage started and when I turned motor off immediately. Usually fixed by disconnecting battery and adjusting the starter solenoid if that didn't fix it (I recognize this isn't a real fix)
I have a new starter and battery cables (minus negative which was reused from a 1980 truck that's wiring was melting so maybe it's a bad ground cable) which is grounded to the bar that goes from the air intake manifold to the block. But it did the previous issue when I had a new cable in it.
Now I turn the key and there's a click from the starter solenoid and nothing else. Battery voltage reads 12.56 Volts on the battery at solenoid and on block.
Another item of discussion is the starter was loose. Bolts out by an inch or so but I know I tightened the bolts down really good. It was twenty degrees out and I still have scars from my hands rubbing on the metal and bleeding to tighten them down.
First step is to replace the starter solenoid. Sounds to me like its sticking. I got a cheap Chinese one from O'reily that stuck and did exactly what you describe. I returned it and got a duralast one from autozone and never had the problem since then. If it does it again, give the solenoid a hard tap. If it stops, the problem is found.
(minus negative which was reused from a 1980 truck that's wiring was melting so maybe it's a bad ground cable) which is grounded to the bar that goes from the air intake manifold to the block.
You have two different issues IMHO. The above is a problem. The battery ground needs to go directly to the block, preferably in the factory position. Anything less is asking for trouble.
Power is calculated by the the square of the current draw multiplied by the resistance. Even a small amount of increased current or resistance is magnified exponentially. You need to keep the ground path resistance to a minimum. Grounding the battery to the location you described is not good.
The tachometer is driven either by the coil or the ignition system. If the engine's not running, even if it's turning, if there's no spark, the tach doesn't respond. The speedometer is mechanical and driven from a cable attached to the transmission; it will register whenever the transmission output shaft is turning. That explains what you're seeing. 'Course, with a C6 (sorry, I'm not familiar with them), it's possible that even if you shift it to drive and then tow it, with the engine off the torque converter might completely disengage and the driveline won't turn the engine at all.
Why are you towing it with the transmission in gear anyway?
Did the starter tighten up when you tightened the bolts? Maybe you should just put a little loctite on the threads.