1986 XLT won't start when hot.

Hi guys.

So I have this '86 Bronco that I'm building for my son. Runs great and starts great when cold. After about 10 min of running when I turn it off it don't wanna start again. Seems like a low battery. When jumped it starts. First think I did I bought a new battery, next I got a new starter solenoid. Still won't start when hot. Checked battery with multimeter it shows 12.5 v. Started the truck and checked battery again this time it read 14.5 v which tells me that alternator is working. Turned off the truck, got under, put positive multimeter on starter, black on frame. Told my son to crank it. Multimeter showed 9.6-10v. Truck started. Told my kid to **** it and try again. This time it registered 5.6v. Truck didn't start. I'm running out of ideas. Any of you guys had symilar problems? Please advice.

Welcome to the Zone!!

Possibilities:

 Any of the major ground connections may be corroded (the starter grounds through it's connection to the engine block), (the engine block grounds to the battery (-) at the front left side of the lower block), (the cable from the battery to the frame and block may be corroded where the cable and lug connect)

Remove and clean all battery connections and reattach.  Inspect the cable to lug connection, if any corrosion replace the cable.

Remove the starter and clean the area that it seats to the block.

If you have bad grounds, Heat can either improve the connection or degrade it due to expansion and contraction of the metals. 

Lastly it could be the starter is weak and when hot just doesn't have what it takes.  The voltage drop indications at the lugs most likely indicate a voltage supply connection problem.

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Don't replace the starter before you do the cables.  

I have the same truck.   It is common for heat soak to indicate when the starter is on it's way out.   The hotter the engine is, the slower the starter will turn.   Take the starter in to be checked at your local parts store.

My 79 Bronco did that too. I changed out all the electronics. It ended up being the brand new distributor.