Mustang has NO BATTERY power when HOT

I have a 1969-1970 (mismatched parts) with a 351W that will not turn over when hot. The starter won't turn over at all.. It sounds like it's trying but even with a short cool down and jumper cables it will just barely get going. Once it cools overnight it will start up perfectly with lots of power to spare. What could the problem be??  

Bad/worn starter is the first suspect.  Ford starters usually "die" like that - spin cold, no spin hot.  Especially with steel tube headers on the car.  

Also corroded/high resistance connection between starter and block / block and body / negative side of battery and body.  Weak battery, reduced charging capacity in alternator. corroded/high resistance positive cable.  
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.

Chris hit the main two culprits. Unless it's a big-block or has aftermarket headers I usually look for the resistance/bad cable issue first. High resistance in the ground cable, say, from corrosion or internal breakage, or even just loose connections, can positively ruin your day once the car gets hot.

Ensure all cables are clean, tight, and do not feel broken in the insulation. If it were my car I'd pull the cables off and measure them with an ohmeter just for piece of mind. Standard two-gauge stranded battery cables are around .000162-ohms per foot, give or take a bit for temperature. If you have four-gauge cables you might also want to consider upgrading to two-gauge or even zero-gauge...

HTH...
Mark  
Mark Houlahan
Editor
Modified Mustangs & Fords Magazine

1965 FFR Roadster 427W/TKO-600
1966 Mustang 289-4V/C4
1968 Mustang 4.6L Three-Valve/5R55S auto
1990 Mustang 347/AOD
1998 Escort 2.0L SOHC/F4E-111 Auto
2002 Explorer Sport Trac 4.0L V-6/5R55 Auto

got a digital volt meter?????    set it to 20 volts DC...  all tests on this post are with the volt meter at 20 volts DC

make sure the battery is fully charged...  engine off.. head lights on for one minute.. then off.. measure the battery voltage..
12.65 volts dc is a full charge...
12.45 volts dc is a half charge..
12.25 volts dc is a quarter charge..

with the battery fully charged...     hook up the the digital volt meter to the negative terminal...  the other to the block...   have someone who knows how to do it...   read the meter while the engine was cranking...    should be less than 0.4 volts..
this tests the ground cable from the battery to the engine block..

can you reach the starter post??????  the starter post to the the battery positive post should also be less than 0.4 volts..  anything higher than that ... perhaps thicker cables...

with the engine running...  head lights on.. test from the battery negative post.. to the engine block...  less than 0.02 volts

then the tester leads between the body and the negative battery... less than 0.02 volts..

one last test... positive battery  to the alternator output terminal...
there should be less than 0.08 volts...

these are voltage drop tests...  the volt meter only looks at differences in voltage... if there is resistance in the wire...  it will show up here better than trying to ohm it in place...

some chain parts stores will come out and test your battery and starter...     with the amp clamp on one tester lead... you can load test the battery...

with the amp clamp on one battery cable... you can load test the charging system and see how many amps the starter is taking...

hope this helps....  

Excellent post there Wayne  
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.