Low voltage in Hot Weather!

[Hammocklife88]

1999
Ranger XLT
3.0L

Having low voltage anytime I drive it in hot weather. On a cool morning the starter and engine turn over just fine. I drive it to work, turn the truck off and if I try to turn it back on it sounds like the battery is low and the starter is barely able to turn over.
If I am able to get it to turn over, the voltage is around 11 volts. Does not increase while driving. The battery and alternator were pulled and tested good.

any advice?
could the battery cables be the issue? Are they old and getting too hot with the alternator trying to recharge the battery?
Thank you for any help!
Bryan

 

[BroncLander]

Maybe the belts are getting hot and loosening up? Man I don't know what else would do that. I know my phone hates getting too hot.
Doesn't Ford use an external voltage regulator? Maybe that's the culprit

 

[Uncle Gump]

If your alternator tested good... it's not charging the battery in your truck. You should get close to 14 volts with the engine running.

Does the charging/battery light come on when running?

Is the connector on the alternator good and tight?

 

[RonD]

+1 ^^^

And yes, it reads like a bad battery cable as well, and corroded terminal on the 175A Mega Fuse, it's on the outside of the engine bay fuse box.

Similar to this: https://www.explorerforum.com/forums/attachments/bjbmaxifuse-jpg.75239/

Disconnect both Positive and Negative battery terminals
Disconnect both terminals on the Mega Fuse
Clean them all
Reconnect all

Key off
Test battery voltage and remember it or write it down, the exact voltage
Say it's 12.7v<<this is made up you need YOUR VOLTAGE
12.2v or less is a dead battery, charge it, 11v is a really really dead battery

Leave volt meter on Positive and move black probe to alternator's metal case
Should read EXACTLY the same volts(12.7v), if not your Negative battery cable has internal corrosion, or external that you missed or can't see, change the cable
If you get 12.7v then test Positive cable

Put black probe on battery Negative and Red probe on either Mega Fuse terminal
Should be exactly 12.7v
Test the other terminal just to see if Mega Fuse is OK, should also be 12.7v

You can also test the starter motor's positive cable, hard to get at some times, but should read exact battery voltage, if lower then that cable is bad, but more on this later, because could be bad starter motor

Alternator
Start the engine
Battery should now read 13.8v to 14.8v, usually 14v+ just after start up
If so then all is well
If its under 13.5v then alternator is dead, shut off the key
Test alternators wires
Test battery Volts again, remember it
B+ terminal is on the back of the alternator, stud/nut terminal
Use volt meter's red probe on B+ and black probe on alternator's metal case
Should see battery volts exactly, if not, B+ wire to Mega fuse is bad replace it

Unplug 3 wire connector on alternator
Test Yellow wire, black probe still on alternator's case
Should be Battery Volts, if not ALT SYS FUSE 30amp is blown, its in engine bay fuse box
Test light green wire, should be 0volts
Turn on the key
Retest Light green wire, should be battery volts but can be a few "points" lower since key is on
If not then Fuse 15 in CAB FUSE BOX is blown

If all 3 wires test as OK then alternator is bad, period, faulty test
But plug back in the 3 wire connector
And make sure the White jumper wire is connected to the single spade terminal on the back of alternator, it can slide in "wonky" and not have a good connection

Start engine again, same as above, 13.8v to 14.8v, if lower alternator is bad


Slow cranking can of course be low battery voltage
But slow cracking only when cables and starter motor are hot can be corroded cables or failing starter motor
You tested the cables, except the bigger one that goes to starter motor
But this would NOT effect alternator's ability to output 13.8+ volts

 

[BroncLander]

This is my solution to a 40 year old gauge cluster with questionable reliability