1989 v6 with 5 speed.
When I start up at -5 or colder the alternator gauge only shows about ten volts. No electrics work including turn signals and heater blower fan. After I drive it say 15 miles and stop and wait then start it pops back up to 14+ volts and electrics work. Kind of weird, anyone know what is going on?

 

First guess is your alternator belt is slipping when it's that cold, starts working when it gets warmed up.

It's probably an old and glazed belt and doesn't squeal.

 

Well I watched it spinning and not putting out more than 10 volts. Two days ago Green River, Wy. Minus eight started but only 10 volts drove to Rock Springs 2 degrees and only 15 mi away. Let sit for a few minutes engine off. Then started and 14 volts. Weird

 

Not that it matters but 1986 Ranger should have an AMP Meter, so Discharge or Charge only on gauge, no Voltage reading?
Are you testing with Volt Meter?

Clean the wires on the Starter Relay's Battery positive post(AKA: Starter Solenoid)

Remove each wire, clean it's end, put them back on post and secure
Clean battery negative terminal as well

"No electrics work including turn signals and heater blower fan"<<<these would work even at 8volts, so probably a wiring issue, the Battery post on the starter relay is where ALL 12v power comes from for the whole vehicle, and its where alternator's output is connected
So alternator is not "putting out 10volts", its not working at all

Could be the Voltage Regulator in the alternator is not working when its cold, but its not a mechanical setup like external voltage regulators were, its electronic so "likes the cold", does NOT like heat
The Brushes and rotor in alternator are mechanical, so could be the brushes are worn down and "freezing" in place so not making good contact with the rotor in start up, so no output

 

New battery and same symptom. Cold temps it does not show but 10-11 volts and no electrics until running for 15 mins

 

Alternator not working at all should show Battery Volts, new battery would be 12.8volts

How are you testing alternator?

Test is
Test battery volts before starting
12.3v to 12.8v is expected
Start engine
Test battery volts again
13.5v to 14.8v is expected, if lower than 13.5v then alternator is OFF

If you want to test the alternator wiring then key off
Test battery volts, REMEMBER it, lets say its 12.6volts
Test B+ terminal on the back of alternator, use alternators metal case as the ground
Expected is Battery Volts, 12.6volts, exactly, if lower then there is a bad connection

Unplug the 3 wire connector on alternator
Test the Yellow wire, should see Battery Volts, 12.6v, if not then bad connection
Turn on key
Test Green Wire, should see Battery Volts, slightly less is ok, say 12.4v, if not fuse is blown in cab fuse box

If all wires test OK then truck system is OK
Plug back in 3 wire connector, make sure White jumper wire is connected to it single tab terminal

Start engine
Test battery again, 13.5v to 14.8v is expected, if lower than 13.5v replace alternator, even if its new, its still bad