Low voltage truck dies

[Mhfco6]

I went to start my truck a few days ago and no start, and no headlights and no dash lights. While it was sitting I replaced the thermostat. After the thermostat I jump start the truck and took it for a spin. Headlights and dash lights slowly got dimmer and dimmer then truck finally died. I jump started it again and made it home and died again, luckily in my drive way. I jump started it again and checked voltage on the battery. It read around 11.90, but slowly creeps downward. I checked both the alternator fuses #35 and #38, they look good. I disconnected the battery while the truck was running and the truck died. Hooked battery back up and it started back up. I noticed the in dash voltage gauge reading low and high sometimes after the truck stops at lights and then when I take off again. It's always been reliable and starts right up until now. Not sure if I should get a battery or alternator first. What about an inline fuse? Any ideas? Thanks. I left truck idling for about 35-40 minutes and battery voltage dropped from 11.80 to 11.40

 

[RonD]

As you have surmised the alternator is not outputting any voltage, 0, none, nada.

In a '91 the voltage regulator is in the alternator so it is best to just put in another alternator.

If you want to check if fuses or Fusible links are blown you do that at the alternator.
On the back of the alternator will be the B+ terminal, it will have 1(or 2) larger wires held on by a nut.
Key off but battery connected
Test that B+ terminal for battery voltage, should be the same as the battery, it is a direct connection to the battery thru a fuse or fusible link.

There will also be a 3 wire connector on the alternator, unplug it
The Yellow wire should also have battery voltage, if not then a fuse is blown.

There will also be a Light Green wire, this should also have battery voltage but ONLY when the key is on, so turn on the key and check if green wire shows battery voltage.

Also while you are there the White wire needs a good connection, it is just a loop but make sure it isn't damaged or frayed.

If voltages check out then all fuses or fusible links are good, replace alternator.

Alternator, with engine running and working, will show approx. 13.6volts at battery, you can turn on the lights and heater fan to high and voltage will drop and then come back up to 13.6v, that s the voltage regulator working.
Just after starting engine regulator will recharge the battery, so voltage at battery will be as high as 14.9volts, after a few minutes it will start to drop down to the 13.6v area.
If voltage stays too high battery will "cook", fluid inside will go to vapor which will shorten the battery life

Battery could be damaged now, you will just have to wait and see.
Or if you want, and have a battery charger slow charge it over night and then unhook it from charger and test voltage, needs to be above 12.2volts, that 50% life, then let it sit for a few hours, unhooked, and test it again, if it is below 12.2volts shop for battery sale, it may last a bit longer but will not start engine one cold morning.

New battery is about 12.8volts fully charged, some are 13.2volts
When they get down to 12.3volts they are about 5 years old and near their end of life.

 

[Mhfco6]

As you have surmised the alternator is not outputting any voltage, 0, none, nada.

In a '91 the voltage regulator is in the alternator so it is best to just put in another alternator.

If you want to check if fuses or Fusible links are blown you do that at the alternator.
On the back of the alternator will be the B+ terminal, it will have 1(or 2) larger wires held on by a nut.
Key off but battery connected
Test that B+ terminal for battery voltage, should be the same as the battery, it is a direct connection to the battery thru a fuse or fusible link.

There will also be a 3 wire connector on the alternator, unplug it
The Yellow wire should also have battery voltage, if not then a fuse is blown.

There will also be a Light Green wire, this should also have battery voltage but ONLY when the key is on, so turn on the key and check if green wire shows battery voltage.

Also while you are there the White wire needs a good connection, it is just a loop but make sure it isn't damaged or frayed.

If voltages check out then all fuses or fusible links are good, replace alternator.

Alternator, with engine running and working, will show approx. 13.6volts at battery, you can turn on the lights and heater fan to high and voltage will drop and then come back up to 13.6v, that s the voltage regulator working.
Just after starting engine regulator will recharge the battery, so voltage at battery will be as high as 14.9volts, after a few minutes it will start to drop down to the 13.6v area.
If voltage stays too high battery will "cook", fluid inside will go to vapor which will shorten the battery life

Battery could be damaged now, you will just have to wait and see.
Or if you want, and have a battery charger slow charge it over night and then unhook it from charger and test voltage, needs to be above 12.2volts, that 50% life, then let it sit for a few hours, unhooked, and test it again, if it is below 12.2volts shop for battery sale, it may last a bit longer but will not start engine one cold morning.

New battery is about 12.8volts fully charged, some are 13.2volts
When they get down to 12.3volts they are about 5 years old and near their end of life.

Thank a TON Ron, you always pull through for me. I'll let you know what I find out.

 

[Mhfco6]

I unplugged yellow,white,green plug from alternator. Yellow has NO voltage UNLESS key is ON. Then it reads 11.16. Green wire reads 11.10 with key ON. I did not see the large wires or a nut in the back of the alternator. I took pics.. Waiting on them to load.

 

[RonD]

Ford used the Light Green wire for many years as the Key On wire
And the yellow for the reference voltage wire, 24/7 voltage

Diagram here: http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_charging_1991_2.JPG

If charging system was working before then I would leave it as is, and just remember it.
If B+ had battery voltage then you are good to go for the fuses, it all checks out.
Get another alternator

 

[Mhfco6]

Ford used the Light Green wire for many years as the Key On wire
And the yellow for the reference voltage wire, 24/7 voltage

Diagram here: http://www.therangerstation.com/tec...ow supposed to have power WITHOUT the key on?

 

[RonD]

Alternator bearing was probably going out, causing belt to squeal

You have a 2G alternator(2G = second generation) in a 1991

The 3 wires are A, S and I
A is usually the yellow wire, it should have voltage all the time.
S is usually white, it is the short jumper wire back to single connector near the 3 wire plugin
I is usually the Green wire, it has power only when key is on, it comes from the battery light on dashboard.


B+ is the big wire(s) on larger terminal on alternator, it will have voltage all the time

 

[Mhfco6]

I replaced many alternator before but I don't see the main lead on this one. There should be a heavy gauge wire with 24/7 voltage like you said, and it should slide over a bolt stud on alternator then the nut should tighten that wire on the alternator. I don't see that on this alternator. I tried started truck again this morning and all the lights worked, but truck didn't start. Not even a tick.

 

[RonD]

It will be there, looked at replacement alternator for 2.3l early '90's and it looks to have two connectors, a two wire and three wire so could be the two wire is the B+ on the 2.3l version

 

[Mhfco6]

Yea there are 2 connectors. I bought an alternator today. I'll see what happens. Thanks again Ron!

 

[Mhfco6]

Alternator has been replaced. All is well again. 14+ volts at battery while truck is running. Thanks again Ron!