Electric problems alternator or battery

Hello Guys
I got a 1994 2.3 Ranger already for 12 Years. The truck isn't my daily driver any more. Lately I noticed that the indicators are very slow and it the starter motor appear a bit tired. As I haven't moved my truck for a while Is just thought the battery is low. So I took it out cleaned all the contacts (they looked awful) and charged it and reinstalled the battery. However the problem didn't go away. Without the engine running the battery shows 12.5 volt with the engine on and lights on its 11.8 volts when I increase the rpm it will got up to 14.5 Volts. So I assumed the alternator is alright. Or what do you guys think? Battery is already 4 Years old.

cheers from Germany Tom

You have a bit of a double whammy.

Battery voltage with key off at 12.5v shows battery may be OK

Engine running at idle RPMs(700-1,000) should show minimum 13.5volts at battery

And just after starting up to 14.9volts, even at idle RPMs.

It reads like you have alternator Field problem.

An alternator has 3 fields in its case, each field has 2 diodes
As the alternator's rotor spins each field generates AC voltage, the diodes convert that to DC volts.
It is not unusual for a field to fail as alternator gets older, leaving only 2 working fields.
Sign of that would be head lights dimming at idle RPMs, but it would still charge battery OK.
If 2 fields fail then alternator couldn't produce enough voltage at idle RPMs to even maintain battery voltage, at higher RPMs it could, above 2,000.

IMO your alternator is the issue.

But you can also do a load test on battery.
You want to test the AMPs left in battery.
Open engine fuse box and pull out the EEC Relay, or EEC fuse, this will cause a no start
Put volt meter on battery, 12.5volts
Crank engine for a few seconds, count to 5
Watch voltage while cranking engine, it should stay above 9.5volts

It being a 94, the regulator may be separate, mounted on the drivers side inner fender.

I have had a problem that was similar, but the voltage at idle would vary from battery to 14.5-ish, depending on how I looked at the dashboard. I replaced the regulator, and the idle voltage came back to normal readings. It had been very irregular.
If you have the shop manual, you can test the alternator output and, indirectly, regulator function. You could also have some sticky brushes or slip rings that need to be 'worked' to get their conductivity back. Things that sit don't seem to wake up quickly.
tom

Hello Guys,

thank you for your quick responses. That helped me as I was about to buy a battery. Is there any chance I test if it is the alternator or the regulator? I have the haynes manual.
I certainly will do a bit of higway miles to see if it might just be rusty brushes. However isn'T it normal that an alternator will break some day? It is 22 years old...

best wishes Tom

If there was no higher voltage, i.e. the 14volts at higher RPM, I might suspect voltage regulator or external wiring for alternator.

Alternator works like this
When you turn on the key the Battery Light will come on, this is an important circuit, it is sending the Voltage regulator "startup" voltage.
Voltage regulator sends 7 to 9 volts to the brushes/slip rings, this powers the rotors wiring to generate an electric/magnetic field
When alternator starts spinning(engine starts) this 7 to 9volts creates a 13-15volt output from the 3 Field coils in the alternators case.
Once this 13-15volts is being produced the voltage regulator switches over to internal power and the Battery Light goes out/off.

An alternator can't start producing power unless it is given that startup voltage.
And if you just left alternator's rotor power "ON" 24/7 then battery would drain when engine was off.
1 wire alternators use an RPM ON/OFF, the rotor/voltage regulator gets "startup" power from the alternator's B+ hookup to battery, 24/7 12volts, but RPM switch inside cuts off the 12volts below a set RPM then turns it back on above a set RPM.
No battery light circuit needed, and no battery drain when engine is off.

It does surprise me that your battery light is not ON at lower RPMs, does it come on with the key?

Voltage regulator monitors system power, it is set to maintain about 13.5volts when battery is fully recharged from starting, just after starting battery is drained and voltage regulator will increase system voltage to above 14 volts, but stay below 15volts.
It adjusts the output voltage by varying the input voltage to the Rotor brushes, the 7-9volts, 9 volts to rotor would produce higher output voltage.
So say you were at idle lights off, and battery voltage was 13.5v, when you turn on the lights that 13.5 volts would drop momentarily and then come back up to 13.5v, that's the voltage regulator responding to the extra power needed by lights on and it is sending rotor more volts to bring voltage back up to 13.5v.
When you increase engine RPMs voltage would go up so voltage regulator would start sending less voltage to rotor to maintain that 13.5v.

A 100amp alternator can generate 100 amps at high RPMs, that is it's Maximum power output, at idle RPMs it can only generate about 50% of that.
That is with 3 working fields, when a field fails then you lose 1/3 of the power generation.............BUT, voltage regulator just turns up the rotor voltage to compensate for the lost power generation, but at idle RPMs you would still notice a drop in voltage. i.e. head lights dimming.

When 2 fields are lost then you get what you have now, you only get voltage above battery voltage, 12.5volts, when engine RPMs are high, and it would be above 14volts because voltage regulator has maxed out Rotor voltage because battery is drained by running all the electrics at lower RPMs, i.e. just like after starting engine drain.

It could be just the diodes that have failed, and on most alternators you can change those, and the voltage regulator.

But I think you are right in changing out the whole alternator, the second most common cause for alternator failure is one of the bearings for the rotor goes bad, and putting money into the electrics of 22 year old bearings is not money well spent, IMO.

afternoon Gents

so I charged my battery and reinstalled it. The Truck started quite good but the measurement was still less than 12.5 volts when the truck was running. However I let it sit for a week and tryed to start. the engine did crank rather well and continued to do it for quite some time. But it didn't want to start. I tryed at least 25 times and the truck was still cranking very well. Then I jumped wired it. And it took some time until it started. After that I decided to drive the truck for some time to see what happens. And stoped after around 5 miles, the voltages was 13.54 with lights on and truck running Eventhough I cranked it just 10 minuts ago around 30 times.

I already was quite happy and thought everything is solved now. Just to be sure I took the truck on the higway for some miles. After a while the indicators went very slow, the radio turned of, the headlight was as bright as a the light in my glove compartment. This is when I decided to turn on emergency blinkers and truck home with the maximum rpm possible. Even the emergency lights didn't work and my voltmeter at the dash droped below 8. surprisingly the truck still ran quite well. When i arrived the battery had 9,5 volts left.

I changed the alternator and recharged the battery. everything runs fine now. Thank you for your help and support.

Thanks for the update

tomkyle said:

afternoon Gents

so I charged my battery and reinstalled it. The Truck started quite good but the measurement was still less than 12.5 volts when the truck was running. However I let it sit for a week and tryed to start. the engine did crank rather well and continued to do it for quite some time. But it didn't want to start. I tryed at least 25 times and the truck was still cranking very well. Then I jumped wired it. And it took some time until it started. After that I decided to drive the truck for some time to see what happens. And stoped after around 5 miles, the voltages was 13.54 with lights on and truck running Eventhough I cranked it just 10 minuts ago around 30 times.

I already was quite happy and thought everything is solved now. Just to be sure I took the truck on the higway for some miles. After a while the indicators went very slow, the radio turned of, the headlight was as bright as a the light in my glove compartment. This is when I decided to turn on emergency blinkers and truck home with the maximum rpm possible. Even the emergency lights didn't work and my voltmeter at the dash droped below 8. surprisingly the truck still ran quite well. When i arrived the battery had 9,5 volts left.

I changed the alternator and recharged the battery. everything runs fine now. Thank you for your help and support.

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cheers from Germany Tom

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Maximum RPM and you're in Germany......Do we want to ask how fast you where going? I'm guessing pretty darn fast!

Glad you have it sorted out!!

S-

Tedybear said:

Maximum RPM and you're in Germany......Do we want to ask how fast you where going? I'm guessing pretty darn fast!

Glad you have it sorted out!!

S-

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well shes an old lady so we usually do not go over 75 I kept the Truck in 4th gear to keep the RPM up. The Truck also feels funny over 85. For fast travel I got another car.