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Ford Ranger bad voltage regulator?

here its goes.

94 ford ranger

4.0L OHC

4x4 5 speed manual


so the other day i was driving home and i noticed my battery light was coming on each time i stopped at a red light. Soon after it remained on regardless and the voltage gauge started to drop till it hovered right at the “N” in normal but thinking if my alternator had gone i was best to keep driving rather than having to stop and restart the car again. I shut the radio off and made it home. The next day i grabbed my haynes manual and started going through all the tests of the charging system and everything checked out. The battery had a full 12.8v at rest and when idling showed 14.3, whats going on? The car didnt have a problem starting the next day and the battery, even now another day later, has a solid 12.8 on it so i dont suspect a drain anywhere. The only thing that i notice is when i tested the resistance on the voltage regulator the manual calls for at least 2.4ohms and mine read 4.0ohms, is that normal? Could it just be that my actual gauge is bad and im getting a false reading? My only other thought here is maybe the drive belt is a little loose and its slipping when the alternator is under load, but i dont hear a squeal and when i measure the deflection between the alternator and powers steering pump i get a little less than a 1/2". whats going on here? im stumped…

A new belt and tensioner would be the first suspects.

you have given us the voltage.
now what is the amperage at idle and at 2000 rpm.
what is the alternator amperage rated at?
you should be getting about 90% of rated amperage

have a quick update here - i took the car around the block a few times today and just like clock work after 5 or 7 minutes the light comes on and the gauge just drops. THEN! i pull over, shut the car off, immediately start it back up and the gauge reads normal…now im really stumped…

hey thanks for the responses, how do i check the amperage?

Next time the light comes on check the battery voltage with the enigine idling. Should be ~14V.
If it’s OK there’s a problem with connections to the dash.
Otherwise it could be a slipping belt, worn alternator brushes or a flaky regulator.

The alternator delivers whatever amperage is needed to maintain system voltage.
A quick test is to turn on headlights and the heater blower on high, then check voltage with the engine at fast idle (1500-2000rpm), should stay up near 14V.

i haven’t checked the voltage when the light has been on, that’s a good idea I’ll do that tomorrow. I did however check it when the system was under load, i.e. everything on and the engine at 2k, and it was giving me 14.6 if i remember correctly, but again this is when the truck had just been started and the light hadn’t come on yet. I’ll get it nice and hot tomorrow and see what readings i get - thanks for all the help guys, i’ll keep you posted!

You may just need new brushes. Take the alternator apart and have a look. Near a quarter of an inch and they’re done.

another good idea, and for $10 or so it should probably be done anyhow. thanks!

Take a socket/ratchet and put it on the alternator pulley nut. Try to rotate the alternator pulley. If it rotates easily the belt is not getting a good bite on the pulley and the belt and/or the pulley is worn.
Another possibility is a weak belt tensioner spring.

Serpentine belts generally do not squeal when slipping like the old V-belts are prone to do.

quick update: first thing i did this morning was check the brushes, i included a picture but i’m not sure if it will show up or not, anyway when i got the brushes out they’re were all but gone and one had worn so far the spring was starting to come through and had scored a line in one of the slip rings.

Fixed! replaced the brushes and took it out for a quick 20min drive under load with no problems. Thanks to everyone for the help and hopefully this is the last you hear of it!

I got some alternator training from Glenn Belmain in Caribou Maine. It was a friendly shop at Glenn’s Auto Repair(Glenn’s Auto Sales?). His Dad installed a radiator for me in 1974.