I have a 97 Ranger with 4.0 V6 with about 120,000 miles. The battery light came on today so I took it to Advance Auto to check the battery and Alternator. They said both were good. I checked the fuses & relays and nothing was blown or out of place. Could it be a bad ground?

 

Well lets think about this some more. Since the battery & alternator output check ok, I suppose the diode in the alternators regulator, that operates the warning lamp, could be shorted, as that would turn the warning lamp on, even though the alternator was still outputting. The diode in my 85 Renault alternator regulator was open circuit, so the alternator lamp would never turn on, but if I grounded its wire at the alternator connector, to test the lamp & wiring back to the lamp, it would turn on, like it should, if the wiring & lamp were ok.

SO, at KOEO, alternator regulator electrical connector Disconnected, does the lamp go out???? It should. If not, you likely have a short to ground in the warning lamp wiring, so look for scuffed up/cut, or other wise damaged insulation where the wire is exposed to, or crosses metal or something hot thats metal.

At KOEO, if the lamp goes out when the connector is off the alternator, the warning lamp wiring is ok, so suspect the alternator regulator warning lamp diode is shorted.

With the ignition key OFF, if you use your multimeter on the ohms setting, at the warning lamp pin/socket connection point on the alternator, the other meter lead to a good clean, unpainted, unrusted, bare metal engine or body ground, you'll probably get a reading to ground, if the diode is shorted & you shouldn't if the diode is good.

If you don't feel comfortable doing the ohms test with your multimeter, run this puppy by any alternator rebuild/repair shop & they'll probably do the check at no cost. Maybe give you a good deal on replacing the regulator, if you don't want to mess with it.

Some thoughts for pondering, let us know what you find.

 

I will check that out and report back. I am still leary of the battery, I had to put water in it and it is a 2004 72 month battery. Thanks.

 

Do the KOEO alternator electrical disconnect test first to see what the charge lamp does.

If no joy & they didn't electrically load check the battery, you can sorta do it by turning off all electrical loads & measuring the battery No Load SOC voltage across the posts with your multimeter. If fully charged it should read 12.6 volts or higher.

If you have removable battery caps & own a Hydrometer, you could check each battery cell electrolyte specific gravity, for each cells state of charge (SOC).

Or do a home made load test by turning on a heavy electrical load, like head lights, blower motor on high speed, push the cigar lighter in to heat up & quickly check the battery under load voltage across the battery posts. It should read more than say 10.5-11 volts.

If the engine cranks slow, or the dash lights dim a lot when cranking the engine, or the head lights dim noticeably at idle, suspect the battery, cable connections to the battery posts, or their connections on the other end, maybe loose, corroded, rusted, or the cables could have a problem internally where the lugs are molded or crimped onto the battery cable wire strands. An under load voltage drop test across the connections would tattle on a culprit.
If your not familar on how to do a voltage drop test with your multimeter, visit the Battery Tutorial link atop this forums index page & poke through it to find out how to do it & a lot more useful battery info there, that'll be helpful in trouble shooting/testing, or getting the most battery for the buck if you find you need to replace it.

Let us know what you find.

 

I put a different known good battery on it and the battery light stayed on and never flickered. What connection do I unplug on the alternator? There are 2 electrical plugs or is it a bolted electrical connection? I used to have a hydrometer but I don't know were it is.

 

If you put the know to be good battery in it then replace the alterator. it is failing. You can spend 5 hours testing or ten minuets taking out the 3 13mm bolts and replacing the whole part. Hope this will help.

 

Pull the plug that goes into the alternator regulator & see if the light goes out. If it doesn't, you likely have a wiring problem/skinned wire insulation & short to body/ground, between the dash warning light & the connector. If it does go out, then the problem is likely in the regulator diode that operates the dash warning lamp being blown/shorted internally.

 

The battery light came on my 2005 at low idle. Battery tested good, installed a new life time NAPA alternator and fixed the problem. Had the alternator tested and it tested good off the truck????

 

Battery light is off for now. I think I found the problem. The negative battery terminal clamp band is broken on the bottom and not causing good contact sometimes. I bought a replacement and as soon as I get it on I will report back.