Battery light flickering... who's the culprit?

[Devin]

So after I fixed my car, got my emissions test, got tabs and was in line for fast food, my battery light started flickering. Turns out that when I diagnosed it, I am getting 14v to the battery when there is no load. Add the heater, headlights and fog lights and I get 13v. The engine bogs down and the headlamps dim once every couple seconds. This is repeated but faster or slower depending on RPM. Something is definitely sick.

With that does it sound like the battery or the alternator? I wanted to see if I could determine a likely culprit before tearing into the front of the engine. What's the best way to test it myself? Or should I drive it down to O'Reily's and have them test it on the car?

 

[sperold]

The O'reileys way is a good start.
It is usually the alternator that gives you grief, but your numbers should have kept the light off. I think a real healthy alternator gives you near 15V.
Is this the 90Amp model or the 120 Amp model. I think the little guy is more expensive than the bigger one.
If it is the alternator, see if a good shop can rebuild yours

 

[hawkeye18]

If the engine is bogging down under load, the alternator is *****. I had an alternator do that once, and it seized up so hard under load on the test bench at AAP it killed the machine - burnt up the motor. Not exactly sure what causes an alternator to lock up like that under load (and only under load), but it sounds like that's what yours is doing.

 

[Devin]

I'll get an update tomorrow hopefully. I can't take it to work because 86 miles round trip makes me nervous with a possible bad alt...

I wonder if this is part of what caused my earlier issues with lack of power and codes faulting MAF and SPOUT.

 

[rubydist]

I think a real healthy alternator gives you near 15V.

A healthy alternator will output 14.4V. over 14.8 or 14.9 will cause the indicator light to be on for over-voltage.

Typically on these cars, the indicator light will not come on until the voltage is under 13, by which time you are already at risk of not having enough battery capacity to get the car started.....

 

[Devin]

I'm still at work and have no update on the test, but maybe it'll help if I clarify.

When I said that the battery light was flickering, it was not steady at all. It would flicker one and off and at different intensities. There was nothing steady about it. When I did the measurements above the light was not on. It definitely did not look like a normal error light.

 

[hawkeye18]

I'm still at work and have no update on the test, but maybe it'll help if I clarify.

When I said that the battery light was flickering, it was not steady at all. It would flicker one and off and at different intensities. There was nothing steady about it. When I did the measurements above the light was not on. It definitely did not look like a normal error light.

And I repeat: your alternator is failing under load. I just gave a fairly lengthy description of how the batt light circuit works in another recent battery light thread, but I'll give a brief synopsis here - the circuit is a tug of war; the batt light is in the middle and the positive batt terminal and alternator are at either end. They're both pulling (pushing, actually, but I'll not get into semantics here), and should be pulling equally as long as everything is working right. If either end pulls more or less than the other end, the rope will move, and the light will turn on. If the rope moves quickly (large imbalance), the light will be bright. If one side slips just a little, the light will be dim. If it's a dim flicker, one side (99% chance it's the alt) is faltering in its voltage output.

 

[Devin]

Video

So Mr. o'Reilly's dude checked the alternator and the tester said it was A OK, but I don't believe it. So I figured you guys would provide better analysis (Ok Hawkeye, I just don't want to tear the Alt out and rebuild it and have issues!). I promise if you confirm from the vid that it is the alt I'll get it rebuilt! :sun:

Alternator vid (WMV file)

 

[hawkeye18]

Well, assuming about a .75-1v loss from taking measurements from wiring inside the cabin, the voltage at the alt seems about normal. That voltage dip when you let off the throttle is what worries me though. It dips to 12v and then recovers. What are the two lights on the dash? (I don't have a SHO handy to reference)

 

[Devin]

The brake and battery light (the battery light is the one on the right). And those are the same measurements that dude at the auto store took. It ranged from 13.2 to 13.7 at the battery.

 

[hawkeye18]

13.2 is too low. It needs to be at least 13.5 at the battery to call it good. If it varied by half a volt, I'd say the regulator is on its way out. Its job is to supply 14.5v at a constant rate to the battery and shouldn't vary by more than .2-.3v. And I saw that nasty dip down to 12, it should definitely never do that.

 

[Winterhawk]

My lights were flickering ever so slightly. I replaced the alternator and it's been great ever since.

I just used an Autozone one. The SHO has a built in regulator. I wouldn't take the time to rebuild one as I didn't trust the bearings.

 

[hawkeye18]

My lights were flickering ever so slightly. I replaced the alternator and it's been great ever since.

I just used an Autozone one. The SHO has a built in regulator. I wouldn't take the time to rebuild one as I didn't trust the bearings.

I've rarely seen the bearings on an alternator fail. I mean yeah, it happens, but only maybe 10% of the failures I've seen were bearing-related. The other 90%+ were regulators; they go with alarming frequency. I suspect inadequate cooling. The internal cooling blades do little, I think. perhaps one could mount an external cooling fan solution somehow? I suspect it would greatly increase the life of the regulator.

 

[Devin]

Is that why the regulators were external on later Gens?

 

[Devin]

If I unplug the alternator's plug and take the single wire off the post, will I get battery *only* power? Or will that **** the ignition circuit? I'm at work thinking of how to diagnose it when I get home.

 

[jonheese]

Yes, that will work fine, but be careful with that charge wire (the big one on the single post), because it's still got battery voltage going to it.

The only thing I'm not sure about is the green/red (hot while running) wire on the alt harness plug... I'm not sure if the alternator makes that wire hot, or if something else (the ignition switch?) does, and the alternator is watching it to know when the car is running...

Anyone know?

 

[hawkeye18]

Yes, that will work fine, but be careful with that charge wire (the big one on the single post), because it's still got battery voltage going to it.

The only thing I'm not sure about is the green/red (hot while running) wire on the alt harness plug... I'm not sure if the alternator makes that wire hot, or if something else (the ignition switch?) does, and the alternator is watching it to know when the car is running...

Anyone know?

The Y/W goes directly to the BATT+ terminal via a fused link and a connector, and is powered by the battery to tell the alternator what the battery is putting out so the regulator knows how much voltage to put out. The LG/R wire also goes to the battery, but via the battery light in the dash and the starter switch. Both sides are powered; the battery's side by the battery, and the alt's side by the alternator. As I said earlier, as long as both of them are putting out the same voltage (and they should be), the light will not be lit.

I've attached the readout. This is for a 95 MTX, but other years are similar.

 

[moobypets]

eu guy i dont want to **** anybody here but

i had the same problem replace the alternator 3 time

and found out one off the fusible gauge wire that was feeding from the starter relay to the alternator was bad and was giving me bad voltage

you could unplug your conector and check that how i found my hidden problem

plug the tester on one side and you should read the same as the batt then have some one play with the wire pulling and moving and see if you have any voltage mouvement then you will be sure

 

[jonheese]

The Y/W goes directly to the BATT+ terminal via a fused link and a connector, and is powered by the battery to tell the alternator what the battery is putting out so the regulator knows how much voltage to put out. The LG/R wire also goes to the battery, but via the battery light in the dash and the starter switch. Both sides are powered; the battery's side by the battery, and the alt's side by the alternator. As I said earlier, as long as both of them are putting out the same voltage (and they should be), the light will not be lit.

I've attached the readout. This is for a 95 MTX, but other years are similar.

Aha, that's right, I remember that now. It is the same in the '90 EVTM (or at least close enough as it makes no difference).

So yes, you can disconnect the alternator entirely purely for testing purposes, just be sure to isolate the wires you disconnect, as at least two of them will be hot (all the time in the case of the big charge cable, and when running in the case of the red/green wire). Of course, you will get a battery light on the dash while doing this.

 

[moobypets]

think the one connector is name "a"

and then check for bad wire

it took me 3 week to fix that prb