Hi,

Lights dim with the truck at road speeds at approximately 2K rpm or less when I hit the button to close the windows, once they close all the way you see a dim in the headlights.

The truck has 138K miles on it, It had a new alternator at about 120K miles. I don't have anything other than the stock stuff on this truck so and don't kn ow that I noticed this before.

I don't know that I noticed this before. Is this normal behavior? If not is there anything I can check?


If its normal, maybe I'm just starting to pay attention!

Thanks.

 

I know it's new, but get the alternator and the battery load tested. Parts stores do this free.

 

I took it to Advance on the way home, they said the battery needed to be charged and the alternator output failed the test. I put a charger on the battery last night.

They ran the alternator test with the engine at idle (700-800 rpm), A/C, cabin lights (door was open) stock radio (old man music) and headlights were all on..

Its been probably 20 years since I had an alternator tested at a store like that, but I remember them wanting me to have the car at about 1800-2000 rpm with their meter hooked up. I asked the guy from the store running the test and he said it didn't matter... implied that I was old and to get with the times.


Does it truly matter if the alternator test is done at idle or not? They are over driven right?

I guess where I am going with this is that with the load from the lights, radio, AC etc.. and a discharged battery, doesn't seem that the report of an alternator failed output test in that condition is conclusive of anything other than indicating that there is some unknown issue with the charging system.

My plan is to replace the aux belt and tensioner as discussed in a separate thread since it is needed anyway. Do some electrical and wire checks (grounds) at home before fully charging the battery, finally complete a retest.

Any other suggestions on what to check? Perhaps I'm lying to myself but, the charging system must be working sometime as the battery is maintaining enough of a charge, enough to start / operate etc..

Suggestions welcomed!

 

Replace your alternator - it should have a warranty.

 

I'll check to see if it has a warranty. I just assumed I'd be on the outside of any warranty.

I'm hesitant to just be a parts changer based upon what I know today. I'll give you full credit for the "I told you so" if I end up going this way though!

The battery with the vehicle off is showing 12.68 volts, 24 hours after the maintenance charge. Truck has only run for about 25 minutes in that time. And after running the battery was at 12.48 volts.

Fired the truck up and using a fluke the alternator puts out around 14.5 VDC, drops to ~13 VDC for about a second when anything kicks on then right back to above 14 VDC. Stays solid above 14 VDC across a variety of RPM ranges and with everything I can turn on- turned on.

Voltage measured from Alternator Frame to positive Terminal was same as battery post-to-battery post.

Switched to measure alternating current, observed a ripple of 16-20 mV AC I don't know what the spec is but from what I have read and seen on the forums for people with bad alternators, this seems very low and well within what people are generically calling "spec"

I'll monitor my voltage and check the battery daily for a while to see, probably continue to think to hard on the issue. But I don't see any outward signs with of the alternator. I have some hard driving planned this week so I'll take my meter with me and test it after a couple of hours on the road to get some data with the truck hot.

Maybe I'm chasing my tail, but I really don't want to spend a few hundred on a new alternator.

 

KPCREW, your battery and alternator readings seem normal to me.

A new "resting" battery that is fully charged will read about 12.7V, so your battery seems just fine in my opinion. And, you don't have any serious parasitic draws in your truck either, or the voltage would be quite a bit lower after sitting 24 hours.

A voltage of around 12.5V shortly after engine shutdown is entirely normal too.

Your alternator output voltage and the battery voltage when they're connected should always match - exactly. I don't really think they CAN be different. And, I don't think the tiny voltage ripple you are seeing is significant either.

But, if the lights are dimming at higher engine RPM, then it points to a reduced alternator output at that speed.

I think you should connect your multimeter to the battery terminals and run the wires into the cab, so you can see the actual alternator voltage when you are driving at highways speeds, at night with the headlights on. You'll probably need jumper wires to make the connection(s) if you do it this way. Alternatively, you could probably attach the multimeter to some "hot" source inside the cab, such as the wiring to the cigar lighter, which should give you the same information.

Either way, you'd be sure to see any voltage drop when you notice the lights dimming.

I know in my old truck (a 2005 KR), the alternator voltage would start out at around 14.1V with the engine cold, but would drop to about 13.8 if the engine was warm and the truck had been driven a couple hours. My new truck seems to hold around 14.2V all the time.

I don't know that the dimming you describe is particularly worrisome, unless the lights dim way down at night when you bring the truck to a stop and the engine is at idle.

- Jack

 

You can buy a digital voltmeter that plugs into the lighter socket or power port, they aren't expensive.

http://www.walmart.com/ip/47930243

They are on Ebay and Amazon dirt cheap, but they ship from China.