Engine/Transmission (1994 - 1998) Alternator / GEN Dash Light

[GROBBINS]

Hello Folks,
I haven't posted here in years, thankfully to a reliable truck. My truck (1997 12 valve 4x4 all stock) sat for a couple months and the battery's were almost dead. Turned over a couple times but no fire. I charged them each overnight, took the truck out and after a mile the GEN light came on. Noticed the volt gauge at low at 12 volts. Took home, cleaned the battery terminals, checked the batteries with a voltmeter (12.33v) and took out for a drive again. Same result, GEN light comes on after a mile of driving. Could I have fried something while charging the batteries? (optima red tops -10yrs old) How do I check to see that the alternator is operating properly?

 

[Topzide]

@GROBBINS, You'll need a voltmeter (analog or digital). Put the leads on the battery and note voltage, start truck and then note the voltage. If it drops below 10 volts while cranking, batteries are probably toast. With truck running, you should see the voltage above 13 volts when charging. Could be as high as 15 volts. If you're not seeing above the standing battery voltage with truck running, Alternator will need to be rebuilt or replaced. Hope this helps, let me know if you need more info.

 

[GROBBINS]

@GROBBINS, You'll need a voltmeter (analog or digital). Put the leads on the battery and note voltage, start truck and then note the voltage. If it drops below 10 volts while cranking, batteries are probably toast. With truck running, you should see the voltage above 13 volts when charging. Could be as high as 15 volts. If you're not seeing above the standing battery voltage with truck running, Alternator will need to be rebuilt or replaced. Hope this helps, let me know if you need more info.

Thank you for the detailed step by step procedures. I'll give that a try.

 

[GAmes]

The PCM will kick the alternator off line if there is a constant high amperage draw, like the grid heaters. The grid heaters gets constant hot power and the PCM sends it a ground signal during cold starts. Stock relays for the grid heaters have a habit of failing. You could disconnect the little wires to the relays just to make sure they are not the problem.

Edit: I think it is possible that the 10 year old batteries are a high amperage draw.

 

[BigPapa]

I think it is possible that the 10 year old batteries are a high amperage draw.

My spidey senses tell me those batteries are toast.

 

[SheldonBates]

GROBBINS, Just some more thoughts. I think Scott and GAmes are correct. If after charging overnight and a short drive your batteries are only showing a rest-state, no-load charge voltage of 12.33v then I believe they are essentially dead. Or at least at a state where they will no longer take a full charge due to sulfation, the result of being left in a discharged rate for a prolonged time, e.g., sitting for several months exacerbated by the constant parasitic voltage draw these trucks all have. GAmes point about a high amp draw is probably the key. Your trucks charging system is just not happy with those 10 year old, possibly sulfated batteries that won't charge.

As I understand it a fully charged 12v Lead-Acid battery (OPTIMA AGM in your case) should read 12.6 to 12.9 volts. Fully discharged at approximately 11.4 to 11.8 volts. So you are only dealing with about 1.5 volts from 0% to 100%. Your 12.33v reading would indicate about a 50% or less charge for your AGM batteries.

Topzide's excellent battery check procedure should show their true under full-load condition.

Also, I believe AGM batteries in particular do not like being left for any length of time in a discharged state, even just slightly discharged. They like being keep at a full or nearly full (12.6 to 12.9v) state of charge for a long and productive life.

Sheldon

 

[GROBBINS]

Well, I just checked the voltage. Just over 12.08v before, just above 10.42 while cranking, and then 11.82 while running. I'm ordering the new alternator from Napa. It's not a Denso, but "Denso Type" whatever that means. Let me know if there are better options out there. The original Denso alternator made it 275k miles, so I might not out live the new one.
Also, the truck has sat in a heated garage for the life of the batteries. I know that in the 10 years, they have only been discharged a couple times by operator error or lack of use, but have always came back and operated like normal with a recharge. I don't really get excited about putting $500 bucks into new batteries now if I'm still getting use out of the current ones. Any advice there is welcome too. Thanks again to all, for the wisdom.

 

[BigPapa]

I'll wager you're wasting your money on an alternator.

 

[petersonj]

He mentioned the alternator he is replacing has logged 275,000 miles. If that is correct, I wouldn't consider it a waste of money. It could be as simple as the brushes are stuck (from the truck setting for a couple of months), or the brushes are just worn out. I am not saying that he doesn't have a problem with the batteries, just noting that the alternator has given him a long service life.

GROBBINS, Just some more thoughts. I think Scott and GAmes are correct. If after charging overnight and a short drive your batteries are only showing a rest-state, no-load charge voltage of 12.33v then I believe they are essentially dead.

If the alternator wasn't charging as soon as the engine was started, then the low 12.33 battery voltage could easily be normal after driving over a mile and then returning home. Just the grid heater operation, starter draw, and the post cycling of the grid heaters (as well as other electrical accessories) without a charging alternator could easily bring battery voltage down to 12.33 volts with good batteries.

My battery testing procedure would be as follows: Disconnect the ground at each battery and fully charge each battery with a battery charger. After they are fully charged, leave the grounds disconnected and let the batteries set for at least 24 hours and then check each battery separately with a voltmeter. There should be less than .1 volts difference between each battery.

I would expect a voltage reading of around 12.4 to 12.5 volts for good ten year-old batteries. Connect the grounds, disable the engine from starting, cycle the grid heater, and then crank the engine for 15 seconds. If the engine cranks at a slower than normal speed and voltage falls below 10.5 volts, then the batteries need to be replaced. This test assumes that all wiring connections are in good condition.

- John

 

[Matt42]

The battery test that John described is what I did with the six year old AGM batteries in my 1996. (Six years here is a good, long life.) I charged the batteries separately, and then let them set. One dropped to about 10 volts during the setting time. The other held to about 12.

GRobbins: I'm curious about what a "Denso Type" alternator turns out to be. Did the NAPA counter person mention whether it was new or rebuilt?

 

[GROBBINS]

I's a new alternator. Napa lists this in the description:
Part #: NRO 1N8615Brand: NAPA
"Alternator - New,Nippondenso Type; w/ 120 Amp Alternator; w/ 8 Groove Pulley"

 

[GROBBINS]

Good morning,
Just wanted to update all of you with my truck. When stuff works you tend to forget about the people that helped you get it working. So, thanks for all of you who helped me in this thread. I installed a brand new alternator and the voltage went right back to 14v +/- The Gen light was gone and I've been back in business even with the 10yr old batteries. Which gets me thinking......it would be hard for me to throw these away if they are still operating like normal. Is that crazy for me to keep running them?

 

[GAmes]

Depends on how far from home you roam.

 

[petersonj]

The Gen light was gone and I've been back in business even with the 10yr old batteries. Which gets me thinking......it would be hard for me to throw these away if they are still operating like normal. Is that crazy for me to keep running them?

Old batteries will always appear to be "operating like normal" because they are not being tested anywhere near capacity when the engine starts quickly. 10 year old batteries heading into a Wisconsin winter - for me, that would a no-brainer - replace the batteries, as they have served you well.

Thanks for posting the fix.

- John

 

[BigPapa]

Glad I was wrong and that you got it fixed.