Engine/Transmission (1994 - 1998) Odd Charging Problem

[JoeBioDiesel]

Last night I noticed my system voltage was dropping to 12V at idle. Raising the RPMs to 1500 or so would make it go back to 14+ volts like it normally is. Today I tested the alternator and got some strange results. I am hoping someone here has seen this before.

At idle the alternator puts out 5-10 amps at 12. 7V.

Increasing alternator load does nothing to change amperage output.

Raising the idle casuses the amperage output to slowly increase until reaching about 1500 RPMs where the output peaks.

The alternator load tested OK, delivering 160 amps at 14 volts when loaded with a carbon pile tester.

Checking data with a scanner it shows desired charge rate at idle to be 13. 7 volts, and actual 12. 3 volts at idle.

Raising the idle makes the desired and actual numbers agree with each other.

Battery temp sensor reads 57 degrees (ambient).

Full fielding the alternator with the scanner works OK, so the PCM seems to ba able to control the alternator OK.

Batteries load test fine.

No parasitic draw.

No voltage drop from the alternator to the batteries on either the + or - side.

I've never seen an alternator act like this.

Any ideas?



Joe Rappa

 

[Loren]

Joe,



It sounds like you know more about electricity than I do, but I'll give you my theory on your system. You can decide whether or not this makes sense.



Theory:

Your system is working properly.



When your battery is fully charged, or only slightly below fully charged, it will not require nor accept amperages above 5-10 amps. The only time your alternator will acually produce the full 160 amps is when the battery is dead and the truck needs additional current for electrical loads.



The voltages you read sound like they match my numbers, although I haven't tested as extensively as you have.



What do you think? Does this sound like a reasonable explanation?



Loren

 

[lil red cummins]

joe,



you didnt say what your idle speed was. you also didnt mention if your idle was lower than "normal" or being slowed by some other load or unusual condition. most alternators can not put out full current at idle but can maintain 14 volts at around 30 amps or less load. the higher the output rating of the alternator, the less it puts out at idle (generally speaking). idle speed and alternator pulley size have a bearing on this characteristic. suffice it to say, i wouldnt expect the alternator to have full output at idle, even if the load exceeded 160 amps. it will just do the best it can till you raise engine speed.



if this is a new "problem", then there is obviously something we havent thought of or discussed yet.



other than that, what you described seemed to be a normal condition.



jim

 

[JoeBioDiesel]

Hi Jim and Loren,

Thanks for the replies. My idle is pretty low compared to most others, around 650 or so right now, but even lower in the summer. It's easier on the brakes that way.



Jim,

The problem is a new one. It used to maintain 14V at idle as long as the heater grids weren't on. I don't have AC or any power accessories on this truck so there is not much to load the alt at idle, other than the heater grids or deeply discharged batteries. After reading your comment on pulley sizes it reminded me to check for belt/pulley slippage. Although it's uncommon on serpentine belt setups it can happen. No evidence of a problem there.



Loren,

It makes me feel better to know that your truck acts similarly. When I raise the idle on the truck it charges the batteries at about 40 amps after the heater grids have run. I thought maybe that's why there was a change... now that it's cold the batteries are being discharged by the heater grids and the alternator now needs a little more speed to recharge them. However after a 60 mile ride the voltage still drops to 12V at idle. That's plenty of time to recharge the batteries.



I am going to take the"wait and see" diagnostic approach on this one. I'll continue to drive it until something breaks or changes.

Thanks again for your help.

Happy Holidays.

Joe

 

[JoeBioDiesel]

I found the problem today, and the truck is back to normal. The battery on the passenger side had a shorted cell. It would load test fine, and then fail a load test a minute later. Static voltage would be either 12. 6V or 9. 6V, depending on whether it was acting up or not. Any time the truck was running, and vibrating the batteries, it must have been shorted.

Anyway, two replacement NAPA batteries(old ones weren't that old) and it's back to normal.



Joe

 

[lil red cummins]

yup, that falls into the category of abnormal condition. it is not unusual though. i have seen this atleast 2 times over the years. the first was on a 78 chevy 1/2 ton with dual batteries. i should have remembered that, but didnt until you mentioned it.



glad you are back at 100%



jim

 

[Loren]

Joe,



See! I told you that you knew more about electricity than I did.



I did manage to discover the single bad battery in my 1995 truck when it went dead, but I can't remember what exactly tipped me off. I'm sure that I didn't notice the voltage problem that you mentioned.



For what it's worth, the remaining good battery from that truck is still in service. It's currently in my Ford tractor, and doing a fine job too. So far. (I'd better be careful saying things like that in public. It may go bad tomorrow. )



Glad you found your problem.



Loren

 

[JoeBioDiesel]

Hi Loren,

What year Ford do you have? I've got a '53 Jubilee that I use regularly. I'm working on a deal to get an early 60s 871 Select-O-Speed Diesel. I'm selling my '56 International 300 Utility in order to get it. I like those old Fords... and I definitely need a Diesel



Joe



My old Jubilee