hi all and thanks in advance for any answers. i have an 02 ram pick up 4x4 with a 4.7 l. daughter was driving it today and she broke down. i went down to get her and she had a dead battery. hooked up jumper cables and it started; as soon as i took off cables it died again. put in a new battery and hooked up my voltmeter it read 13.6 volts. should the alt. be putting out 14.6 volts? or is 13.6 enough? i am going to charge and load test the battery tommorrow. do you think it sounds like battery or alt???

 

This is almost always bad alternator.

When you hooked the multimeter while the battery was in, You were getting voltage from the battery and not the alternator. If the truck can't stay running w/o the battery connected, You have a bad alternator and it needs to be replaced.

 

13.6 is a new battery voltage only. it will get less and less as you turn stuff on on the truck. alternator charges at 14-14.8 or so.

 

The battery is only used to start the vehicle whereas the alt is used to keep it tunning. That why you can start the vehicle and then take the battery completely out. But if you start it and then pull the battery out and it dies, it is because it was running of the battery and not the alt, hence the dead alt.

 

I don't know about the older trucks, but the newer trucks do not run directly from the alt. The pcm on the 07 pulls from the batt and if you disconnect the batt, the truck will die no matter how good your alt. Yes, this is contrary to conventional wisdom, yet it is what it is.

 

Just take the battery out and take it down to a checker or oreily or autozone and have them do a load test on it and see if its charging fine, if it is, pull the alternator and take it down for the same test. Its not that hard to remove either and is not worth towing it in anywhere to have it looked at. That is what I would do.

 

 

That is very good to know.

 

Thats new to me. Ill have to try that on my 05.

It was always my understanding that the whole system had a charge on it. Whether that charge comes from the battery or the alternator doesn't matter. That charge comes from the battery when the alt isn't running and when the truck is running that charge is supposed to come from the alt, It also provides charge to re-charge the battery.

Now for the ecu to run off the battery and only off the battery, It would have to be wired nearly directly to the battery with diodes in line to prevent bleed from the rest of the system.

However, It does make sense to me why they would do that. It is a great way of protecting the ECU.

 

Isn't it because the alternator needs a battery voltage before it will charge? if the battery is dead the truck won't run because the alt isn't charging, and if the alt isn't charging in the first place, the truck will wear down the battery and stall.