So my truck has been barly starting lately (and now not at all). Volts reads just above 8 when I cycle it before starting. When I drive volts go from about 14 down to the line after 8 (i guess 10 or 12) depending on if I'm at high or low rpms. THen if I let it sit for more than a week at a time, it's totally dead. If i start it and idle for 10 min or so, Its still dead. If I drive it around for a while it will start up again but just bairly!
My batteries are Excide Select by Orbital with 850/690 cca.

So to make it more complex, when I had the remote start installed (over 2 years ago) They originally tapped into the voltage regulator. Then he found the tach wire and did it correctly. So that's also a possibility. But ofcourse It could be an alternator too.

So what do you think it is/what should I do to check it to make sure that's what it is?
How much is the part I should replace and how hard is it to do?

If it's the batteries, what should I replace it with?

 

If it shows up to 14 volts when running at speed, I'm guessing the alternator is working.

You also said that it's flat dead after sitting, which would make me think the most likely cause is something is causing excessive draw when the truck is off. If the batteries are flat dead, it will take a long time for them to fully recharge, and if they have been down to zero volts very long or more than one or two times that likely shortened their lives...

I guess I'd start by unhooking the batteries and trying to bring them up to full charge with a good charger, then either take an ohm meter reading between the battery cables or an amp meeter reading between the battery and truck (with everything off) to see what kind of draw there is...

 

Im going to say that your alt. is toast. Your alt should not very in voltage due to high and low rpms. That is most likely causing your batt to drain. But depending on how old your batteries are then they could be bad too. Have your alt and batteries checked. Both will most likely have to be replaced now.

 

Yeah, I kind of thought that too, but thought perhaps the varying volts might be due to having nearly fully discharged batteries (assuming he jump started it and was running with them dead).

The thing about it being totally dead after sitting pointed me to suspect a draw, but you could be right about the alternator, or it could have multiple problems.

I think Volly818 has a good point, and it would be quick, free and easy to pull the alternator and have it bench tested.

 

My bet is the batteries are toast. Disconnect them ftom the truck and charge them up over night. Let them sit 6 hours are put a voltmeter on them. If they are less than 12.3 they are toast. Replace them with Optima Red Tops

 

Alright, First and foremost:
THANK YOU FOR YOUR HELP!!!

Secondly, here's what happened. I take the batteries to the second checker auto (1st one was really rude!). Mngr says, "lets test them." well they are dead. She pulls two new ones off the shelf and plops them on the counter. Says hope these work, and on my way!

Put em in and it fires right up!

However, as you suggested, I think i also need an alternator.
So my question, Should I drive it right now, leave it disconnected, .....? WHat should I do in the mean time?

 

Why do you feel you need an alt. too.
Check the battery voltage eng. off. It should be 12.0 to 12.5 volts.
Run the eng. and check it again. It should be 13.2 to 14.8 volts.
If not, then you could need an alt. It`s very rare that the batteries and alt. are bad. If the alt. is bad, then the batteries are dead due to lack of charging. Charge them on a charger and retest.

If it is a bad alt. and you continue to drive it, the batteries will go dead again.
Find out what the real problem is before you get stranded somewhere.

 

I would say there is still the possibility of something drawing power while the truck is off, too, though one of the batteries could have had a dead cell or otherwise have been self-discharging.

I think I would still take a reading to check for excessive draw while the truck is off, and if you wanted to for peace of mind, you could still have the alternator bench tested.

Then you would have covered all the bases.

 

If you wan't to check to see if you have a draw on the batteries is first check to make sure everything is off in truck. Unhook the main negatives and when you hook them back up the size of arc will tell you if you have a draw. A tiny arc is normal. Anything bigger you have a draw and causing your batteries to die. Also would check alt at auto part store to be sure. Doesn't take much to be out of spec.