k5 battery drains while running

Nissan11

Nissan11

Well-Known Member
Joined
Nov 29, 2005
Location
Marston, NC
Ive got a 85 chevy k5 Blazer. I put a new battery in it a couple weeks ago. When I drive the truck during the day it runs fine, but when I drive it with the headlights on the truck will sputter and die after a few minutes. I got stranded on Groometown road tonight because it cut off at a red light and the battery was completely dead when I tried to start it. I got a jump start and drove home in front of a buddy with the parking lights on and it ran good. When I got home I turned on the headlights while it was at idle and the lights got dimmer and dimmer until I cut it off and took the battery out to charge. I put a new alternator in it this afternoon because I thought that was the problem but apparently it is soemthing else. Does this sound like a wiring problem between the alternator and battery? If so, where should I take it to get it wired right? Its hard to find a good mechanic that really knows what he is doing. Im located in Greensboro BTW. Thanks!
 
Check your cables to make sure you have a good connection to both 12v and Ground.

Will your batt charge without running the lights?

If you put a Volt meter on your batt while running you should have 13.5-14.5 volts. and you should always be above 12v running with all of the lights on and heater and A/c on.
 
Crank your truck , then disconnect the positive lead from the battery. if it cuts off you still have a bad alternator.
 
I know this is for an '85 but in newer cars you don't dissconnect the battery. I read somewhere that an easy way to test is pull it to where it faces a wall. Turn on the lights, if they dim when you turn off the car then your alternator is OK, if they get brighter than the alternator is shot. It actually works. If the alternator is charging then the lights will have more juice and be brighter, but if the alternator is toast the engine will be a drain on the battery dimming the lights. Thus when you turn off the car the engine load disappears leaving more for the lights, and making them brighter. Plus you don't have to even pop the hood to test it out.
 
new alternators can be bad, have it tested. problem may lie in the wiring to the alternator not giving it proper signal to charge.
 
Ok thanks for the help. When I get out of class this evening I will disconnect the negative terminal and see if it cuts off. Im hoping it will cut off and prove to be a bad alternator because I dont want it to be a wiring problem. Im glad I saved the reciepts for that alt.
 
Get a cheap multi-meter (measures Volts, Amps, Resistance) to test things. You're going to need it for other stuff eventually anyway. They usually have them for like $10 or less at that Harbor Freight store on High Point Rd. Or any auto parts store will have one.

Test the Voltage (DC) across the battery posts. With the engine running, you should see over 13 volts, like 13.5-14.5 like Ridgerunner said. If not, then the alternator is not working right.

If this test shows the alt to be bad, take it off and have it tested at the parts store. If they say the alt is good, then start checking the wiring at the alt...
 
It *sounds* like a bad alternator.
But it could also be a shorted battery.
Or even really crappy battery terminals.
They can fool you.

The advice you have gotten so far is good. Although I am not crazy about disconnecting the battery with it running. That can throw a spike and damage electrical components. But it does work in a pinch.

Best is to put a voltmeter on it.
If you don't have one, now is the time to get one.
You can pick up a half decent one for under $20

With the battery charged.

Engine off.
Battery should show around 12.5 volts
Turn on headlights, key, wipers, and heater fan on high.
Should stay above 11 volts.

Start the engine.
Running it should be 13.5 - 14.2 volts at idle.
Turn on headlights, wipers, and heater fan on high again.

It should still maintain at least 13 - 13.5 volts.
At the VERY LEAST, it should be higher than battery voltage was with engine off.

Just thought of something else. Fan belt could be bad or slipping too.
With the engine OFF - can you turn the alternator pulley by hand?


HTH
 
The fan belt is new and I have ajusted it a couple times recently. But, even when I tighten the belt as tight as I can, I can still make the pully turn on the alternator if I grip it with both hands.
 
are you prying on the alt. with a pry bar when you tighten it?

Is your fan belt the right width to fit the groove properly?
 
Just got home. I tightened the fan belt with a crowbar and it is much tighter now. With the baattery freshly charged I put the volt meter on it and it read 12v exactly. While at the truck was at idle the meter read 14.5v. While the truck was at idle with headlights on, heat full blast and radio on the meter read 13v. While at idle I disconnected the negative battery terminal and the truck continued to idle without missing a beat. Whats the deal? The only thing different is I tightened the fan belt but it appeared to be working fine yesterday when I had the problem.
 
It could have just been a loose belt. It is amazing how long a car will run on the battery while other loads like the lights are not one. We had a alternator belt brake while we were towing my truck to Reno on the way out of Phoenix. We made it over 50 miles before the towing vehicle died. That was without lights on, but the radio was full tilt.
 
I took it out for a test drive and and had advance auto test the alternator and it was charging good. I drove 15 minutes home with the lights on and it ran great. I cut the motor and started it up again at the house and it fired immediately. I guess it was the belt. Thanks to everyone for helping me with this!
 
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