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Truck started when I brought it home In June and battery, gradually deteriorated (over a few weeks) so I replaced it with a new cranking battery (that I bought for my boat and planned to replace - so it’s a marine battery). It’s been in for about 2 months now, and today is totally dead.
I couldn’t charge it enough (through jumper cables) to get it running, so I put the old one back in and charged it up with jumper cables. So old battery cranks now, but barely.
My guess is bad alternator.
Anyone had this happen? Sound like the alternator?
But, my wife gave it a bath today, so looking nice.
It could be anything and you will need to track down what it is.
Its easy enough to check, take a good old test light with a normal bulb and hook it up between the + of the battery & the cable.
Depending on how much of a draw there is the light will be dim (low draw) or bright (big draw).
Start pulling the fuses 1 at a time till the light goes out or vary dim.
Don't forget the door light when the door is open.
If you have all the fuses out and the test light is still on pull the large wire off the ALT. out put and see if it goes out.
Dave ----
I have a '79 that my dad bought new and it has drained the battery overnight since it was new. just did it again last month. may do it twice in a year mat not do it for 2 years. I have absolutely no idea why.
If yours is a slow drain just shut everything off and unhook the battery. then touch the cable back on the post and see if you get a spark. if you do then you have a draw so start eliminating everything 1 thing at a time until it stops. if it doesn't, then that's a PITA to diagnose.
Use a 12 volt test light , key off ,unhook ground off battery use test light from negative cable end to the negative post on battery to test , if it lights up you have something draining the battery , how bright the light is will tell you how bad the draining is , you can pull fuses one at a time to see if you can narrow it down ,,you can take alternator off and have it checked if you want . If you have a radio hooked directly to keep clock set could cause some drain or anything thats directly powered from battery source , so unhook anything that uses a direct source for power will also point you in the right direction .
Stick a voltmeter on your battery, it should register around 12 volts. Start your truck, it should register around 14 volts. Does the charge gage work on the dash?
Use a 12 volt test light , key off ,unhook ground off battery use test light from negative cable end to the negative post on battery to test , if it lights up you have something draining the battery , how bright the light is will tell you how bad the draining is , you can pull fuses one at a time to see if you can narrow it down ,,you can take alternator off and have it checked if you want . If you have a radio hooked directly to keep clock set could cause some drain or anything thats directly powered from battery source , so unhook anything that uses a direct source for power will also point you in the right direction .
Only thing about taking the ALT off and having it tested is it can test good as it does not run the battery down if driving it.
Originally Posted by lexpression
Stick a voltmeter on your battery, it should register around 12 volts. Start your truck, it should register around 14 volts. Does the charge gage work on the dash?
See above answer why checking the ALT out put will not show a drain. Has any one seen the charge gauge move? I know I have not on my 81 F100.
Using the test light, it CANNOT be an LED test light, between battery & cable is the easiest way to check. Could try using a AMP meter between battery & cable but not many have one handy.
Dave ----
If you are able to get it running, take it to an auto parts store - they can test the alternator in the car, and check your battery at the same time. To me, sounds like your alternator is bad and causing the battery drain, but only because when mine went bad that is what it did. Could see the drain with the test light but was unable to find where it was, but the store picked up on the alternator being bad in about 15 seconds.
If you are able to get it running, take it to an auto parts store - they can test the alternator in the car, and check your battery at the same time. To me, sounds like your alternator is bad and causing the battery drain, but only because when mine went bad that is what it did. Could see the drain with the test light but was unable to find where it was, but the store picked up on the alternator being bad in about 15 seconds.
Then your ALT was not putting anything out and just pulling from the battery till dead and why they could find a bad ALT.
Originally Posted by meangreen92
Yep, a bad alternator diode can cause an otherwise seemingly good alternator to drain the battery.
Yep If you were to test out put like the parts store would do, it is putting out volts and charging the battery when running.
Now turn the truck off and let it sit over night a day or 2 and battery is dead?
ALT tested good but a bad diode, it is a check valve for electricity lets it flow ONLY 1 way, will now let volts flow from battery thru ALT and kill the battery.
You can find this using the test light between battery & cable and if you get a light and pulling fuses does not get the light to go out pull the large wire off the ALT out put and if the diodes are bad the light will go out.
So if you don't want to change out the ALT because it is charging just disconnect the battery when you shut the truck off each time.
I think some rebuilders have a way to test if the ALT has bad diodes but I am pretty sure the parts store cant but could be wrong.
Dave ----
The 2 most likly suspects are the Alternator as mentioned and the voltage regulator.
I would just unplug the regulator and see if that cures it. If not then suspect the alternator. If a diode is leaking it should show up in an AC ripple test.
To do an AC ripple test, make sure the bat is fully charged. Check the battery voltage with your multimeter set on AC volts, with the engine off and see if the meter reads voltage while in the AC mode. It should not show any voltage if DC is blocked. If it does read AC try swapping the meters leads between Pos and Neg
If it does not read with the leads swapped be sure to use that orientation during the test.
Start the truck and let it idle down (choke off) then rev to 1500RPM and hold it there, then load up the elec system headlights on, or heater motor on etc. Connect the meter between the pos and neg on the batt, anything more than 100mv (0.1V) AC indicates a bad diode in the Alt. Replace the Alt.
If you are able to get it running, take it to an auto parts store - they can test the alternator in the car, and check your battery at the same time. To me, sounds like your alternator is bad and causing the battery drain, but only because when mine went bad that is what it did. Could see the drain with the test light but was unable to find where it was, but the store picked up on the alternator being bad in about 15 seconds.
I charged the battery and put it back in the truck, took it to AutoZone. Their report said “voltage regulator failed, diodes passed”. So I bought the regulator they recommended, put it on the truck, and the voltage dropped from 12 to 9. Soooo, I was thinking they gave me the wrong size?
I was also looking to replace a part that attaches from the pigtail of the voltage regulator (on one end) and attaches to the back side of the voltage regulator and attaches to the radiator frame with the same screw that holds the regulator in place. Does anyone know what this is?
WHile I was under the hood I noticed the bolt that held the positive batter cable to the starter solenoid was rusted, and I accidentally knocked that bolt off trying to clean the rust. So now I need a new solenoid.
Was hoping to undercoat the frame today but not going to happen.
Autozone report Old regulator What is this part? Attached to pigtail for regulator and then to back of voltage regulator. Same part attached to the pigtail. Some kind of igniter or capacitor? No one knew what it was.
I charged the battery and put it back in the truck, took it to AutoZone. Their report said “voltage regulator failed, diodes passed”. So I bought the regulator they recommended, put it on the truck, and the voltage dropped from 12 to 9. Soooo, I was thinking they gave me the wrong size?
I was also looking to replace a part that attaches from the pigtail of the voltage regulator (on one end) and attaches to the back side of the voltage regulator and attaches to the radiator frame with the same screw that holds the regulator in place. Does anyone know what this is?
WHile I was under the hood I noticed the bolt that held the positive batter cable to the starter solenoid was rusted, and I accidentally knocked that bolt off trying to clean the rust. So now I need a new solenoid.
Was hoping to undercoat the frame today but not going to happen.
Autozone report Old regulator What is this part? Attached to pigtail for regulator and then to back of voltage regulator. Same part attached to the pigtail. Some kind of igniter or capacitor? No one knew what it was.
One more post on this tonight. I replaced the solenoid, put the old voltage regulator back in , and the voltage at the battery is back up to 13. Where I started this am.
Im guessing the only thing left is the alternator or the belts - but belts look pretty good.
That helps with engine noise in the radio ,it helps filter engine noise or whine out of the radio , may be called a condenser cant remember , on these old trucks there is a lot of interference in the system when its running thru the electrical
