1992 F150 charging problems

[jade_morgan]

My truck will jump and run but then if I shut it off it is completely dead. Replaced the alternator, have tried 2 different batteries and I know the wires are good? The last owner had an alarm system put in the truck, I'm not sure if that could be pulling it? Any suggestions?

 

[kiwi f150]

Welcome to the site - I've found that with this sort of thing it's best to start at the basics , have the battery load tested to see if it's any good , charge the battery and fit it back in then start the truck and check to see if the alternator is charging - you should get around 13.8 to 14.3 volts across the battery terminals if that's working ok (if you're battery is buggered or really flat then you're alternator will never charge it) .
If that's all good then you need to start checking for power drains - here's how I do it.

Take the positive lead off your battery and make sure it isn't touching anything that is grounded then get your digital multimeter and switch it to DC amps (you'll have to switch the +ve lead around on the meter to the symbol that looks like this "-" and use the highest range socket first ) - Set the meter to the highest range which is on most meters 10Amp then connect the negative lead of your meter to the positive battery lead (which you remembered to disconnect from the battery right ) make sure that the positive lead of the battery isn't touching any earthing points.
Then touch the positive meter lead to the positive terminal on the battery and you should see the current drain (if any) displayed in amps on the meter - if there is a drain then start removing fuses and plugs to see which circuit has the drain.



Hope this helps.

 

[Bruners4]

I use a slightly different method to check for battery drain.
1. Key OFF. Remove negative battery cable from the battery.
2. Use a standard test light. Place the test light between the negative battery post and the cable. (Note: check that the light works)
3. If the light comes on and stays on, there is a drain on the battery. Light comes on and then fads out, OK.
I've used this method for quite some time. The light bulb will limit the current so this can be used to find shorts safely (NO SMOKE!)