lms2013

I hope someone can help me....please. My 1988 F-150 5.0 L will charge the battery....the gauge shows normal position, an external volt meter at the battery indicates approx 14 volts. Leave it overnight and the battery is drained to where it just clicks. No lights are on (that I can see). I also believe the fuel pump relay is ok having replaced it a little over a year ago. I believe the alternator has an internal voltage regulator. Could that be defective and draining it? The vehicle only has 77,000 mi and the alternator is original.

Anyone have any ideas or suggestions?

Fast Hampster

Welcome to the forum! To check your system for "leaks" you may connect 10Amps measuring capacity ampermeter between battery (+) terminal and battery (+) wire. If you'll get readings below 0.15A it will mean that your battery is dead. Otherwise, pull out fuses one by one until you'll get approx. 0.1-0.3A (depends on what electronical devices you have installed in your truck). Just for your information - one 3Watt lamp (lot sure if we have any of those installed) consumes 0.25A, and it will take about a week for it to discharge a battery completely.

lms2013

Thanks for the info. Pulled each individual fuse and still got a full 12 volt draw. My Chilton's manual, rather useless for troubleshooting, alluded to the fact that the only direct connection with the key off is between the battery and the alternator. So I'm going to pull the alternator and have it tested. I'm betting on a blown diode bridge. Anyone have any other suggestions?

Fast Hampster

Blown diode bridge usually drains the battery completely in less than an hour, but anything is possible...

87Ford

you could posibley have a short some where along the line or your battery is bad how old is the battery sometimes its the simple things that get us. my previous owner of my 87 f150 302 couldnt get it started ever week it wouldnt start changed a bunch of things turned out it just needed a new starter and a good battery. i would replace the alt anyway if its the original id get a better amp too

DixieDarkhorse

have the battery load tested as well

lms2013

Thanks guys, I charged the battery (1 yr old) yesterday and left it disconnected overnight. Re connected it and the battery was still charged and the truck started right up. This time the volt meter indicated it was charging at 15+v the highest ever. So I'm going to take the alternator off and have it tested. It's the original alternator, 21 years, 77K mi. Not sure what it is rated at. I'm the original owner and she's been a most reliable truck. I'll update this thread when she's fixed.

lms2013

Well, I took the alternator in and had it tested. "Toast" was the word the technician used. Put the new one in and it seems to have solved the problem. Two days now and the battery isn't draining. Seems to be OK.

Thanks to all of you who took the time to answer my questions!

Rich

Do!nk

I recently found a burnt fused link that made my electric fan stop working (on my '88 f150 xlt lariat 5.0). I cut out the fuse and temporarily wired it together without the fuse and the fan worked fine, switch it on/off at the dash like it should. Two days later, truck won't start and battery is dead.
I trickle charged it for 20 hours and only got 3V when battery was tested with multimeter.
The extra wire (from the battery) for the fan is a constant draw and shows the same 3V as the battery terminal to terminal. Help!?