Well, the project Disco has given me the first test of loyalty. This is something I rebuilding for my brother, as such he was driving it when this happened so I don't know if anything strange preceded it. He didn't notice anything though. 1995 Disco 3.9.

The situation: Running just fine, no issues or warning lights. He parks it for the day, next morning it's 100% flat, not even a dome light or chime. Battery is just a year old, and always starts right up and hasn't had a sign of low battery at all.

I went out later to jump it, double checked nothing was left on that would drain the batt. Hooked up the cables and it fired right up - with a catch.

The issue: The speedometer was going nutty. In park, idling, it was bouncing all over the place. I turned on the headlights so the dash would be illuminated and I could see better. The bouncing stopped. Turned off the lights, the bouncing started again. Turned the lights back on, and the low and high beams strobed back and forth, and went dim. Pulled the switch for high beams and it was dim, put it back to low beams and the dummy light for the brights went off but the brights came on.

The question: I'm thinking it's not a failing ground strap because the battery fully discharged, so maybe a short somewhere? What on earth would connect the speedometer and headlights and cause a battery discharge?

 

Sounds like bad alternator or frame connection. To test, disconnect alternator (it has no fuse link, must take cable off back or in the underhood fuse box front terminals) and crank truck just on battery to see if this repeats. Because if volts are going high you are cooking the electronics. If does not repeat take alternator out and to parts store for bench testing as one option.

But a loose ground lead to the frame can cause this. Check ground strap to frame, it can be seen from under the bumper on the passenger side.

You would want to read battery charging volts. If above 14.4 (like 15 plus) the regulator in alternator has failed and it is zapping the electronics in the truck. If high volts shut down truck at once.

Was it the speedo or the tach that was bouncing? Tach bouncing or not running is another sign of alternator regulator failing. Alternator is connected with a heavy wire to battery via a fuse link, and a bad one is quite capable of draining the battery. Speedometer has a circuit board inside the instrument cluster that smooths out signals from the vehicle speed sensor for display. Would start with wiring bolts on back of alternator (the small plug in lead is the tach signal point). Would have a good look at the underhood fuse box for tight and clean connections, including that small wire that runs to the plus battery from the box.

 

Another quick test of the frame ground is to take a single battery jumper cable, and attach one end to (-) battery teminal, and the other to a clean stout piece of metal on the engine, bypassing any loose connections. The engine may also have a bonding jumper to the frame, perhaps near the starter.

 

my tach is bouncing, gonna change it out this week. I had the alt checked but guy at oriley said he couldn't test, tester read check connections. She has been shorting out bulbs. Good thing I got the warranty.

 

I agree with Buzz....you have classic bad ground symptoms. It is not that something is draining the battery that causes it to be dead, it sounds like the alternator just doesn't get to charge it properly due to a faulty ground.
Good luck!

 

Buzz spent quite a bit on the tuition for the loose ground strap years ago - had a two way FM radio go out in the shop truck. Pulled it out, put in a second radio. It promptly stopped. Decided to put in a third. It stopped. Turns out loose ground, alternator was putting out about 18 volts and Motorola Micor radios really don't like this and the radio shop really does....

 

It is common for the ground connections from the starter to the frame as well as the battery and alternator to frame connections to corrode and give problems.

AntiChrist (Tom Rowe) manufactures the best power and ground cables i have ever seen. Contact him for a set that will fix multiple issues.

 

Tom's cables are certainly amped up! You could weld with them.

 

Thanks for all the replies. I'll definitely check those leads. It was dark out when I came to look at it so no trouble shooting at the time.

To clarify, it was the speedometer, NOT the tach. That's part of what led to my confusion - I can't think of a circuit that would have both the speedo needle and the low/high beams coming together.


I do want to mention again why I am thinking this is more along the lines of a short than a bad ground - it has always fired right up without a problem, even 4-5 times the day the problem occurred. It showed no signs of low battery and nothing was left on, but the battery was so flat not even the dome lights or dummy lights or anything came on for a second when attempted to start, it was 100% discharged. I can't see something running just fine, and 8 hours later not even having a faint glow from the dome light. I think something had to discharge overnight, not just fail to charge during the day.

 

For a short: Use a test light or a simple bulb with two wires from a socket. With truck off, remove battery lead and put bulb in series with it. Bright bulb means short, remove fuses or fuse links until it stops.

The bad ground can make the alternator reference be lost, and it will put out a high voltage, above 15 volts, causing other circuits to go waco. Don't turn radio on while this is going on.