Hi All:

I'm a rank amateur trying to deal with a bizzare electrical problem on my '99 F-550. The batteries have been running down overnight. When I turn the headlights on I lose the entire back end from the high lights on back (running lights still work). With the headlights on and the flashers on I get the back end to light up (all lights). With the headlights off and the either blinker on I get flashing on both sides in the back. With the headlights off, the blinkers on, and the brake depressed I lose the whole back end again. On a bumpy road or in the rain, the interior lamps will start flashing.

I've pulled the interior relay and that shuts down the disco effect. I've also disconnected the air ride pump power and the tool box lights to eliminate them from the equation with no effect.

Obviously I've got at least one short, but really don't know how to diagnose. I'd like to learn, does anyone have advice??

Thanks,

Jeff

 

Yes, lots of Asprin! On the serious side, divide and conquor is likely the best way to go here. One question which comes to mind is do the batteries run down every night or is this just something that crops up ofter a run down a rough road or in the rain?

As to finding the source, the first thing to do is to see what kind of draw you have on your batteries when the key is out. I use an inductive pickup on my meter but it's not strictly required. You will need an amp meter with at least a 10 amp capability however. What you will want to do is to measure the current draw from the primary (battery on right side - the one connected to the engine & frame) to see how much current is being drawn. Next start pulling fuses, one at a time; replace each one after noting if the current draw changes. When you get one that results int he draw going down or being completely elliminated you've narrowed you area to trouble-shoot. Keep in mind that some things will have a draw (such as keep alive for the computer(s) and radio, hood light and courtesy lights. I usually pull the hood light and then make sure the doors are closed long enough for the interior lights to be out when doing this kind of work.

Your likely culprit will be the circuit that gives you the biggest drop in current draw. for example, on my 2000 F350 (with some added gear that needs keep alive current) I see less than one amp draw. Your problem will be somewhat more than that (or your batteries are in really bad shape).

Good luck and let us know how things go for you!

 

The first thing you can do is measure the voltage at the battery with the engine running. If the charging system is ok, you should have around 14 volts.

Here's how you can test to see if you have a drain with the engine off, and a testlight connected as below. You can pull fuses like the other poster said, and if the light never goes out, you can pull the wire off the alternator to see if the light goes out.


Your lighting problems sound like ground problem. I am assuming you probably have some sort of aftermarket bed on this truck, and I bet it doesn't have a good ground to the frame.