my 84 F-150 LX 4x4 manual, with the 5.0 has developed a electrical short that will completely drain the battery overnight. Key off and truck parked. I did the test where you pull the neg battery cable off and hook a test light up, then pull fuses out one by one until light goes out. The light never went out. but I did have a blown fuse for the heater/AC.
So I know its between the battery and the fuse box. I have completely unwrapped the harness under the hood to where it goes into the cab. and inspected each wire. found a couple of wires going to the alternator with rub spots.
I have pulled the radio out. disconnected the wiring to the heater/AC switch. and still the short remains.
Can any one give me a thoughts of which circuits that have hot wires ran to them that are known to give problems? lighting, starting ???

 

Probably bad diode(s) in the alternator. Remove the "bat" cable at the back of alternator, then retest with your "test light method".

 

X2
Dave - - - -

 

Possibly a circuit off the Fuse Links?

(Voltage Regulator, Alternator, Fuel Pump, ECA, Ignition Switch, Body Marker & Trailer Circuits)

So, try disconnecting these one at a time and check your Batt draw... If the alternator does not pan out..

 

The alternator diode is a common failure mode. It even still charges like this, though not as good.

 

Thanks for the suggestions, I will be checking the alternator out this morning. then from there?
the fuel pump on this truck is mechanical.
I though about the Ignition Switch. but have not looked into it yet.

I will see where the day takes me.

 

If you disconnect the Bat term on the alternator and still have draw, then remove the wires (the fusible links) from the starter solenoid terminal - then connect them one at a time while testing for draw until you find the wire that has the draw.

Also don't be fooled by the small draw that a computer or radio will pull.
The test light method is inaccurate in this regard (as it is difficult to tell how much the actual draw is) and an ammeter is more helpful.

 

Computer and radio memories will not light the testlight. It's a pretty good way to tell if you have a draw.

If you do have a ammeter, what do you use as a threshold of too much or not enough? I have been using up to .070 amps as being good, anything over is bad. The testlight is simple.

 

50 milliamps has long been the standard. Maybe a little higher these days, up to 70 mA say. Less than a tenth, of a single amp, is what we're talking about, for modern government school graduates. .05 amperes indicated on the display = 50 milliamps

 

Good point on the low current not lighting the test lamp.

See this article https://www.motor.com/magazine-summa...ter-june-2016/ good for modern cars (measuring "voltage drop" across the fuses).

Of course, our trucks are not modern by any stretch of the word - so the old test light method is good enough for us.

 

I did the checks today and found
1. pulled the wire (blk/org) that runs from the alternator to the solenoid, light stayed on
2. replaced the 1st wire and pulled the (orn/blu) wire that runs to the regulator light goes out.
3. replaced 2nd wire and pulled wire that goes to the choke, light stayed on.

In the process of taking an amp reading my meter sat on the 20 amp setting, started out at 7 went to 14 and shut down. and now the light is out and did not come back on after reconnecting the battery, starting the truck, and then disconnecting the battery. weird!!

 

I removed the alternator today and had it tested, and it tested out OK, so now I think I will look at the Ignition Switch.

 

So short is in the "Orn/Blu" wire circuit. But there isn't an Orange/Blue coming off the positive term of the starter relay. Or are you disconnecting wires somewhere else? EDIT - Oh you said you pulled the wire off the regulator. That's not where you should be testing. Start with the wires coming off the Starter Relay. Once you find the circuit with the draw, we can narrow down the tests for the items on that circuit.


Are you removing the battery cable and putting the ammeter "in series" with the bat cable (neg terminal to battery positive, and positive terminal to battery cable. Or vise versa if your working off the neg battery terminal)?
The amp draw you saw could have been something like a courteously lamp that turns off a few seconds after you close a door - or a capacitor charging in the computer. IDK. Sounds odd.
But since it went to zero that doesn't sound the problem circuit. When troubleshooting these things you need to wait a little for circuit to stabilize before drawing a conclusion.


See page 15 of this link http://www.garysgaragemahal.com/char...e-engines.html Look at the wires coming off the Starter Relay (the Fusible Links). You may not have all the wires shown, but those are the ones you are to remove, one at a time, to see which circuit has the drain.

 

Its been a few days and I wanted to give an update to everyone who has offered suggestions.

I have had this truck a little over 2 months, I got a good deal on it knowing it had a key off short. I have temp installed a shutoff switch for the battery. Its a retirement project.
The 1984 owners manual is still in the truck which is nice. The door sticker shows it was manf 08/83 in Dearborn, wb 117, axle 19 which if I read the chart correct is a FORD 8.8-INCH, 3.55, 4 speed manual.

I still have not found the complete electrical problem, but lots have been found and corrected. and still more to do.

Rewired the after market radio the PO had installed. all the wires were put together using wire nuts, plus the power and memory power wires were reversed, causing the radio to be on all the time. fixed.

Found the instructions for testing the light switch, and found the IP lights circuit in the switch did not work. I pulled out the IP and made sure the light all worked. I replaced the switch. IP lights back on.

When I turn on the headlight the radio goes out, standard headlights. I have everything except the circuit breaker to do the relay upgrade.

The other thing I found on the headlights circuit is that most of the wires are hard, another reason to do the upgrade.

I also tested the wiper switch, it tested out OK.

Tested the HVAC switch and it only runs on high need to get a new HVAC fan resistor.

I pulled the alternator out and had it tested it tested good, two different parts stores.

The trailer light wiring harness was a mess, I just removed it.

Now for a couple of questions;

Does anyone know where I could find some of the round 4 wire connections, used in this model truck? Salvage Yard, I'm guessing.

Can a fusible link be replaced with a circuit breaker? I have not found a bad one just something I wondered about.

the resister on the back of the IP for the gauges how do I test it I have not been able to locate the procedures.

Next Steps; complete the headlight modification, waiting on the resister for the HVAC,

 

Have fun with this link for the connectors. http://www.fordservicecontent.com/pu...torcatalog.pdf

You can replace a fusible link with a fuse or circuit breaker. I do not recommend it. The fusible link is small and weather proof, fits right into the harness and handles ALOT of current before it fails. If you use a circuit breaker or fuse, they need to be VERY large. They still use fusible links in the newer vehicles, but have gone to mega fuses for some of the circuits. You would need 60-100amp circuit breaker or fuses to replace some of the fusible links. These are large and clunky and clutter up the real estate under the hood, and then you have to worry about insulating and protecting such large circuits.

I am not sure what resistor you are talking about on the IP circuit board. There is a voltage regulator back there, is that what you are referring to?