I have a 92 f150 4.9 l 5 speed and if I leave my truck sit for a day without starting it the battery will drain down to about 8.50 volts and won't start. I know where stuff is but not sure what to check. Does anyone have any ideas? I know that there is nothing being left on in the truck. Help if you can please.

 

even if nothing is "on" you have something thats causeing the drain, either by always being constent on (hot) or that is shorted compleating the circut draining the battery, do you have any mods to this truck of yours, have you installed any thing that can be left on all times? is there ANY thing thats on a circut with NO switches? IE do you have a GPS plugged in the power port which is always hot that you leave running (charging battery when "off") that remains plugged to the power port?


if so things like that could be the cause, so if we know what mods if any were done we could at least start somewhere, did you add a train horn/air horn? they are items that could be possibilities if added in.

also check the key is fully "off"IE it isnt in ACC mode or run mode

 

Do you usually park the truck on a hill? And if so-do you have a factory underhood light with mercury switch?

Do you have an aftermarket alarm system?

 

With the engine off and a fully charged battery disconnect the positive cable from the battery and see how big the spark is.

Connect an ammeter in line and see how much it is drawing, or use an inductive ammeter -around- the battery cable if you have one.

First, try unplugging the alternator. if the draw stops the rectifier diodes in the alternator are bad and allowing current to flow to ground.





If you have a second generation alternator (2G...with an external fan on the front) check the condition of the charging plug. (the rectangular one with 2 black/orange wires and 1 white & black)
If it's melted, or has ANY signs of heat discoloration replace it with a new harness pigtail before it sets your truck on fire.
NO JOKE!

Any GOOD autoparts store should have one of the following in stock for under $10.

Borg-Warner PT-780
Standard Motor Products S-542
Power Plug 4628011
KEM Parts 350-220
Pico 5711
Dorman/Motormite #85123 / 08622
etc.....

 

First off thanks for the help. @ Jaime the only aftermarket item is a CB and is hooked to I beleve the radio circuit that shuts off with the truck. I have recently put a new alternator on but this problem was happening before that. @ phoneman I park the truck on flat groung but I did chek the light under the hood and the bulbs not in it. I am going to try and do an ammeter test. To add to the situation. I went out this morning and turned the key on I had no start-up lights or guages move. It was light I had no power on the dash. I unhooked the Negative cable and when I hooked it back up the dash was working. Strange!!

 

The battery was recovering while you had the cable disconnected.

It doesn't take much. Lead acid batteries are very robust.

If it is the plug a new alternator will not fix it.
FWIW, most alternator rebuilders will not warranty without your replacing that plug.
Many of the better rebuilders include one in every box.

 

A poor mans alt test is to first thing in the am before starting the truck is to feel the alternator, if it is warmer than ambient temp then the diodes ( as Jim had said) are leaking and drawing current.

 

Connect an ammeter or dvm between the neg terminal and the Start pulling fuses till you have no draw. Unplug alt, relays ect. Then you will know where the problem circuit is.

 

Ok, I hooked up an ammeter and I have a 3.89 draw. I unhooked the fuses and relays both inside and outside the truck. the only ones that made a difference was the fuse which controls the clock memory. i unhooked everything in the engine compartment and the stuff under the dash that i could reach, all with no change. I hooked the battery back up and the dmm is just slowly counting down?????????????

 

Disconnect the black/orange charging wires at the starter solenoid and see if it stops.

The solenoid itself could be shorted to ground through the mounting plate but IME that is very rare.

 

There is a doubloe white wire on the starter soliniod that runs to the headlights and over to the other side. if I hook the ammeter to that connector to positive post that I believe is the draw. the black and orage wire made no dofference. I left the negative terminal off so it would start this morning.

 

Have you traced those white wires and looked for frayed or chafed areas that could be shorting out?

I only have schematics through '88 so I can't say with any certainty where those wires go.

 

I'll be doing that today and let you know whats up. ty

 

Does the fuel pump prime, or run for a sec with the key off,when you reconnect the neg terminal?

 

Do the fuel pumps draw 4A? (I don't know, mine's mechanical)

Could it just be a stuck relay?