Ford F150 - truck won't start after sitting overnight

i have a 1991 f-150 that runs fine all day but when it sits over night is is DEAD and i am really DEAD.
please help so i can get too work

Do you mean dead as in battery is dead? If it is, check the fuel pump relay and se if it is staying energized and letting the fuel pump run while the truck is shut off. This was a very common problem in those types of trucks. Shut truck off and go back to the fuel tanks and listen for the pumps running. You will hear a whining/buzzing noise if they are. Good luck

my friend had the same issue with his 1990 f-150 and he said something about a relay. he said if you are looking under the hood the relay is on the right side is this correct?

Right side if looking under the hood but actually left ide if sitting in drivers seat. There should be a little black box thaat haas at least 3 or 4 relays. The green one is the fuel pump relay and the brown one is the EEC relay. You may want to replace both but the Green one should fix it if that is the problem.

what is the eec relay.

Electronic Engine Control Relay. It powers up the PCM(computer) and also supplies power to the fuel pump through the fuel pump relay.

what athor relays are in that box.

I think there is only 3 relays---EEC(PCM),fuel and WOT(Wide open throttle cut out). They should be WOT(black)EEC(brown)fuel(green). They are on top of the drivers side wheel well sometimes housed in a little black box type piece--that is unless somebody broke them and didn't put them back in the right place.

Did this fix this problem?? My truck is fine if I drive it every day. But over 36 hours it is under 9 volts and won't start. This has just started out of no-where. Anyone have an idea. Battery is new, charging system working AOK. FYI - did it with the old battery and the new one. That was the first thing I replaced as it was the original.

What electrical mods have you done? Radio? Radar detector? Cell cradle? Winch? Alarm? RKE?

How do you know the alt is OK? Has it been tested recently for a bad diode?

The Battery is new and exhibitting the same problem, so very doubtful that it is also bad.

No mods since new (7 years ago). When it was new an auto start and alarm were installed. Seem to be working fine.

Alternator charges the battery if you use the truck daily. Output is at 13.8volts. Would these things be true if there was a diode short? What are the symptoms of a diode short?

Thanks for the help. Tom

A simple draw test needs to be done to find out what is drawing away the voltage from the battery. An alternator can be causing a draw even with the engine not running and can also still be charging ok. Without doing a draw test, pulling fuses,unplugging the alternator, etc. etc. You will never be able to keep guessing what the problem is.

OK - I didn't know that about the alternator. It is the original with 98k on it, so I could see something like that happening.

The Draw test - I am assuming this is putting a Milli-amp meter inline to the battery and measuring current draw as you disable systems - correct? And is diconnecting the altenator simple? I honestly havn't looked yet.

I was also wondering if anyone else has had their RADIO fail in such a way to cause this problem, so that was part of the reason for asking these questions. It all happened at the same time. Radio going dark with only on\off, static and volume and the battery issues. Seems coincidental.

Correct on the draw test----Ford uses 50 milliamps as a rule of thumb. Anything over 50 is too much and can cause a battery problem. You can unbolt the main battery cable from the alternator and also unplug the the connector. Not real bad----but removing the belt and then the alternator would probably be easiest.
A radio can cause a draw on our battery. Actually a radio cause some draw always because of the voltage to keep the memory for the clock and the radio stations---but not enough to cause battery issues---it falls under the 50 milliamp spec. But, a radio can draw more then 50 and cause a problem too. Try disconnecting it also.

13.8V is slightly low. Many parts stores test alts. free in the parking lot - call around.