Truck was running great and then this happened: it would start once per day. For example: I wake up in the morning, turn on ignition, check engine light is on, crank it, truck starts, runs smooth with no problem, turn of ignition, turn it back on, no check engine light, can crank, but it won’t start. Next morning same thin so on.

what have I tried:
1. replace EEC and Fuel Pump relays, no change
2. swap computer (PCM, I have 2)

Another thing I noticed that if I disconnect battery, wait for few minutes, connect it back, it will give me opportunity to start the truck one more time

Help will be appropriated
thanks

 

Welcome to the forum

No CEL with key on is, of course, the problem, so focus on that

Battery terminals are always the first stop, and the cables to vehicle
When you start an engine battery cables(BOTH) must pass 60amps to power the starter, this heats up these cables and can cause partial disconnected if they are not clean and tight
Once engine starts the Alternator is powering EVERYTHING, Battery is not used at all, its only purpose is to start the engine
So when you shut off the engine and try to restart the battery can't provide power thru its cables
Removing cable and re-installing reconnects it, so it works again


The CEL Bulb gets 12volts from instrument cluster with key on, like most of the indicator bulbs
It is Grounded by the Computer(PCM) to turn the bulb ON
PCM does this when its powered up, by PCM(EEC) relay in engine bay
It could be the ignition switch(under steering column) is not always turning on the PCM Relay, ignition switches can wear out, not common but possible

Test battery voltage, 12.3v to 12.8volts is expected, remember it, that's the "12v" voltage referred to below

Pull out PCM relay
Key off
Test each slot in relays base, one will have full time 12volts from fuse 4 in engine bay fuse box, 30amp
Turn key on
Another slot should now have 12volts from the ignition switch, leave volt meter or test light hooked up to that slot, cycle key off and on repeatedly to see if it sends 12v to that relay slot each time its on
If voltage is lower/dimmer then there could be corrosion under the engine bay fuse box on that wire, so not enough to activate relay each time

 

battery cables should be okay, starter works fine, but it doesn’t start when there is no check engine light, it’s just cranking

I checked relay terminals as you recommended, it has 12.6 volts all the time on one terminal, when I turn on ignition same terminal gives me 12.3 volts , another terminal gives me 12.26 volts , third terminal 1.8 volts and fourth terminal 0

Before this problem occurred I had battery light turning on when I was driving I don’t know if it’s related

thanks

 

That all reads as correct for EEC/PCM Relay

Should be a Mini-Relay so slots look like this: http://www.gtsparkplugs.com/images/m...iring-840x.jpg

30 and 87 are the LOAD, one should have 12v all the time, the other 0 volts, in this relay the 0v wire goes to the computer, and other devices that need 12v with key on

85 and 86 activate the relay, they cause 30 and 87 to be connected together, one will be 12v with key on only, the other is a Ground, so almost 0 volts

You can use a jumper wire between 30 and 87 and that willpower up the computer and engine systems, see if CEL is on now, if so replace that relay, and you can try to start engine now, with jumper in place

87A may not even be there, no slot or no tab on relay, its not used in this application

The difference with the 12.6v and 12.3v is that with the key on battery voltage will drop some when things power up, but 0.3v seems like a lot of drop, but battery may be weak from trying to start engine

The Battery light coming on indicates the alternator may be failing, but not related to no CEL, unless battery was dead, under 12volts
After you get engine start test battery volts with engine running, should be 13.5v to 14.8volts now, if lower than 13.5v alternator is failing

 

Jumping relay helped. The CEL light is on and It will start the truck.

I have 4 spare relays, but they do not work or will work for one start, after time I can use same relay to start it once

previous truck owner installed fuel pump switch
I don’t know the reason, I’m thinking maybe this is the problem and I just need to rewire the pump

few questions:

is something frying relays?
is it safe to drive with jumper ?
is there a way I can test a relay to know if it’s good or bad ?

thanks

 

Swap it with a similar relay in the engine fuse box, like the Fan relay, if fan doesn't work then relay is bad

People put in fuel pump switches to prevent theft of vehicle
Without knowing HOW he wired it, just leave it alone if it works
Nothing to do with CEL not coming on

You can drive it with jumper in place but battery will die if you leave it in place, engine off

 

I tried swapping relays, same thing

what I noticed one of the EEC terminals connected to a diode, when I removed diode it has black residue, maybe that’s the problem?

 

Yes, could be
That diode protects the 85-86 circuit that activates the relay from ignition switch, prevents a backward power spike

A diode is a One Way Street for DC Volts, i.e. 12v can only flow in the direction of the arrow >
The line | means volts from that end are blocked

12v in---->|-----12v out

0v out----->|------12v in

You could put a jumper wire in to replace the diode and see if the relay works all the time
If so then you will need to replace the diode, using it without the diode could burn out ignition switch
Pretty much all Fords used the same diode over these years so not hard to come by, and they rarely failed, so even used is a safe bet

 

I installed a jumper instead of diode, now CEL is on every time I turn ignition on, so diode is the the problem, I looked up online to find a replacement diode, couldn’t find anything. Do you know by any chance where I can buy it ?

thanks

 

Local auto parts should have them, Ford and GM used the same ones, in the 1990's

Does it look like this one?:
That's 4 of them, singles usually run $6-$10 each

In the 2000's they changed them to a --..| setup so they couldn't be put in the wrong way, like this: https://s3.amazonaws.com/rp-part-ima...a8c2daa1a7.jpg

It won't hurt it to put it in backwards, relay just won't work

 

I attached picture of diode I have, maybe I can modify the one you recommended, so it will fit

 

You have the 90deg Legs so Ford Dealer will carry it, about $5, or any auto parts store

Bring the old one with you