hI ALL HOPE YOU HAD A GOOD CHIRSTMAS. well the truck will still not fire up. never heard the fuel pump come on. we swithed the relays out and still would not come on so we thought for sure it was the fuel pump. pulled the bed spent 315 bucks changed out the fuel pump hooked everything back up and still nothing. not a sound from the fuel pump. checked the reset button on the trip it was fine. help ... Tommy

 

check for power and ground at the relay. first check for power, (if its a 4 pin relay its like this...) take a test light, and ground it on the alternator frame (this is a great ground since the alternator is case grounded) Then turn the key to the on position. 2 of the pins in the fuse box that the relay plugs into should have power, there should be 2!!!! If this is good, then what you want to do is take your test light, or volt meter, either works, and put 1 side on the positive voltage side( either of the 2) and then put the ground lead into what you determined to be the negative... or ground route of the relay... there will be 2.

You may need to use a paper clip or something of the sort to get in there because you dont want to cram a test light or test leads in there.

If you can get yourself a wiring diagram it will help you a lot as soon as you figure out if you do not have power, or do not have a route to ground.

Im not sure if when you turn the key to on, if you are applying power, or applying ground, they do not always work the same.

If you test enough you should be able to determine which (relay connections run diagonal because that way you cant put a relay in backwards) is the voltage signal, or ground signal that allows the fuel pump to turn on. So if you are confinent, you can jumper a certain terminal to power(battery) or to ground.... and the fuel pump should turn on.

Did you check for voltage at the fuel sending unit when you had it out?

I want to say that a module, probably the PCM switches to ground allowing the fuel pump to turn on(signal voltage at the relay is grounded, allowing high current to flow to the fuel pump and activate it..... but if you had a wiring diagram you could check for sure.

 

thanks for the info hawk. we are going to do some more checking tommorrow. when you say check at the fuel sending unit are you talking about where it plugs in? as far as relays what we done was swapped the horn relay that we knew worked and put it in plae of the pump relay. but we did not have anything to check voltage with so we assumed if we had it at the horn relay we would have it in the relay next to it. i know we never assume but we did. if the pcm went out would it not mess a lot of stuff up? thanks Tommy

 

The PCM could go out without having any other problems. it is possible.

In most cases with electrical problems it is usually NOT the wiring... BUT in this case it might actually be the wiring.

There is at least 1 connector at the fuel sending unit, chevy trucks have 2. I believe that on the 2 plug systems 1 is for the fuel pump and 1 is for the fuel level sensor(float)

The idea behind a relay is that a small amount of current controls a large amount of current.... Your fuel pump needs a good ammount of current flow, so if you were to just have a switch to turn it on, you would need a large switch to handle all the current. With a relay they use a small amount (mA) of current to flip a switch inside the relay. This can be done in 2 ways, a pull up or a pull down. Pull up is applying power, Pull down is applying ground. So when you turn the key to the on position, either the key switch applies power(mA) to the relay-pull up, or it applies ground somewhere else-PCM most likely- pull down. When current flows through the "coil" side of the relay it switches the relay on and high current flows through the other side of the relay through the switch. 2 powers, 2 grounds, small current controls large current.

SO if you have power at 2 terminals at the relay, and you dont have ground- check a couple of things... for power at the fuel sending unit, 1 terminal HAS to have power. 1 of them also HAS to have continuity to ground. No idea which color should be which. (Do these tests with the relay installed) but if you dont have ground at the pcm the relay wont even click over, is it clicking over? it may be hard to tell though you probably have multiple relays engaging when you turn the key on.

Did you replace the entire sending unit? or just the fuel pump?

 

hawk it ame with the float and everything attached all we done was take the fuel lines off and reatache them to the new one. also had to do the eletrial conetion plug. by talking about the ground check are you say if i have power by grounding to a source outside the relay plug but if i use the ground thats in the relay plug and i do not show power is that saying i do not have a good ground?

 

Exactly, if you ground your test light to the alternator and have 2 terminals with power, you have something bad downstream. The power is sitting there waiting to be sent to the fuel sender, and ultimately to ground.

You did replace the entire sending unit, which is good, because often the wiring within the sending unit can be fried.

I can try to look up a GM schematic, and give you reference as to where the sender and the realy switch grounds to. let me do that and get back to you.


come to think of it you may actually have constant power at 2 relay terminals at all times, and then when you switch the key to on you allow the power to reach ground, possibly through the PCM and or ignition switch. (your problem is probably not the ignition switch)

 

Ok, here is the Chevy equivelant.

first there is a fuse(under hood fuse block) DID YOU CHECK THE FUSE?!?!?

in chevy the VCM (vehicle control module) applies turn-on power to the realy when you turn the ignition to on. Allowing the high current constant power at the realy to flow to the fuel sending unit.

Here is what is interesting!!!! In chevy BOTH of the grounds are grounded to the chassis somewhere. The fuel sender is grounded somwhere under the truck bed, and the relay turn on signal is grounded somewhere under the hood.

So if this is the same as ford, it makes me wonder if you have a bad ground, either after the fuel sender(you may have power at the sender, but it is not able to ground) or you do not have ground for the turn on signal of the relay, which wont flip the relay on.

I dont know if ford has a vcm, it may as well be the PCM, I would really try hard to find a wiring diagram or a simple description of the system so that you can do some checks to figure out whats going on.

THe easiest thing to check will be the ground after the sending unit, just follow the wires from the connector at the sender, and 1 HAS to go to ground. GM has a Black grond wire, if this ground is corroded or loose or there is an open-your problem may be that simple.

 

hi hawk and thanks for your response. this is what we found out today. i went back to the relay to check with the relay out i had good voltage across the power circut but when i checked for the secondary circut for the coil i had no voltage. so i jumped out the relay and the pump came on. i thought great it was something so i tried to crank the truck and it would not fire up. so after scratching my head went back to the fuse panel and check again. i found the 30 amp fuse going to the pcm module blowed. replaced it and it blew again. SO NOW WHAT. i hate to let something like this kick my butt but it looks like its winning. tommy

 

I hate to say it, and I think you know what I'm about to say.

Unplug the PCM connector from the PCM. You might have to remove the battery and the battery tray. Its like an 8 or 10 mm bolt that holds the connector on to the PCM. (Im pretty sure they are all in the same location, in the firewall on the passenger side right next to the battery, there is a big clump of wires going into it.

With the PCM unplugged, insert the fuse. If the fuse blows your PCM power wire has shorted to ground.
If the fuse does not blow.... replace PCM. (I hate this) there is a short to ground somewhere within the PCM

GM only shows 1 power wire going into their VCM, and it has 2 ground wires. When it comes to modules, power wires are almost always red or orange, and grounds are almost always black.

See how it goes with the PCM unplugged, and before you buy a new one tell us the results.

 

hawk would it be possable if any of the sensors were bas to blow a fuse like that? are saying with the pcm unpluged try to crank to see if fuse blows or just turn the switch

 

just turn the switch on-dont crank, if the pcm is unplugged, the battery voltage wire is not going to make contact with anything. So if the fuse still blows there is a short to ground, if it doesnt swill blow the PCM could be bad.

Right now is the pcm fuse blowing whether the key is on or off? or only when you turn the key on?

 

only when you turn the key on

 

unplug PCM, insert fuse, turn key on. see what happens.

 

hi all sorry i did not post back. well the truck is running again after towing it to the ford dealer. it was in the wiring harness. a clamp holding the harness up broke and let it drop onto the exaust. it melted 3 or 4 wires. they spliced it back togather and all is fine now. hope this might help someone out. thanks for all of you guys helping out. Tommy

 

Thanks for letting us know what fixed it