my 87 bronco (4.9 with a 4speed) decided to give out on me yesterday, RIGHT AFTER I PUT 80 BUCKS IN GAS IN THE PIG. she ran perfect (which is funny because on the way to the gas station i made note of the engine having to only crank once, maybe one second on the starter if that, to fire up). shut her off, filled her up, but when i went to leave, i cranked it over for a minute and she wouldnt start. hold on the gas and she would run on one cylinder for a brief time, but then it wouldnt even do that. quick troublshooting revealed the fuel pump wasnt pumping. so we towed it to the shop, and started working. checked all fuses, none were bad (come to find out the pump isnt even fused). tested for 12 volts at the pump- nothing. figure its the inertia switch. switch is good. but no voltage. but we looked and the plastic around the wire coming from the relay switch was melted... break out the haynes manual, look at the wiring diagrams. trace it to a relay switch next to the air cleaner box. pull it off, look at it and find the pin for the wire to the inertia switch has been arking... test the relay, and its bad. 8 bucks later for a relay, were getting 4 volts from the ignition to switch the relay, we get 12 volts to the relay going back out to the inertia switch. wierd part is we get 12 volts into the inertia switch, but only for about a second after we turn on the key. whats going on here???? its driving me crazy
wierd part is we get 12 volts into the inertia switch, but only for about a second after we turn on the key. whats going on here???? its driving me crazy
That is the way it is supposed to work.
The Computer times it out after one second after the key is in the run or start position and the computer does not see a PIP pulse saying that the engine is turning.
if thats the case, why does the fuel pump prime when you turn on the key if it gets 12 volts for only a second? i dont know about you, but i was taught that a load can only run when it recieves ample power...
or do you mean the computer gets a pip from the pump running (which doesnt make sense to me because the pump grounds out instead of returning to anything)
When the key is turn to the run or start position the EEC Power Relay is picked.
When this relay is picked it sends power from the starter relay battery side to the EEC Computer, Ignition Coil, Fuel Injectors and the Fuel Pump Relays coil.
When the EEC Computer receives this power on pin # 37 & # 57 it grounds the Fuel Pump Relay coil and that picks the Fuel Pump Relay sending power from the starter relay battery side to the Inertia switch and on to the fuel pumps.
If the in one second the computer does not see a PIP pulse from the Distributor it removes the ground from the Fuel Pump Relay and that lets the Fuel Pump Relay drop out turning the power off to the Fuel Pump circuits.
In other word the computer will not let the fuel pumps run if the engine is not running or cranking.
i understand the wiring aspects of it- ive spent a good long time studying the wiring diagram of this truck.
but if everything goes down just how you say it does- that does not explain WHY or HOW it is possible that the fuel pumps prime the lines if power only goes to them for 1 second.
and that is my problem- the pumps DO NOT recieve power even though the circuit is in working order.
The fuel lines should already be full of fuel unless you have a bad check valve in the FDM.
Then one second is long enough to re-pressure the fuel system and the pressure should hold for a few hours while slowly dropping.
If the fuel lines were empty then you would need to turn the key off & on about three times to pressure the fuel system.
You say "the pumps DO NOT recieve power even though the circuit is in working order".
If they do not receive power at all in this one second time period then we have to take a look at that.
OK if you are not getting the power out of the Inertia switch and you are getting power in with a test light the Inertia switch is open and if it will not reset then you need to replace it.
The meter will fool you so use a test light in this circuit on a go or no go bases.
Make sure you have a good connection where you found the plastic around the wire coming from the relay switch was melted.
If you need the power on for longer than one second to test then ground pin #6 of the EEC self-test connector with the key on and the power to the pumps will be on as long as you need it to be.
problem solved. even though this thread completely confuised and frustrated me, we got her this morning. we checked every possible connection, wire, component, and everythingl ooked good- which left the computer to blame. we even swapped out fuel pumps come to find out the old one was good (and i took a nice gasoline shower because the lines were pressureized...) so as a quick fix while i looked for a computer, we went "oldschool" and plugged a wire into the fusebox, to a switch, to the hot wire for the fuel pump- and voila, it ran for the time being. but then i got to thinking that maybe it just a case where it needed a little jump start. so i plugged everything back where it should be and she ran just fine. the only thing we replaced was the relay.
but to clarify things now--- when I say we only got 12 volts to the inertia switch for 1 second, i mean ONE second. literally. you put the meter to it, turn the key, and it jumps to 12 and then back to 0. i think when you say one second you mean several seconds, because the pump on any truck ALWAYS runs for several seconds when you turn the key to prime (or repressurize, as you say). you can hear it buzz. with my problem- i got nothing. so the wiring was right, your explanation of things is right, but how it went down isnt right. and i think there was just some miscommunication.
but anyways, the big bronc is back up.
thatnks for the help.
The Ford specification for the running is one second. I think one second is longer than you are thinking. A flick of the meter movement is not one second long.
The next time you turn on the key without starting the engine say one thousand and one. See if that is how long you hear the pump run. I bet it is.
This forum is owned and operated by Internet Brands, Inc., a Delaware corporation. It is not authorized or endorsed by the Ford Motor Company and is not affiliated with the Ford Motor Company or its related companies in any way. FordŽ is a registered trademark of the Ford Motor Company.
