Truck wont start low voltage at fuel pump

I fixed the vacuum lines and now the truck wont start.

The fuel pump is not turning on. 7.1 volts at the inertia switch. 7.1 volts at the fuel pump relay and 7.1 volts at the #8 pin on the ecu orange/lite blue wire. I disconnected the inertia switch and still 7.1 volts. I unhooked the fuel pump relay and still 7.1 volts. Finally i checked the ecu and it only has 7.1 volts at the #8 pin orange/lite blue wire.


I guess the ecu is bad or am i missing something.

How does fixing vacuum lines/leak end up with low voltage at the fuel pump.........


Edit. I have continuity from the fuel pump relay orange/lite blue to the ecu connector pin #8 orange/lite blue. Both the ecu and the relay were unplugged from the connector. The grounds at the ecu are good. There are two that i could see right next to the ecu on the body. I had taken it to bare metal when i first bought the truck.

Strange you you'd have 7.1 volts on a circuit that has no power running to it. Even if it was a short and you were testing to ground, you'd be testing ground to ground. #8 FP I believe is a ground and not a hot. I think there's something fishy going on with your meter.. Do you have a test light to diagnose with?

Psychopete said:

Strange you you'd have 7.1 volts on a circuit that has no power running to it. Even if it was a short and you were testing to ground, you'd be testing ground to ground. #8 FP I believe is a ground and not a hot. I think there's something fishy going on with your meter.. Do you have a test light to diagnose with?

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Pin #8 goes from the ecu to the relay then to the inertia switch then to the fuel pump. It is an orange/lite blue wire from the ecu to the inertia switch. The it changes colors to black/pink to the fuel pump.

The other wire from the ecu to the fuel relay is pin #22 which is tan/lite green. Then it goes from the relay as a red wire to the fuel injectors. and sensors.

I have the wiring diagram in front of me. I am using a blue point dmm.


Why would pin #8 have no power when it goes to the inertia switch? This is on an 89 ranger 2.9L We are talking about the same truck right?

hkerekes said:

This is on an 89 ranger 2.9L We are talking about the same truck right?

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No you're right, pin #22 is the grounded side, sorry didn't have a diagram in front of me at the time. I have one from an '88, but it's doesn't look like it has a fuel pump monitor.

Have you tried the fuel pump test on the diagnostic connector? It supplements the ground between the ecm relay and may help isolate where the problem is. Hard to tell if it's the computer, might be able to test the continuity from the fuel pump test on the diagnostic connector to ground to verify if it's ground the relay or not, but I am not sure if that would work.

Psychopete said:

No you're right, pin #22 is the grounded side, sorry didn't have a diagram in front of me at the time. I have one from an '88, but it's doesn't look like it has a fuel pump monitor.

Have you tried the fuel pump test on the diagnostic connector? It supplements the ground between the ecm relay and may help isolate where the problem is. Hard to tell if it's the computer, might be able to test the continuity from the fuel pump test on the diagnostic connector to ground to verify if it's ground the relay or not, but I am not sure if that would work.

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With the relay unhooked there still only was 7v at the ecu. That is where the voltage starts and it gets its ground internally right?

I have not done the fuel pump test and i dont know how either. =(

hkerekes said:

With the relay unhooked there still only was 7v at the ecu. That is where the voltage starts and it gets its ground internally right?

I have not done the fuel pump test and i dont know how either. =(

Click to expand...

Well, I was thinking on the lines that the voltage was coming from the ecu. If you unhook the relay, you loose the fused power (yellow) that actually feeds the fuel pump o/lb. Later on Ford added a fuel pump monitor, pretty much a wire spiced in the main fuel pump wire to the ecu. But I am not sure if it's normal to see 7 volts from there when it's powered..



Do you have a Helm Inc diagram? I am pretty sure that is the correct diagram (minus some wires), but the wire color based in the manual would probably be the best bet.

Edit:
My diagram says it's the same as between the relay and ecu, t/lg...

Are the fuel pump and the Ecu relay the same? I had replaced the fuel pump relay not to long ago. I swaped them this morning and no change. Is pin 8 the monitor circuit and 22 the switched power to turn the fuel pump on which pin 8 powers the pump and monitors the voltage since it is the same color wire that splits at the relay? I guess the relay could be shot again or both of them are bad.


I had zero volts at pin 8 today. I have to test if pin 22 has voltage, if that is what runs from the Ecu to the fusebox to the switched side of the relay which turns on the pump and feeds the monitor circuit.



I wish u knew the history of this truck so I could strangle the owner.

I believe they are interchangeable, but I've never really looked into it. FWIW - The green plug is fuel pump and brown is computer relay, but you probably know that by now..

The computer relay needs to work for the fuel pump relay to work though; There's a red wire from the computer relay that powers some other sensors, injectors, and also the positive of the switching side of the relay. The ground side of this is the t/lg wire from pin #22. Once you have both, relay makes the connection from the main fused yellow wire to the o/lb wire to the pump.

Then there's an additional spice from the o/lb wire to the computer so that it can monitor the voltage actually going to the fuel pump for diagnostic purposes (#8).

Psychopete said:

I believe they are interchangeable, but I've never really looked into it. FWIW - The green plug is fuel pump and brown is computer relay, but you probably know that by now..

The computer relay needs to work for the fuel pump relay to work though; There's a red wire from the computer relay that powers some other sensors, injectors, and also the positive of the switching side of the relay. The ground side of this is the t/lg wire from pin #22. Once you have both, relay makes the connection from the main fused yellow wire to the o/lb wire to the pump.

Then there's an additional spice from the o/lb wire to the computer so that it can monitor the voltage actually going to the fuel pump for diagnostic purposes (#8).

Click to expand...

So 22 is a ground and the other side of the fuel pump relay is a powered red wire which powers the sensors. I either have a bad relay again or a bad ground at pin 22. I do have 12v at the yellow / black wire at the fuel pump relay.

I have replaced that relay before. I am going to replace both of the relays and hope that is the issue.

If the fuel pump relay is shot the red wire wont power the ecu relay thats how they switch power to the ecu relay correct? I could have two bad relays or just one fuel pump relay that is shot.

hkerekes said:

If the fuel pump relay is shot the red wire wont power the ecu relay thats how they switch power to the ecu relay correct? I could have two bad relays or just one fuel pump relay that is shot.

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Close but backwards - the ecu relay needs to come on first. If the ecu relay is bad, you won't have any power on the red wire, which is the positive side of the fuel pump relay, and pin 22 is ground as you've stated above. Ecu relay is switched on from the ignition key.

This may be more useful..

need some advice

hi to all, well i am from mexico, and been a lurker for some time now her, this is a old post but iam in the same bot right now, will chequink the relays i found out the the relay of the eec relay is getting hot and the relay of the fuel pump is 7.4 volts with a voltmeter, so unplug from the eec relay and plug in the relay for the pump will click onces and the will show 12.78 volts and than goes of with 7.4 volts again, and i got the haynes repair manual, did some one got it fix and can help me i will real appreciated.

p.s sorry for bad English