Can someone please help me clarify exactly what voltage I should expect to see if I check it at the fuel pump on my 1990 Bronco 5.0? I'm only getting about 7 volts. The car basically runs OK, but is 7 volts normal?
Thank you

 

Quote from rla2005
"Power is applied to the fuel pump via the fuel pump relay for 1-2 seconds when you cycle the key from the Off to Run position. This is a pre-programmed function in the computer (PCM). This means you will only get power for 1-2 seconds when you turn the key to Run then power drops out. To keep the fuel pump relay energized jumper Pins 2 & 6 on the diagnostic port then turn the key to the Run position. The fuel pump relay will remain energized (that means power will be applied to the fuel pump) until you turn the key to the Off position."

 

I don't have access to wiring diagrams for Fords older than 1992, but I'd be quite surprised if it's not supposed to see full battery voltage when the pump is activated. As mentioned in post #2 above, you can enable the pump to make the measurements.

Does your 1990 Bronco have a fuel pump inertia switch and/or a fuel pump selector switch? If so, you may want to check the voltage feeding the switch(es) as well as leaving the switch(es) to determine if maybe the switch or switch connections have some excessive resistance. Also, if you're taking your voltage measurements at the fuel pump assembly, are you sure you're measuring the pump supply versus a good ground and not measuring either the fuel sending unit output or possibly using the sending unit output as your ground reference?

-Rod

 

Its reading 7 volts entering and exiting the inertia switch. I am measuring the supply at the fuel pump, but I am suspicious of possibly a bad ground and I'm not exactly sure how to check that.
I found a lot of corrosion at the relay switch under the hood, and I thought that was the problem. I replaced the relay and I cleaned the corrosion thoroughly, but still only getting 7 volts.

 

You must have a bad ground on your meter because 7 volts will not run the fuel pump and you said it basically runs ok
In and out of the inertia switch is just that. Straight thru once the plunger is down
Run wires up to the front or jumper cables from the battery so you have a good ground to deal with when using your meter and taking voltage measurements
Used a jumper like torq showed you to drain the gas out when changing fuel pumps for years. Do not leave it on for too long

 

If you're reading 7 volts at the inertia switch and 7 volts at the pump, the ground issue would not be at the pump. If you use the same meter and measure directly across the battery, do you get an expected reading of something like 12.2 volts? What happens if you measure from the battery negative terminal to an expected good chassis ground, do you read much for voltage that way? If so, then there's an issue with your main ground from the battery.

My stash of service manuals only go back to 1992 so I'm not sure how this compares, but in 1992 the fuse for the fuel pump that feeds the inertia switch via the fuel pump relay is fuse 'O' in the Engine Compartment Fuse/Relay box, 20 ampere rating. You could/should check the voltage at that fuse as well.

-Rod

 

The 7 volts is an error as the pump will not function at 7 volts
He said it runs okay so the meter or his ground for the meter is at fault not the trucks fuel pump circuit ground

 

I'm still learning when it comes to electricity and ground testing so there is a reasonable chance I am doing something wrong here, but I definitely get the 12.2 ish volts if I read directly across the battery. I'll try and check voltage at the fuel pump relay ASAP.

 

Frankie,
What I mean about a bad ground on your meter is when you get back to the tank ( it is just insufficient )
There are no good grounds back there so wherever you are hooking the black lead is not getting a good ground
That is why you have 12 something at the battery and only 7 at the pump, but it runs (so the pump must be getting 12)
That is why it is sometimes easier to just run jumper cables to the rear of the truck when checking voltage
Then you can use the black jumper cable to be your ground while checking the pump voltage ( and you can use the red one to check the ground at the pump )
Then you should have 12 for a voltage reading at the pump
Good luck you can do this and I am 62 and have been learning juice all my life. Us pros call it chasing electrons

 

Go back and re-read Post# 2. Battery voltage is applied for only 1-2 seconds to the fuel pump when you cycle the key from Off to Run. When the fuel pump relay drops out you will typically see a 6-7 VDC value displayed on a meter. Some say it is stray voltage on an open circuit others will attribute it to the fuel pump monitor circuit in the computer (PCM).

Without jumpering the fuel pump relay to stay on while the key is in the Run position it is VERY difficult to diagnose this circuit. Dozens of folks have been tripped up on fuel pump troubleshooting because power is applied for that initial 1-2 seconds then they see the stray/phantom 6-7 volts on their meter when the relay opens.

 

If he got any voltage at all, he was there with the meter at the right time. (during the fuel pump fire up)

 

When I tested at the inertia switch, I grounded to a door hinge bolt that was painted, so I could see how that ground may have been poor.

I think I understand what you're saying about 12 volts for only 1-2 seconds, so I was definitely testing voltage after the initial turn on.
My plug as 3 wires going to it, I assumed one was positive, one was ground, and one was for the sending unit to the dash. I'm sure I was not connected to the sending unit wire. I connected black lead to what I thought was the ground wire, and red lead to what I thought was voltage. Does that sound right?

 

I can clearly hear the fuel pump relay come on, then turn off about 2 seconds later after I turn the key on.

My thinking is that maybe I should reconnect black to ground, and red to voltage, then have someone turn the key on and see if I get a 12 volt reading for the 1st 2 seconds. Is that reasonable?

 

If you only have three wires at your fuel pump assembly, then yes, one would be power from the inertia switch, one would be ground, and one would be the signal to the fuel level gauge in the instrument cluster. And you're thinking is correct in how to make the measurements. You can either use a second person to turn on the key while you are monitoring voltage, or you can do what's mentioned in post #2 above to keep power applied to the fuel pump by jumpering the EEC connector.

After again confirming your meter reads proper battery voltage when connected directly to the battery, and to confirm approximately how much voltage you should be reading at the fuel pump connector, I'd suggest measuring both from power to ground at the connector as well as making another measurement with a known good, clean, ground. If you get a different reading with both of those checks, then you have a loose/dirty/corroded ground somewhere for the pump. Although since you claim your truck runs fine, exactly why are you trying to make these measurements?

What colors are the three wires at the fuel pump assembly? If they happen to be Black, Pink with Black tracer, and Yellow with White tracer, then those would correspond to Ground, Power to pump, and Gauge Signal respectively. You'd want to connect your black meter probe to the Black wire and your Red meter probe to the Pink/Blk wire.

-Rod

 

Thank you for the input.

I dont remember exactly what colors the wires are at the pump, but I'll check next time i'm there and I'll try and report back. I do know that I wasnt testing the wire to the sending unit because when I replaced the pump I could clearly see which of the 3 wires went to it.

I'm concerned about voltage to the pump because the pump failed out about 2 months ago. When I checked voltage to try and diagnose the problem, I got the 7 ish volts (probably checked it wrong), and I thought maybe if voltage was poor it might have caused the old pump to fail and may cause the new pump to fail. Now it seems as though the new pump would not have fixed my problem if 7 volts was the true reading, so I'm feeling that my concerns about voltage were probably not accurate. I'm pretty good with mechanical things, especially just remove/replace jobs on cars, but diagnosis of electrical problems is not my strength. The whole purpose of my original post was to try and clarify if my concerns (and diagnosis techniques) were reasonable before I start chasing my tail trying to fix a electrical problem that never really existed.