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1987 Bronco II extremely low voltage only at in-tank fuel pump connector


Shran

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Have you checked the wiring from the inertia switch back into the dash? I’ve had issues with it melting at various spots.

I’m assuming you are grounding out the fuel pump wire on the ECM test connector when you are checking for voltage at the pump? Grounding that powers the relay constantly when the ignition is on, not just for a few seconds when the pumps prime.

Another common failure is the ground wire coming off the negative battery terminal, from the factory it would be one or two small wires that run off towards the firewall. Those provide ground for the fuel pump and computer.
 


TastierCash

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In a lot of "lost power from here to there" cases, you need to de-pin the terminals from the connector body for a complete inspection.

There are usually two places where a wire is crimped to a terminal- 1) the bare copper wire strands, or the conductor part, will be crimped closest to the terminal contact end; and 2) there is also a "strain relief" next to the first crimp, on the end that wire comes in, in which the plastic insulation, with the copper strands within, are crimped in a similar fashion.

With age and exposure, the copper strands can corrode (rot) or structurally fail (break). When the harness was made, the equipment which automatically makes these crimps must be adjusted precisely or the crimp isn't perfect and will end up failing from time, exposure and vibration. Something I have seen before, is that the strain relief is a little too tight, and with time the copper strands inside the plastic insulation are stressed and finally break. The insulation looks perfect, but there is a conductor break within. You have to get the terminal out of the connector; the conductor crimp can be visually inspected, and flexing the wire-to-terminal junction will usually show an internal break at the stress relief by being "wiggly". You can also use a pin to pierce the insulation upstream, and use a meter to check conductivity (or resistance in ohms) between the wire strands and the terminal itself.

Refer to the diagram and look at it this way: there are only three physical components making the power circuit from C161 to the tank pump: a terminal, a piece of wire (note no splices in it), and a terminal. The terminal at C161 will have two wires crimped to it (both pumps' power wires), and the terminal at the tank pump will have just the one wire. So logically, you have can only a break in the wire, or a failed connection at the wire-to-terminal on either end, IF there's +12V at C161 but no volts at the pimp.

Hope the info helps. It's not difficult to de-pin a connector, just be careful as the old plastic will be fragile.
Thank you for the response, when I go to check the wiring again and de-pin the connector would it be viable to pierce the wire with pins at two points and check for any power at parts of the wire by measuring from between the pins or will it not be accurate or work at all (measuring in vdc specifically)? I'm not the best with wiring, just with mechanics like engine or system work/repairs as I have only recently started working on wiring. Luckily I know how to be somewhat safe around electricity due to some IT knowledge. Also, is there a tool made to assist in de-pinning the connectors? I don't want to risk damaging the wiring or connectors beyond repair if possible. I would also appreciate any tips you may be able to provide in terms of wiring different parts together or repairing wiring as I no longer trust myself with a soldering iron after last time and the first wiring I did for the truck was the inertia switch which ended up kind of just thrown together using spade connectors. As always, any help is greatly appreciated and thank you in advance.
 

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Also, is there a tool made to assist in de-pinning the connectors?
Yes there are special tools, but there is usually a work-around. Just don't use a hammer. :)

Connectors vary; if disassembly is not obvious post a pic and I'll try to help. I'm not sure off the top of my head which types the '87 will have in it.

Anyplace you pierce insulation, be sure to seal the hole when you're done with either a liquid tape or good electrical tape.
 

TastierCash

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Have you checked the wiring from the inertia switch back into the dash? I’ve had issues with it melting at various spots.

I’m assuming you are grounding out the fuel pump wire on the ECM test connector when you are checking for voltage at the pump? Grounding that powers the relay constantly when the ignition is on, not just for a few seconds when the pumps prime.

Another common failure is the ground wire coming off the negative battery terminal, from the factory it would be one or two small wires that run off towards the firewall. Those provide ground for the fuel pump and computer.
Thank you for the suggestion, I have not checked the wire that runs back into the dash but I figured it's probably good since when I measured voltage between the two I had around 12v, never less than 11.8v. Also, yes I ground the relay to be permanently open through the test connector each time that I measure the voltage. As for the ground, I don't believe that it would be the problem as grounding the fuel gauge sending unit to the ground pin for the pump resulted in a measure of 12v but grounding the positive of the pump to the negative of the gauge resulted in no voltage. However I will say that I believe I have seen three or four grounds from the battery total or branching off from it at some point, only one of which is disconnected but I've attempted using alligator test leads to connect it to the frame to complete the ground. It may also be worth stating that this ground in question bolts to the intake manifold, however I have removed the intake manifold for the time being (that being why it isn't connected to the correct piece at the moment) and the issue with the pumps not working had come before I removed the manifold and its ground from it.
 

TastierCash

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Yes there are special tools, but there is usually a work-around. Just don't use a hammer. :)

Connectors vary; if disassembly is not obvious post a pic and I'll try to help. I'm not sure off the top of my head which types the '87 will have in it.

Anyplace you pierce insulation, be sure to seal the hole when you're done with either a liquid tape or good electrical tape.
Good to know and thank you for offering your help. Would the previously stated method of piercing the wire in two places and measuring for voltage work to check for breaks/12v or would it display no current between the two pins?
 

Shran

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When my inertia switch wiring melted, it had shorted out in several places and kept blowing the fuse. This happened at random and I really chased my tail for a month in several parking lots where it left me stranded… very annoying. I eventually found it and clipped out a bunch of those two wires. Maybe not your issue but something to check anyway.
 

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Assuming that it has it (most do) use "continuity test" on the meter. Some call a "diode tester".

Or measure on the meter's ohms setting between the two points. "0" ohms = zero resistance; numbers show resistance, and the higher the number is higher the resistance.

With a conducting circuit you want to have the least amount of resistance, and even distance in a wire will create a small resistance, all other things being perfect.

No circuit, or an open, will show dashes or an infinity symbol on the meter.
 

TastierCash

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Assuming that it has it (most do) use "continuity test" on the meter. Some call a "diode tester".

Or measure on the meter's ohms setting between the two points. "0" ohms = zero resistance; numbers show resistance, and the higher the number is higher the resistance.

With a conducting circuit you want to have the least amount of resistance, and even distance in a wire will create a small resistance, all other things being perfect.

No circuit, or an open, will show dashes or an infinity symbol on the meter.
My meter does have a continuity option but for some reason for both ohms and continuity it shows just the number 1 on the far left when not connected to a circuit, probably cause its a cheap meter. I assume the two pins will work for checking between sections of wire. What numbers should the continuity show when I check the connection? And should I check the pump positive and negative at the tank again? One last question for now, if I get the in tank pump working but the high pressure doesn't start working, what should I do?
 

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Honestly, the easiest way to troubleshoot the circuit to test for voltage. Meter Red lead to the tank pump terminal with the Black/Pink wire; meter Black lead to chassis ground. Use test jumper leads with alligator clips to set up the meter and free up your hands. Power up the fuel pump circuit like @Shran described (and thank you going over that, Shran).

Once you are set up, start at C161 and inch-by-inch work your way back to the tank, looking at every inch of wiring, as you push, pull, wiggle and shake the harness. Watch the meter for scale jump.

If that doesn't produce any results, check the Black/Pink wire on the tank pump leg about a foot downstream of C161, by using the pin method and check for +12V, meter Black wire to chassis ground. If the test is bad, disassemble (de-pin) C161 and determine where the open is (twin wire crimp on the downstream terminal would be the number 1 suspect area. If the test is good (does have +12V on the Black/Pink, tank pump leg), then do the same test at the tank pump connector end.

If the Black/Pink wire has +12V at about a foot from the tank pump, and the tank pump connector terminal does NOT have +12V, the open will be in that foot between those points. Again, the connection of copper wire strands to the terminal will be the prime suspect, and keep in mind what I described about copper strand breaks inside the insulation.

So yes, resistance testing can be done, but voltage testing as I described is a lot faster and easier. And less confusing.

A simple way to describe the best troubleshooting is, is that you know where you have the power, and where you don't have the power. Your testing is simply narrowing down the area of the fault until it becomes obvious enough to find, and correct. If you reach a point where you confirmed power at the pump, then you'll look at the grounding side, and the pump itself.
 

gaz

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@TastierCash ,
- Have you checked the fuel rail pressure?
- The high pressure fuel pump can be checked without a multimeter. Unplug the pumps output hose, set the ignition to run; fuel should pump out visibly with gusto.

NOTE
Also the high pressure pump is only on for 2-3 seconds unless the engine starts
 

TastierCash

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@gaz
I have indeed checked the fuel rail and not only is there no pressure but there is not any more than about 0.01v at either pumps power connectors with KOEO and I have the ecm test jumped to constant run the pumps every time I test.
 

TastierCash

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@RobbieD
For the chassis ground do I just need to clip it to any part of the chassis (like right at the bottom of the rear frame where the metal folds under) or is there a specific part that I need to ground it to? Also what do you mean by the tank pump leg? Also how would I determine where the open is if I de-pin c161, would I have to check resistance or continuity between sections of wire using the pin method or is there another way? Based on your response I'm assuming that it wouldn't show 12v if the negative for the pump was bad or in general? Correct me if I'm wrong.
 

gaz

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@TastierCash ,
If you are uncertain whether your high pressure pump works, provide a good 12v source and ground wire and test it.
 

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@RobbieD
For the chassis ground do I just need to clip it to any part of the chassis (like right at the bottom of the rear frame where the metal folds under) or is there a specific part that I need to ground it to? Also what do you mean by the tank pump leg? Also how would I determine where the open is if I de-pin c161, would I have to check resistance or continuity between sections of wire using the pin method or is there another way? Based on your response I'm assuming that it wouldn't show 12v if the negative for the pump was bad or in general? Correct me if I'm wrong.

I thought that the pump on the frame rail (high pressure) would run, and tank pump (low pressure) would not run.

I apologize if I'm mistaken on this. The easiest way to troubleshoot one pump, or both pumps not running is to start at the beginning and work your way down the circuit, to confirm where power is and where it isn't ("testing for voltage" method).

My reference of "tank pump leg" was based on my (mis?)understanding that front pump would run, but tank pump wouldn't run.

Using chassis ground for your meter simply takes the device's grounding out of the equation, so that your testing can be focused n the power side of things. Chassis ground is any clean metal on the frame, body or drivetrain and it has to assume that everything is properly grounded. Once it is established that power is being delivered to the device(s), then the ground side f things can be tested, to find a fault or to be confirmed as good. This approach simply turns the elephant into smaller bites.

So, neither fuel pump will run?
 

TastierCash

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I thought that the pump on the frame rail (high pressure) would run, and tank pump (low pressure) would not run.

I apologize if I'm mistaken on this. The easiest way to troubleshoot one pump, or both pumps not running is to start at the beginning and work your way down the circuit, to confirm where power is and where it isn't ("testing for voltage" method).

My reference of "tank pump leg" was based on my (mis?)understanding that front pump would run, but tank pump wouldn't run.

Using chassis ground for your meter simply takes the device's grounding out of the equation, so that your testing can be focused n the power side of things. Chassis ground is any clean metal on the frame, body or drivetrain and it has to assume that everything is properly grounded. Once it is established that power is being delivered to the device(s), then the ground side f things can be tested, to find a fault or to be confirmed as good. This approach simply turns the elephant into smaller bites.

So, neither fuel pump will run?
I had the test connector jumped and when checking for power at either pump I only had 0.01v and neither would run or prime when the key was turned or the test connector jumped, I only checked for the high pressure earlier to make sure.
 

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