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


TastierCash

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I've been working on restoring my old Bronco II to a state in which I can drive it for around two years. Recently (as of this past weekend) I drained and replaced the transmission fluid, after putting the pan back on I ran the truck to get the fluid to get into the transmission and to check for leaks in case I didn't tighten the bolts enough. I had planned on running it longer to ensure the fluid got into the transmission but after about 20 ish seconds the engine died and when I tried to start it again the in-tank pump would not prime and I couldn't figure out why. Well earlier I went and bought a new multimeter to test the wiring. The relay connector, inertia switch, and ecm all had around 12v of power, however when I got to checking the connector for the pump the pins for the gauge unit got around 8v or 9v of power while the pins for the actual pump for some reason only had 0.01v of power. The ecm, inertia switch, relay, fuel tank, and fuel pump are all newly replaced and had been working fine for months. I was going to replace the fuel injectors after finishing the transmission but since the pump wont start I've decided to wait until I can get the fuel running again. Any help would be greatly appreciated. Thank you in advance to anyone interested in providing helpful information so that this truck may see the roads again.
 


sgtsandman

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Check the fuel pump fuse first. My pump died and the fuse looked perfectly fine but for some brown residue on the pins. After a day of troubleshooting and finding nothing wrong, I replaced the fuse and the truck fired right up. The middle part of the fuse looked perfectly fine. No melting, or break. Nothing. Just brown residue.

And welcome to the forum.
 

TastierCash

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Check the fuel pump fuse first. My pump died and the fuse looked perfectly fine but for some brown residue on the pins. After a day of troubleshooting and finding nothing wrong, I replaced the fuse and the truck fired right up. The middle part of the fuse looked perfectly fine. No melting, or break. Nothing. Just brown residue.

And welcome to the forum.
Thank you for the tip, I'll have to check that in the morning. By the fuse do you mean the fusible link? For some reason every fuse in that truck (save for a few for the interior and the lights) is a fusible link, especially for the fuel pump fuse.
 

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A "map" might help you; '87 wiring diagram follows.

If the front high pressure pump on the frame runs, then chances are the problem is between it and low pressure pump in the tank. It's a relatively simply circuit (see the diagram).

At the tank plug, try measuring multimeter (MM) Red lead to the pump Black/Pink with MM lead to a good metal chassis ground on the truck.
No +12V, look for a break in the Black/Pink between the two pumps, especially at the wire join at C161.
Yes +12V on Black/Pink, change the MM Black lead from chassis ground to the pump Black wire; if less than +12V now, the problem is on the ground wire side. Since both pumps share the same ground point, it would likely be the splice or connector 118 (the two wires share a terminal connection there).
Connectors C118 and C161 are both on the driver's front fender apron.

The connector at the tank would also be suspect; it should be closely inspected for terminal corrosion, and for a failed/failing connection between the copper wires strands and the metal terminal within the connector.

I'd get the pump sorted out first, and then tackle the gauge, if there is a gauge problem

Good luck, and welcome to TRS.


87 fuel pumps (1).JPG
 

sgtsandman

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Thank you for the tip, I'll have to check that in the morning. By the fuse do you mean the fusible link? For some reason every fuse in that truck (save for a few for the interior and the lights) is a fusible link, especially for the fuel pump fuse.
My limited experience with fusible links is that they are either good or bad. There is no in between as long as the protective coating is intact. If the coating is damaged, then you are looking for corrosion in the wire. I've seen people use thinner wire that will fail before the rest of the wire harness does if they can't find a proper fusible link wire. The catch is knowing what the proper amp rating is for that system. Use it as a last resort if you have to replace a fusible link and can't get the right material.

 

TastierCash

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I checked the fusible link earlier after I got home and it seems to be fine, pulls 12v when I check between the connector pin at its end and the connector pin that powers the pump (they get connected together and run the pump when the ecm hits the relay they connect to. As @RobbieD suggested i believe it is the wiring and I will be checking that tomorrow.
 

TastierCash

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@RobbieD I meant to include it in my response earlier but do you know how far or where in the wiring/on the truck I would find connectors C161 and C118? Either way, thank you in advance.
 

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@RobbieD I meant to include it in my response earlier but do you know how far or where in the wiring/on the truck I would find connectors C161 and C118? Either way, thank you in advance.
According to the EVTM they're both laying on the driver's side fender apron, in the engine compartment. C118 is a black 12-pin and C161 is a white 1-pin (which sounds kind of odd, but these books do have an occasional typo).

Good luck!
 

TastierCash

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I just got in from trying to check the connectors and the wiring. I believe I found the correct connectors and when I checked their voltage each one had 12v where it should. However, I also pulled out the power connector for the high pressure pump and checked both ends of it to find that it had no power. Im somewhat confused by this as the same color wires in the connectors that I checked (and traced to the high pressure pump connector) read 12v while the end at the high pressure had no power. If yall have any more tips or things that I may be able to check then please let me know cause I'm honestly at a complete loss. Also a bit of added information, while checking the connectors and wiring I had the test connector jumped and the key on to provide constant power. Thank you in advance.
 

TastierCash

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I just got in from trying to check the connectors and the wiring. I believe I found the correct connectors and when I checked their voltage each one had 12v where it should. However, I also pulled out the power connector for the high pressure pump and checked both ends of it to find that it had no power. Im somewhat confused by this as the same color wires in the connectors that I checked (and traced to the high pressure pump connector) read 12v while the end at the high pressure had no power. If yall have any more tips or things that I may be able to check then please let me know cause I'm honestly at a complete loss. Also a bit of added information, while checking the connectors and wiring I had the test connector jumped and the key on to provide constant power. Thank you in advance.
I also forgot to add that I went back and checked the low pressure pumps connector again specifically to check if it would read voltage if I connected to the positive of the gauge unit and the negative of the pump and the other way around (the pumps positive and the gauges negative). When I checked there was around 12v through checking the positive of the gauge and pump negative, but when I swapped to the pump positive and gauge negative I had nothing, so I believe that I have a problem with power itself getting to the pump through the positive and not a grounding issue. I greatly appreciate any further help. Thank you.
 

sgtsandman

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do the connectors look corroded? Not just on the plug in side but the back side as well. Is the wire loom protecting the wire intact or broken? Does it look like any of the wires have been damaged of there is damage to the loom?
 

TastierCash

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do the connectors look corroded? Not just on the plug in side but the back side as well. Is the wire loom protecting the wire intact or broken? Does it look like any of the wires have been damaged of there is damage to the loom?
The connectors look fine and the wire insulation looks perfect except some slight discoloration from time, there's only one wire in the entire truck that I know is broken which is the wire for the oil pressure switch (the wire is in half but I'm gonna replace it soon) every other wire seems to be perfectly fine.
 

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I just went thru a fuel issue. My truck ran great for about 20 miles. Shut it off and it would not start. I could hear the fuel pumps were not priming. Next truck started right up. Long story short, I replaced the Fuel Pump Relay. 250 miles since replacing relay and no problems
 

TastierCash

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I just went thru a fuel issue. My truck ran great for about 20 miles. Shut it off and it would not start. I could hear the fuel pumps were not priming. Next truck started right up. Long story short, I replaced the Fuel Pump Relay. 250 miles since replacing relay and no problems
Thank you for the suggestion, however the relay installed for my fuel pump is brand new and is a known good relay same goes for my inertia switch which I replaced after the original failed and the same also goes for the ecm installed in the truck.
 

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The connectors look fine and the wire insulation looks perfect except some slight discoloration from time
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.
 

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