Fuel pump wiring

[jd4242]

Anyone know how the fuel pump is wired?? Without getting into to much details on what im doing,im just looking for a schematic..

im basically trying to wire a relay to power my BIG pump and need to find a signal wire to turn the relay on..

Ive tested the relay under the hood and doesnt seem to ever go full 12v..it has 4 pins..pin 30 is always hot..87 is 7v**(i assume this is the wire that feeds the pump but never goes to 12v)**..85 is ground..86 is key on constant 12v..also if you trip the inertia switch none of these change..

The wire that runs to the pump(red with black strip) goes to 12v for 1 second then down to 7v when you turn the key on..it never goes to 0v...i know its only spose to prime the system but why doesnt it never go to 0 volts..this wire does go to 0v if inertia switch is tripped...

Without the wire going to 0v it keeps my relay and timers on...my pump does not have a check valve so had to put a timer on it to keep it priming till you start the truck....i want it to be able to trip or kick off in a accident so need to find a signal wire that goes to 0v after after key on prime or a constant 12v till the inertia switch is tripped and not use a timer..

 

[TDG]

The wire that runs to the pump(red with black strip) goes to 12v for 1 second then down to 7v when you turn the key on..it never goes to 0v...i know its only spose to prime the system but why doesnt it never go to 0 volts..this wire does go to 0v if inertia switch is tripped...

There is a PCM monitor connection downstream of the relay before the inertia switch that goes to pin 8 on the PCM - the 7 volts MAY becoming from that pin as a feedback circuit checking for the presence of the pump or status of the inertia switch.

Diagram attached.

 

[jd4242]

There is a PCM monitor connection downstream of the relay before the inertia switch that goes to pin 8 on the PCM - the 7 volts MAY becoming from that pin as a feedback circuit checking for the presence of the pump or status of the inertia switch.

Diagram attached.

View attachment 154649

See thats what i dont understand though. .

Wire 787 going to pump does get 12v for 1 second when key on before it drops to 7v and does go to 0v if inertia switch trips..12v when running

But wire 238 at relay never goes to 12v,it stays at 7v...also why is wire 926 at relay always hot with key on?? That should be the signal wire telling the relay when to send power to wire 238 to the inertia switch and then pump...

 

[TDG]

why is wire 926 at relay always hot with key on??

Are you measuring this with the relay in or out? This appears to be a ground side switched circuit, 12volt power is fed from the PCM relay to 361 and the PCM takes 926 to ground to operate the relay, if you're seeing 926 hot with the relay in the circuit you are reading the voltage through the relay coil fed from 361, removing the relay from the circuit I would not expect to see voltage when measuring from 926 to ground.

Additionally 926 is spliced to the DLC where you can ground the control side of the fuel pump relay to manually energize the coil for testing.

I don't have an explanation for the other issue.

 

[jd4242]

Are you measuring this with the relay in or out? This appears to be a ground side switched circuit, 12volt power is fed from the PCM relay to 361 and the PCM takes 926 to ground to operate the relay, if you're seeing 926 hot with the relay in the circuit you are reading the voltage through the relay coil fed from 361, removing the relay from the circuit I would not expect to see voltage when measuring from 926 to ground.

Additionally 926 is spliced to the DLC where you can ground the control side of the fuel pump relay to manually energize the coil for testing.

I don't have an explanation for the other issue.

View attachment 154660

I've tried both ways..

With the relay out 926 is hot when key on..ground is also always ground,i thought same thing..

Are you measuring this with the relay in or out? This appears to be a ground side switched circuit, 12volt power is fed from the PCM relay to 361 and the PCM takes 926 to ground to operate the relay, if you're seeing 926 hot with the relay in the circuit you are reading the voltage through the relay coil fed from 361, removing the relay from the circuit I would not expect to see voltage when measuring from 926 to ground.

Additionally 926 is spliced to the DLC where you can ground the control side of the fuel pump relay to manually energize the coil for testing.

I don't have an explanation for the other issue.

View attachment 154660

With the relay out..85 is ground and is always ground even with truck off...30 is always hot even with truck off...86 becomes hot with key on and relay out..87 reads 7v with key on..all with relay out..

 

[2000StreetRod]

As TDG explained, the PCM monitors the fuel pump relay output (238) for fault diagnostics. When the PCM energizes the relay via "grounding" 926 if it doesn't measure battery voltage at 238 it knows the relay has failed. When the ignition is on the PCM has power. If the relay is not energized then the voltage at 238 should be about 0.7 volts which is the input voltage for the solid state devices in the PCM.

When the ignition is turned to on the fuel pump is energized for only a few seconds unless the PCM detects via the crankshaft position sensor that the crankshaft is rotating. In the event of an impact the inertia fuel shutoff relay disables the fuel pump even though the fuel pump relay is still energized. Some vehicles also have a roll over sensor to disable the fuel pump.

Does your programmable timer have a user settable input voltage to trigger?

 

[2000StreetRod]

I found the specifications for your timer:
Delay from .1 sec to 400 days
6 - 20 Vdc
5 amp

It doesn't specify if the 6 - 20 Vdc is user settable. Even if you can't set it the voltage at the relay output when the relay is not energized should be less than 6 volts. Make sure your voltmeter is on the correct range.

 

[jd4242]

key off and key on...86 has voltage with key on and 85 is always ground key on or off

 

[jd4242]

I found the specifications for your timer:
Delay from .1 sec to 400 days
6 - 20 Vdc
5 amp

It doesn't specify if the 6 - 20 Vdc is user settable. Even if you can't set it the voltage at the relay output when the relay is not energized should be less than 6 volts. Make sure your voltmeter is on the correct range.

The voltage is not settable but the trigger wire is..i can use a ground or hot to trigger it.

The problem is i cant find a wire that goes to 0v or a ground that is controlled by the ecm...i cant figure out how it engages this stock fuel relay..

 

[jd4242]

Also sorry should've made it clear but this is on my 94 ranger...dont know if that makes a difference

 

[TDG]

All of the diagrams I've posted are for a 93 Explorer which should be the same as a 92... but probably a bit different than a Ranger...

For the edification of others who may come upon this thread in the future - jammy jamin meter probes into a relay socket as pictured can cause future loose connections / contact issues within the socket, especially on a ~25 yr old vehicle.

 

[jd4242]

Sooo million dollar question. .why is 86 hot and 85 the ground??.even if for some reason they were backwards, why is ground still always a ground and not switched with key on or run?

This is for a 94 which is same as posted

 

[jd4242]

Well it looks like they are backwards. .or the relay itself is labeled backwards. .i found the wire on the dlc pin 6 and traced it to what the relay says 85..even though i can connect to 85 always with my meter and to a 12v power source and get a reading,its not fully grounded till the truck is running or key on for first 2seconds..

I grounded it at the dlc and it does in fact kick the relay on and off..but from my understanding this relay stays hot even if the inertia switch is tripped so basically wasted my time..i assumed the inertia switch would tell the pcm to kill the relay,it doesn't. .

 

[CDW6212R]

Can you use the circuit 787 PK/BK wire to trigger your larger pump relay, and use the delay module to make it keep running after the 787 shuts off, for the time needed to maintain pressure? You said that one shows the full voltage and none as needed right?

 

[CDW6212R]

The inertia switch is only a mechanical shut off device, either an impact or being upside down will move the steel ball inside, cutting power through it. I like the idea of placing a relay down stream of that, given higher current than normal.

 

[2000StreetRod]

The PCM has no way to monitor the output of the inertia switch. It only monitors the input that is connected to the fuel pump relay.

You're measuring across the relay instead of from the pins to chassis ground. Relay Pin 85 is connected to the Red wire from the PCM power relay. It should be battery voltage when the ignition key is on. Relay Pin 86 is controlled by the PCM. It should be battery voltage when measured to chassis ground a few seconds after the key is turned on but the engine is not running. At initial ignition turn on it should be much less than battery voltage.

 

[jd4242]

Can you use the circuit 787 PK/BK wire to trigger your larger pump relay, and use the delay module to make it keep running after the 787 shuts off, for the time needed to maintain pressure? You said that one shows the full voltage and none as needed right?

The voltage never drops to 0v..it drops to about 7v and keeps the timer on...

The inertia switch is only a mechanical shut off device, either an impact or being upside down will move the steel ball inside, cutting power through it. I like the idea of placing a relay down stream of that, given higher current than normal.

I think im going to just remove the inertia switch and mount it on the firewall next to my relay. .then wire it in to the lead wire for the relay from key on constant power..

 

[jd4242]

The PCM has no way to monitor the output of the inertia switch. It only monitors the input that is connected to the fuel pump relay.

You're measuring across the relay instead of from the pins to chassis ground. Relay Pin 85 is connected to the Red wire from the PCM power relay. It should be battery voltage when the ignition key is on. Relay Pin 86 is controlled by the PCM. It should be battery voltage when measured to chassis ground a few seconds after the key is turned on but the engine is not running. At initial ignition turn on it should be much less than battery voltage.

I measured to chassis ground also..according to the relay they are backwards, there is no numbers on the box itself where it plugs in..everything works as it should..i assumed the inertia switch told the ecm to cut power..i didnt have a schematic..

 

[CDW6212R]

That'll work too, put the inertia switch in the trigger side.

So thanks for digging into the fuel pump wiring, because I might have to do the same thing, put the inertia switch on the trigger side. I've planned to split my trigger wire to a 2nd relay, for a 2nd pump. I think two 255lt/hr pumps will run okay with the OEM relays.

 

[Roadrunner777]

I'm not really going to dive into this with you guys except to say this: You really have to watch voltmeter readings if you start seeing weird mid-range values like 7-10V on a circuit without a load, like a relay coil socket. It has to do with the semiconductors on the computer outputs. If you put the slightest load on it, it may drop to zero volts. This is where a simple test light is superior to a voltmeter. I have... geez, at least 5 digital voltmeters, but I use a test light just as often. Just saying...