Hey Scotty
My 95 Aerostar (4.0 Auto, 279000 mi.) has bee running fine but now it'll turnover but immediately dies if you let off the gas. OBDI code is 543 (Secondary fuel pump circuit not responding). Pump comes on when you turn the key but the engine only stays running (roughly) if you keep your foot down. Conflicting solutions on the web.
It's starting to have wiring problems. Shouldn't be any bad corrosion. It's a desert van, never been out of Nevada, so hope it's just age related electrical. These are basically a Ranger frame that they poured a van body around it so it's a nightmare to find anything in order to fix it.
Here's what may be happening here.
This diagram is for a 1995 4.0L Ranger but yours should be similar if not identical.
I've highlighted the Primary Fuel Pump Circuit (red for power, green for ground).
So the fuel pump relay's coil is receiving "Battery Voltage".
When you turn the key to "ON" the computer (through pin 80) provides Ground to the relay's coil which energizes the coil and closes the internal wiper (switch).
That sends power (through the inertia switch) to the fuel pump and it runs.
All that sounds like it's working fine.
However, there's a secondary side. (highlighted in Blue) and this may be where your problem is.
After the Computer energizes that coil, it expects to see Battery Voltage at Pin 40. (Fuel Pump Monitor Input)
If it doesn't receive that signal at Pin 40 it may be cutting the ground (from Pin 80) and then your Fuel Pump Relay's coil is de-energized. (No more power to the fuel pump).
Should be pretty easy to test. Check that the computer is getting power at Pin 40 (Key On) when the fuel pump relay is energized.
You'll have to confirm your computer pinout is the same as this
So "jumper" 12 volts (by backpinning) to the PCM connector's pin 8 and see if the Aerostar keeps running. If it does, it's a wiring issue between the fuel pump circuit (downstream of the relay) and the fuel pump monitor input (pin 8) on the pcm.
Looking at the wiring diagram there's no reason for that 12 volts to your Pin 8 to go to 0 with the fuel pump relay energized.
If the jumper works you can either "run down" the issue, or just splice another wire from your fuel pump wire to Pin 8
I'm assuming you already replaced the relay to rule out a weak relay coil
Thanks Jack. Couple errors though. The 95 Aerostar has a 60 pin ECM so the pinouts are different. Fuel Pump Monitor is on pin 8 and the Fuel Pump Relay is on pin 22. Color code is the same.
12v appears briefly on FPM 8 as you start the engine, then goes to 0 and the engine dies. This tells me the wiring to the pump is good. It still whirs to life and hopefully brings up the pressure. Could the 543 be a symptom of something else?
BTW, if anyone has a hard time finding the computer on a 95 Aerostar it's up high on the firewall on the driver side, easy to get to. Internet forums had me diggin' all over the place.
Changed out the relays. Put 12v on pin 8 but still dies. Error 543 is now 542. Could it be the pump itself is not reporting it's OK?
We started on the fuel starvation issue because of the 543 code. Now it's a 542 code. Still looks like a Fuel Pump/Fuel Pump wiring/Fuel Pump Relay Socket issue.
I would confirm that this is a fuel pressure issue because it's real easy on your Aerostar and chasing down electrical issues is harder.
Buy an inexpensive Fuel Pressure Gauge. (around 20 bucks).
Here's a link to walk you through step by step on where to connect the FP gauge and test the FP on your 1995 Aerostar. Watch the FP during start and as you're "feathering" the gas pedal to keep it running.
https://troubleshootmyvehicle.com/ford/4.0L/how-to-test-the-fuel-pump-2
After this, if you see that you have no fuel pressure it's time to test that fuel pump relay socket.
Pull out the fuel pump relay and "jump" that relay socket to see if the fuel pump will run without the PCM being involved in controlling the relay.
If it runs, it's PCM/PCM to FP Relay wiring related.
If it doesn't run with the relay "jumped", you'll have to get to the fuel pump wiring connector and test for 12 volts there. Also check for ground in the connector.
If you have 12 volts and ground there, with the relay socket jumped, it's the fuel pump
This system deleted my edit and half my message.
I did rent a FP gauge from Auto Zone. When I crank it I get 35psi. Very slowly goes down as it sits.
I don't understand how the Secondary Fuel Pump Circuit works. If the pump is working but not 'reporting' that it's working could that be these 542/543 errors? But that would still mean I'd have to replace it.
When you had the Fuel Pressure Gauge connected did you start the Aerostar and do like you did earlier, hold the gas pedal down to keep it running roughly, while watching the fuel pressure to see if it's maintaining?
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The fuel pump doesn't "report". I highlighted the secondary fuel pump circuit in Blue in my original reply. There's nothing else to it
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Code 542 got thrown because of the test you just performed. You jumped 12 volts to PIN 8
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So, in your case, when you turn the key to ON the PCM provides a ground to the fuel pump relay coil (through PIN 22) to energize the fuel pump circuit to run the fuel pump for a few seconds and then removes the ground. It "reprovides" the ground when the rpms reach >400
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It monitors the power side of the circuit through Pin 8
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Code 542 got thrown because the computer shut off the ground to the relay coil, like it's supposed to after a few seconds, (but because of the test you just performed, jumping 12 volts directly to PIN 8), the PCM saw 12 volts at PIN 8 which shouldn't have been there AFTER it shutdown its ground (from PIN 22) to the relay coil so it threw the Code 542
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That's a good thing. You know that the PCM can read the voltage at PIN 8, and it's sending the ground signal from PIN 22, and seems capable of throwing a code if it sees something wrong
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Like I said, it's time to pull the fuel pump relay and jump the relay switch sockets to see if the fuel pump will keep the truck running. That will take the PCM out of the equation.