WJL

If you really like trouble shooting problems, identifying if the pump is working or not is pretty easy, buy a Hanes Manual and a $5.00 Multi-meter. when the car is not starting check for voltage at the wiring connection (As shown in Hanes Manual) if you have voltage to the pump and the pumps not spinning its one of two things bad ground (not likely but possible) or a bad pump

lightscythe

Stevo86, no offense taken, my friend, and I hope you didn't get offended by my ramble as well!

I hope everyone had a good Thanksgiving.

I will be taking my car into the shop sometime this week, probably tomorrow (Tuesday). I'll let you all know of the results when the issue is resolved.

petrock

Haynes manuals suck. They, and other manuals like it, are the Cliff-Notes of the service manual world. Its better to go directly to the source and get the factory service manual & wiring diagrams, or in a pinch you can go to www.alldatadiy.com, pay for a subscription, and get all the wiring diagrams & service info etc you need.


Bad grounds are one of the most common causes of electrical problems. Its actually more likely to be a bad ground then a bad pump.

WJL

I guess he told me, anyhow from what i can tell what he is talking about is the fuel pump relay is equipped with a primary and secondary voltage circuit (two sources) The primary circuit is controlled by the PCM and the secondary is connected to the key (Battery)

with the key in the on position (engine not running) the PCM will ground the pump relay for one second as long as the camshaft Position sensor sends it's position signal all is well, if no position signal AKA pulse the pump will shut off after two or three seconds

I think this is what he is referring to as his problem, could be wrong just offering an opinion. If the Camshaft or the Crankshaft position sensor is not working the car would not start at all.

Although i agree most automotive electrical problems occur due to grounding or the lack thereof i don't think this is the case in this instance, also for $19.95 i have found the Haynes manual very helpful, another also, i think if you search this site Cliffy had a site that had free electrical diagrams at least for fuel injection.

Let me know what they find.

petrock

Kind of. Replace voltage with amps and your closer. The voltage through both sides of the fuel pump relay is a constant 12 volts. The difference between the two sides of the relay is the amount of current (amps) that flows through them. A relay allows a low current circuit to turn off/on a high amp circuit without having the two circuits directly connected. Its all done with electromagnets and spring loaded switch. The low current side of the relay energizes a coil of wire which produces an electromagnetic field. This electromagnetic field pulls the little spring loaded metal switch on the high current side of the relay closed completing the high amp circuit allowing the current to flow. When the low current side loses power the electromagnetic field collapses and the spring in the switch opens up the switch thus turning off the high current circuit. So in the fuel pump case, the coil side of the relay is connected to the ignition switch by way of the computer. The switch side of the relay is connected to the pump & battery.


It will start, it will just die soon afterwards once the fuel pressure from the initial fuel pump priming gets too low.

Stevo86

The FPDM in a Mustang GT modulates voltage to the fuel pump to increase or decrease its duty cycle. They don't have vacuum pressure regulators so that is how they regulate pressure.

Also they won't start without RPM signal from the crank trigger. Don't believe me? Go find a GT and unplug the CKP then try to fire that bad boy up.

WJL

I'm with ya Stevo86

lightscythe

I was able to finally locate my multimeter and attempt to give the plug a test. I found that the voltage to the plug does change when the key is put into the on position. What kind of confused me tho is that the voltage did not go back to what it was before the key was set to on. I remember reading something that said it should only receive an increase in current for about 2-3 seconds before the car is actually started, and then return to its normal current until the car is started.

As of yesterday afternoon, I had my vehicle taken to a mechanic I feel that I can trust to take care of the vehicle. I'll let you all know what the problem is/was when I find out.

Cheers.

petrock

This is correct, however, its not current (amps). Its the frequency of an on/off signal (square wave). The computer sends an on & off signal (voltage square wave) to the FPDM, as Stevo86 mentioned, really really fast to make the pump run. It changes the duration of the on & off time to change the duty cycle of the pump. For example, increasing the on time and decreasing the off time will make the pump flow more fuel. Decreasing the on time and increasing the off time will make the pump flow less. Make sense?

You can’t see these fast on/off changes with a volt-meter. Their refresh rate is too slow, so it will actually display an average (e.g. 6 or 8 volts) depending on the frequency and the meter’s refresh rate and that average will bounce around a lot too. You’ll need an oscilloscope to display the true duty cycle as a graph. But you really don’t need to see it to diagnose your problem. If you see voltage on the computers signal wire to the pump then your (probably) good there. You need to make sure that the main power wire for the pump is also supplying voltage. This voltage should be a constant 12 volts.

Btw, how are you testing for voltage? Are you back probing the wire at the pumps connector while the pump is running or are you testing with the plug disconnected? The former is the correct method (voltage drop test).

WJL

Hey with all of this brain surgery going on here, i have to ask how long has it been since you have replaced the fuel filter. This is the first thing you should do and probably the least expensive. When the filter is clogged it can act just as you have described. Just a wild A** guess