1996 Ford F 150 4.9 L6
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toadyboy1957 wrote: i test all grounds with a VOM or a 12 volt test light from from battery posotive
As long as you are NOT using the ohmmeter. For this type of testing a voltage drop measurement is key, but I do like the test light too.
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If the truck is running fine with the fuel pump relay coil manually grounded, the chances of you having a bad main computer ground are very slim but you should check them anyway.
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toadyboy1957 wrote: i use VOM FOR CONTINUITY testing only
You need to stop using the ohmmeter for that. I can give you many examples as to why. Here is the best lesson
www.youtube.com/edit?o=U&video_id=44dP6X3QS5A
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toadyboy1957 wrote: if im doing something wrong please let me know best way to test all and thanks again
You're fine man. Use your test light and check those grounds I provided for you.
I am concerned that the driver in the ECM for the fuel pump relay is bad. The question is why? What happened? When did this problem start?
A history would help us here for sure. In the mean time, check those main grounds.
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toadyboy1957 wrote: inside the PCM do you know if the fuel pump driver travels through a directional capacitor FROM THE ORANGE BLUE WIRE going into the PCM or where and how does it obtain its ground??
It is a switched ground. The ECM is using a transistor to switch the fuel pump relay coil on and off and it is based on two main inputs.
#1 an ignition feed that powers up the computer
#2 an RPM signal
When the computer first is powered up by the ignition, it will turn that driver (transistor) on for 1-2 seconds. This is your prime.
After that, the PIP signal is your RPM input and as long as it is there, the ECM will keep that relay circuit turned on.
The fact that you have injector pulse tells us this PIP signal is good.
Where does it ground? To the board itself. Where does the board ground? To those wires I highlighted in this thread
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toadyboy1957 wrote: Ok / I ran all the grounds / backprobed all the grounds at the PCM WITH TEST LIGHT on battery posotive . all GROUNDS are present but still HAVE NO GROUNDS AT O2 SENSORS OR SOLENOIDS. IAC,MAF IAT DPFE CRK SENSORS ALL IGNITION SYSTEM MAIN PCM DIODE PCM FUSES RELAYS ALL POWER UP FINE.. I also have a direct short in fuse 16 in the underdash FUSEBOX can you help me with the color wire and any connections so i can trace this down im not sure if this has anything to do with the grounds or not this thing is becoming a nightmare LOL
Fuse 16? Shorted, as in it pops a fuse as soon as it's installed? Lemme grab the I/P fuse box diagram.
For the record, I hate how the OE Ford diagrams combine underhood and underdash fuse boxes in the same diagram at times. :lol: Finally found fuse 16:
Not related to your other issues, unfortunately. If you wanted to chase this short, I'd probably start with unplugging the cigar lighter and recheck for the short. If that didn't work, then you can follow the light blue/white wire for a short to ground.
Back to your PCM... Sure is sounding like a bad engine computer to me.
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toadyboy1957 wrote: even tho the cigar lighter is unplugged and isolated it still pops fuses soon as i install a new 1
Ah crap, I'm sorry man, that was the easy test first. You're down to a direct short now. My go-to test for this kind of thing is to leave the cigar lighter disconnected, and use a test light connected to B+ on the light blue/white wire at the lighter connector. Trace the harness, wiggling carefully as you go, and keep an eye on the test light. When you move the short off of ground, the test light will flicker/go out.
and do the o2 sensor grounds come from inside the PCM OR AN external ground??
These get a ground inside the PCM:
It's worth noting here that, without the engine running, the PCM may not ground the O2 heaters. It may only ground the heaters once the engine is running.
Does the IAC have a ground when cranking? That'd tell us the PCM has the capability of controlling at least one of it's outputs.
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toadyboy1957 wrote: the engine starts and runs fine with a self provided ground to the fuel pump relay where the lt/blue - orange wire plugs in on the relay thats the fuel pump control wire going into the pcm
If everything else works, then I think you're looking at a simple failed driver OR there was a short to power on the light blue/orange wire, either from a wiring problem, or a shorted relay coil. Either way, it seems like the damage has been done.
Have you tried driving around with the fuel pump relay control jumped? Ran well? Honestly, if this was my truck, I'd be REALLY tempted to wire a rocker switch into that light blue/orange wire and put it on the dash. :whistle: You could even hide it somewhere, and now you've got a cheap anti-theft system! :lol:
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