96 Bronco no fuel pump signal when cranking

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tkorolsky

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Hello. I have a 96 Bronco with a 5.8 that sat for over 5 years. Ran before sitting. Recently wanted to get it running again and noticed no prime on the fuel pump when trying to start. Dropped the tank and replaced pump and sending unit, after draining out all old gas and cleaning the tank. Replaced fuel filter also. Added 5 gallons of new gas and she fired right up! We drove it around the neighborhood with no issues, maybe 5 miles max. It sat for a week, then drove it again maybe 3 miles to family's house. It ran fine again. Shut it off there and talked for a while, then went to start and it cranks, bit won't start. I didn't hear the fuel pump prime, so I suspected I got a junk pump. Replaced fuel pump again, and still no start. After a little bit of troubleshooting, I pulled the fuel pump relay and tested the negative trigger. I get the trigger when the key is first turned on (prime), but no trigger when cranking. From what I've read, the ECM needs to see over 200 rpm to send this signal while cranking and running? Where is this signal coming from? Is it something to do with the ignition/distributor? Thanks for any and all help.
 

Tiha

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I am sure someone will correct me, but it either has to be the pip sensor in the distributor or the crank sensor. Maybe that will narrow it down some.
 

Motech

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Fuel pump relay is triggered by the PIP signal. Loss of PIP at PCM = loss of fuel pump relay trigger and ICM trigger too, so you would get no spark either.

A clue is Check Engine light. If it comes on solid while cranking, that's a sure sign the PIP signal is missing.

If not, then check for spark at coil wire. If you still have spark, but no pump, check relay. It obviously works for the key-off prime, but with the added load of cranking, if it's got a power or ground problem, it might fail.

Also, since you're already familiar with the relay operation, bypass it. Jumper the battery power side over to the pump power and crank it. If it starts then, it's not your PIP signal.

Older EEC IV, though similar, were slightly different, and PIP wouldn't take out pump relay controls. But these OBD II setups on sixties Windsors weren't too sophisticated. That's why 96 was the last of them.

And incidentally, crankshaft sensor is unrelated. That's there only to measure crank speed variations for mandated OBD II misfire detection.
 
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tkorolsky

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Fuel pump relay is triggered by the PIP signal. Loss of PIP at PCM = loss of fuel pump relay trigger and ICM trigger too, so you would get no spark either.

A clue is Check Engine light. If it comes on solid while cranking, that's a sure sign the PIP signal is missing.

If not, then check for spark at coil wire. If you still have spark, but no pump, check relay. It obviously works for the key-off prime, but with the added load of cranking, if it's got a power or ground problem, it might fail.

Also, since you're already familiar with the relay operation, bypass it. Jumper the battery power side over to the pump power and crank it. If it starts then, it's not your PIP signal.

Older EEC IV, though similar, were slightly different, and PIP wouldn't take out pump relay controls. But these OBD II setups on sixties Windsors weren't too sophisticated. That's why 96 was the last of them.

And incidentally, crankshaft sensor is unrelated. That's there only to measure crank speed variations for mandated OBD II misfire detection.
Thank you so much for the awesome information! I tried jumping power to the fuel pump through the relay contacts, and still no start. I can hear fuel at the fuel rail, so the pump is working. Guess I'll be researching how to test and troubleshoot the PIP signal now. Thanks again for the reply
 

Motech

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Thank you so much for the awesome information! I tried jumping power to the fuel pump through the relay contacts, and still no start. I can hear fuel at the fuel rail, so the pump is working. Guess I'll be researching how to test and troubleshoot the PIP signal now. Thanks again for the reply

From above:

"A clue is Check Engine light. If it comes on solid while cranking, that's a sure sign the PIP signal is missing."
 

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