Very odd problem!
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New User
Joined: Jan 2005
Posts: 2
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Very odd problem!
Hello guys,
Im working on a 88 F250 with a newly rebuilt 302. I built the motor in class from a longblock. I also dropped it in.
NOW to the problem. This is a very long story.
When we finished the install and went to fire her up, she would fire and run fine ONLY with the key in the start postion. As soon as you took it out of the start postion to "RUN" it would die. We have changed and checked many things. I started with the basic's for hecks sake. Fuel, Spark and Compression. All was good. Got power to the coil in the Run postion. Fuel pump's just click with the key on, cant here them wizz, but I figured that shouldnt matter. I still pulled the line that goes to the filter, and put the key to the run position and nothing came out. I hot wired the pumps (both) and they shoot fine. I even pulled the in tank pump out and benched it. It was fine. I also hotwired both of them and tryed to start the car but it didnt seem to help.
We changed the Ignition control module, hall effect sensor, starter solenoid fuel filter and a few other things.
ALSO another odd thing. I put the map sensor and the TPS on our vantage, and noticed I was getting no reference voltage, ground or signal from the sensor. Im sure its the same with all the other sensors also. BUT heres the odd thing. When you go to start the car, and hold it in the start position, I get a reading of 4.86 volts at reference ON both sensors. The wave forms even look good. So its really odd. Confusing too. Ignition Switch problem?
Sorry for being so long, Just been stumped the last few days. Anyone chime in and get rid of my headache?
BTW- All sensors are plugged in and its got some aftermarket headers with no exhaust.
Im working on a 88 F250 with a newly rebuilt 302. I built the motor in class from a longblock. I also dropped it in.
NOW to the problem. This is a very long story.
When we finished the install and went to fire her up, she would fire and run fine ONLY with the key in the start postion. As soon as you took it out of the start postion to "RUN" it would die. We have changed and checked many things. I started with the basic's for hecks sake. Fuel, Spark and Compression. All was good. Got power to the coil in the Run postion. Fuel pump's just click with the key on, cant here them wizz, but I figured that shouldnt matter. I still pulled the line that goes to the filter, and put the key to the run position and nothing came out. I hot wired the pumps (both) and they shoot fine. I even pulled the in tank pump out and benched it. It was fine. I also hotwired both of them and tryed to start the car but it didnt seem to help.
We changed the Ignition control module, hall effect sensor, starter solenoid fuel filter and a few other things.
ALSO another odd thing. I put the map sensor and the TPS on our vantage, and noticed I was getting no reference voltage, ground or signal from the sensor. Im sure its the same with all the other sensors also. BUT heres the odd thing. When you go to start the car, and hold it in the start position, I get a reading of 4.86 volts at reference ON both sensors. The wave forms even look good. So its really odd. Confusing too. Ignition Switch problem?
Sorry for being so long, Just been stumped the last few days. Anyone chime in and get rid of my headache?
BTW- All sensors are plugged in and its got some aftermarket headers with no exhaust.
Cargo Master
Joined: May 2001
Posts: 3,208
Likes: 13
I just finished an 1991 that would do the same thing and it was the ignition switch down on the steering colume that had paritial come apart would start in start but die in run. Replaced that switch and all is fine
Elder User
Joined: Sep 2004
Posts: 941
Likes: 4
Turn the key to run. Do not attempt to start. Check VREF with a voltmeter or test light wherever it is convenient. If it is 0.0 volts, something is wrong.
It sounds like the "hot in start or run" circuit that goes to the EEC power relay is only "hot in start".
Pull the connector off the IAC motor. With the key in RUN, there should be 12 volts on one or both wires. If there is no voltage, the computer is not powered, and the problem needs to be tracked back to the EEC power relay and the circuit that feeds its coil.
Another way to test: Go to fordfuelinjection.com or troublecodes.net and learn how to pull codes. Attempt a KOEO test. See if the computer is talking to you with the key in run.
It sounds like the "hot in start or run" circuit that goes to the EEC power relay is only "hot in start".
Pull the connector off the IAC motor. With the key in RUN, there should be 12 volts on one or both wires. If there is no voltage, the computer is not powered, and the problem needs to be tracked back to the EEC power relay and the circuit that feeds its coil.
Another way to test: Go to fordfuelinjection.com or troublecodes.net and learn how to pull codes. Attempt a KOEO test. See if the computer is talking to you with the key in run.
Thread Starter
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New User
Joined: Jan 2005
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Well im sure the computer has power, and I checked the map and tps VREF, only when it would be in start would I get a reading for VREF, I also checked signal and ground too. Both sensors only had readings when put in start. Im sure all the other sensors are like that too. I also checked the ref wire in the EEC, and got nothing to it with the key in run. I checked the Keep alive yellow wire, and got bat voltage.
I checked the igniton switch by backprobing the wires, and all gave battery voltage in the run position. So I dunno.
I checked the igniton switch by backprobing the wires, and all gave battery voltage in the run position. So I dunno.
Elder User
Joined: Sep 2004
Posts: 941
Likes: 4
Vref is an output from the ECM, not an input. The ECM has an internal voltage regulator that takes the 12 volt VPWR and regulates it down to +5 volts, then sends it out on the VREF line. You can't check it with the connector removed from the ECM. That's why you need to check it on a sensor.
KAPWR is Keep Alive power. It only goes to the RAM memory on the ECM. The processor and most of the other circuitry run off VPWR, which comes from the output of the EEC power relay. If you aren't getting VREF, it is almost certain you aren't getting VPWR. You can check this indirectly by probing the IAC motor, or directly by removing the 60 pin connector and probing VPWR (which is an INPUT to the PCM). Or you can probe the output of the EEC power relay.
One other distant possibility is that there is a grounding problem. If the PCM is not well grounded, it will have trouble generating VREF. It isn't clear how going to the "start" position would help this, but you can visually check the grounding at the firewall above the PCM, at the stud in front of the battery, at the battery post, and at the engine block.
KAPWR is Keep Alive power. It only goes to the RAM memory on the ECM. The processor and most of the other circuitry run off VPWR, which comes from the output of the EEC power relay. If you aren't getting VREF, it is almost certain you aren't getting VPWR. You can check this indirectly by probing the IAC motor, or directly by removing the 60 pin connector and probing VPWR (which is an INPUT to the PCM). Or you can probe the output of the EEC power relay.
One other distant possibility is that there is a grounding problem. If the PCM is not well grounded, it will have trouble generating VREF. It isn't clear how going to the "start" position would help this, but you can visually check the grounding at the firewall above the PCM, at the stud in front of the battery, at the battery post, and at the engine block.
