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Hey everybody, I've got a 1988 Ford F150 5.0L project car that I've been working on for a little while. I bought this thing with a no start condition as a personal challenge. I'm going to do my best to cover all the diagnosing I've attempted over the last few months. I appreciate everyone's patience for this lengthy recap. Here's a bit of the history on it and what I've done so far:
Last evidence that this car ran was in 2010. It has ford's EEC-IV system with a MAP sensor only (no MAF). I has the dual fuel tank setup and I assume one of them was broken for a long time. When I first started messing with it, I had spark but no fuel pressure on my pressure gauge. I assume I have compression because it will start on starting fluid. When I first started looking into the fuel pump direction, I did have a check engine light with Key On Engine Off but I could not hear the fuel pumps prime on key on—this lead me to go in the fuel pump direction. At this time, between charging up the battery and re-installing it, I made a terrible mistake of hooking up the battery backwards when I wasn't paying enough attention. This fried the ECM: no more check engine light. Unfortunately, the multiple fusible links protecting the ECM did not do their job. I removed the ECM and opened it up and found a few components that failed and leaking capacitors (as is common on EEC-IV computers of this age. Perhaps the leaking capacitors were also a contributing factor to the no start prior to my battery mistake). I also checked all the fusible links and replaced the one to the alternator that had blown. Out of curiosity I dropped both fuel tanks and found one of them to be broken and the other one in working condition when I bench tested them in water. The fuel tank selector valve was also gummed up so I took it apart and cleaned it up best I could. There's a chance it could have been stuck on the fuel tank that had a broken pump, can't confirm. This vehicle has 2 in tank low pressure pumps and and in-line high pressure pump. I can hear the in-line pump hum when I manually ground the Fuel relay, so I'll assume it works for now. I disconnected it for the time being to not burn it up with my testing.
After installing the new computer, I confirmed that the EEC relay and Fuel pump relay are fully functional and that all power and grounds to the ECM are good—less than 10mV on the computer ground too. With my test light, I wanted to check that the ECM is grounding the fuel pump relay to send 12V to the fuel pumps. On my first test it worked perfectly, then when I tested at the high pressure pump connector, my test light did not light up on prime like I expected. I went back to my fuel pump relay test to check it again and this time it didn't work. I unhooked my battery and put it on the charger for several hours. Then, I tried my fuel pump relay test again and still nothing. This lead me to check the 5V reference circuit.
This brings me to my current situation. All of my sensors on the 5V reference circuit that I've checked are showing 1.4V as my reference voltage. This leads me to believe that I have a short somewhere, but I couldn't find anything visually. My justification for believing I have a short is I doubt that my brand new computer is to blame and I think I have unplugged all the 5V reference circuit sensors at this point. In my mind, I see the next paths to take being buying a milliamp clamp and looking for the wire on the reference circuit that has a higher amperage OR cutting my 5V reference wire at the ECM harness to make sure my ECM is putting out 5V.
What do ya'll think? Is this one a lost cause? Did I take the right direction? I can post the schematics from my vintage EVTM manual, my VIN or anything else requested. I appreciate everyone's input.
Saves money from buying an amp clamp. If it does not have 5 volts after cut check powers and grounds. If there good you have a bad new pcm. You could try and feed 5 volts into the wire if you had a power probe adapter for a 5 volt feed
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