Have a 2001 Ford F150 w/ 4.6L motor, automatic transmission, 2wd, and 217K miles. OBD2 codes B1318 and P1000 (codes run after battery has been disconnected/reconnected multiple times while troubleshooting).
I’ve had it for 9 months and it had developed the following very intermittent problem. Motor will turn over but will not start/run. The first time this happened I rotated the full-sized relays (PCM, Fuel Pump, & Trailer Tow Battery Charge) under the hood and it started right up and ran with no problems; I assumed it was a bad relay and I could take care of it later. A couple of days later it happened again and I pushed down of all of the relays and fuses in the power distribution box and it started right up again. At that point I parked it in my driveway to troubleshoot. I’ve reseated relays & fuses and have examined the wires underneath the power distribution box but have not been able to find an obvious open in the circuit.
The one thing that I have inconsistent results on is the power signal through the PCM diode to the PCM relay. On some occasions I measured the standard voltage drop and on other occasions I measured complete voltage drop going through the diode (i.e., normal voltage reading going in and no voltage reading coming out). I replaced the diode (new from dealer) with no change in the intermittent start/no-start.
I just did multiple battery tests with my Innova 5160RS and the results fluctuated between “bad” and “warning”. I’ve taken the battery to two auto parts stores to be tested and was told that the battery tested good at both locations. I am going to slow-charge the battery and replace the cable terminals today and then retest to see if I get consistent results.
So the first question is – is there a situation where a battery can provide sufficient power to crank the starter like there is no tomorrow, but not enough power to overcome the voltage drop through the PCM diode to activate the PCM relay? Can I eliminate the battery issue as a potential cause for this type of non-start.
The second question is – any advice to diagnosing an intermittent electrical issue?
Alternator tests fine.
The truck is in "crank, no start/run mode" at this time. I removed both relays (the PCM and fuel pump relays) and turned the ignition to run. I used my multimeter (set at 20v DC w/ black lead connected to battery ground) to measure voltage at the "constant power" slot, the "activate relay" slot, and the "power to component" slot. It measured ~12.4v at both "constant power slots" and the fuel pump "activate relay" slot; 11.92v at the PCM "activate relay" slot; 0v at the PCM "power to component" slot; and 6.88v at the fuel pump "power to component" slot.
What does the reading of 6.88v at the fuel pump "power to component" slot tell me? I expected to read 0v just like the PCM reading "power to component" slot - what am I not understanding?
Check the alternator and have it tested. You might be showing signs of a bad alternator.
