The Ford Truck Fuel Pump That Didn't

My friend Otto called me one Saturday with a strange sound to his voice. Now Otto isn't one to ask trivial questions and he has been mucking around with cars and trucks and farm vehicles for as long as I have known him. He has a repair shop that would put any professional shop to shame and has enough mechanical savvy to work his way out of any problem. But this one was different. He has the sister truck to mine and had it "stored" for a few months while he drove his "winter car". He had tried to put it on the road the previous weekend but was unable to get it running. I knew I was in for a long day but when Otto calls I know it is going to be an interesting day so I put on my Saturday best and headed for Otto's house.

The hood was open and there were wires and testers all over the fenders. He had disconnected the major connectors and was standing there with a blank look on his face, a volt meter in his hand and the service manual open to a schematic laying on the front seat.

"Soooo, where'e the keg?" I quipped. "It's inside - go get yerself a mug - yer gunna need it! This one has me baffled!"

We stood there mulling over the problem - it would crank, but wouldn't start. Otto had determined that there was no spark by holding a plug wire near the manifold bolt and cranking it over. It should have zapped a fat spark every time. He had also cracked the fuel rail fitting and found that there was no fuel pressure. He then determined that the fuel pump in either of the dual tanks was not making any noise whatsoever - it should hum for about five seconds each time you turn the key on. He had clipped a voltmeter onto the negative terminal of the battery and had tested all the connectors associated with the fuel pump and its relay and the ignition system. He had also determined that the fuel pump relay was clicking!! He had disconnected the connector at the fuel pump and determined that there was even voltage at the fuel pump connector! Must be a bad pump - but both pumps failed to run. How could both fuel pumps fail simultaneously?? Not a chance. He had also determined that there was 12 volts at the coil terminal. So why didn't it start??

Otto had done everything right. He had the service manual for the truck, he had traced the wires by their color codes and had determined that the only possibility was a bad fuel pump in both tanks - not a likely situation. He had resisted the temptation to yank the tank and check out the pump on the bench but had decided to call me instead. A good move as it turned out. He had determined that there was voltage at the coil, but no spark!! A failed coil along with two failed fuel pumps??? Nahhhhhh.

I decided to take the time to repeat Otto's diagnostic procedures and took the voltmeter lead and started back-probing the connectors. Same thing as Otto had found. There was voltage everywhere! But I discovered one thing that Otto had missed. Remember I said that he measured a good solid 12 volts at the fuel pump connector? With it disconnected?? Well I was a bit lazy and when I crawled under the truck with a voltmeter lead in my hand I decided to poke thru the wire at the connector rather than disconnecting it. Otto turned on the key and I measured three volts! Not the 12 volts Otto had measured! Now it doesn't take a rocket scientist to figure out that a 12 volt fuel pump can't run on 3 volts!! I yanked the connector and measured it again - 12 volts!!

Well according to a good friend of mine, Oscar Ohm, his law states that the voltage is proportional to the resistance (E=IR) and the practical application of this engineering wisdom told me that there was a resistance in the supply wire that was far greater than the resistance of the fuel pump! What that meant was that the wire or a connector was bad. Who ever heard of a bad wire? So we started yanking more connectors looking for corrosion somewhere - firewall connector, ignition switch harness - everything was clean.

Back to the bad wire theory. I took the needle probe of the voltmeter and started piercing the insulation of a fat wire which started at the battery and went into the main harness that fed the ignition switch. Might as well start at the beginning, I figured. The voltage at the battery was 12 volts (under fuel pump load) and I moved along the wire six inches at a time. After a foot or three I found a spot on the wire where the probe entered the insulation very easily and went clean thru the wire and into my index finger!! Ouch!! Thinking that I had just slipped with the needle-probe I tried again. I was virtually standing on my head leaning over the fender and reaching down near the steering column so I couldn't see what I was doing very well but one thing was certain. I had found a SOFT SPOT in the wire. We pulled the tie-downs that held the harness in place and stretched the harness over the fender where we could see it better. I took my trusty MacIver knife (Swiss Army) out of its sheath and started slicing. What I found amazed us. There was a six inch length of wire that was no longer wire. There was nothing inside the insulation but copper oxide! Green powder that nature had made out of copper, salt and electricity! This was the main wire that supplied 12 volts to the ignition switch. It was "hot" all the time and must have been nicked so that the insulation was missing near the frame (which is connected to the negative side of the battery). That, along with Rochester salt as an electrolyte had caused an electrolysis reaction inside the wire (24 hours a day) and had done its dirty work where no one could see it.

SNIP - CRIMP - CRIMP - Five minutes later I had spliced a six inch piece of 12 gauge wire into the supply wire. Thirty seconds later we had the harness plugged in again and ten seconds later the Ford truck started and ran like it had a new life!!

The whole job had taken about four hours - the total cost of the parts to fix it was about fifty cents. We both agreed that if Otto had taken this truck to a dealer or other "replacement therapy" repair shop he would have paid for a pair of fuel pumps, a new coil, a distributor, a relay and a new harness, all to the tune of about eight hundred dollars. There is a lesson here - DIAGNOSIS, DIAGNOSIS, DIAGNOSIS. Then replacement!

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