02 F150 XLT, SuperCab, 2wd, 4.2L V6, AT.

I was leaving Lowe's last night, went to start my truck and it lost all electrical power. I popped the hood disconnected the ground on the battery, reconnected it and power was back. I had my wife try to start the truck and the starter relay clicked and power went back out. I repeated the steps several times and sometimes the power would come back prior to disconnecting the ground simply by messing with the wire harness and sometimes power would go out before the key was turned all the way to start and the starter relay clicking. When it dies it looks like all electrical power is cut, however, I have power to the lighter.

I was originally thinking bad starter relay because it looks like it also serves as the main electrical junction but I'm not so sure since sometimes it looses power before the key is turned all the way to start and the relay clicks. Are there any breakers that could be tripping causing this? I will be going to attempt to fix it this evening, any advice as to where to start is greatly appreciated.

 

 

If the battery is good start checking if the battery cable connections are clean and tight.

 

As far as I can tell the Battery is good, I've never had a problem with it, it has always sounded strong when starting and until I attempt to start the truck all lights are bright and gauges function. It was late and dark but I didn't see any signs of corrosion on the battery terminals or cables and all the connections were good. It's almost like there is a weak breaker that's tripping and not resetting until the battery is disconnected. But then there were a few times it reset while moving the battery cables around. If I don't find any thing else wrong I'll pull the battery this evening and have it tested.


I'm no electrician, but from the few wiring diagrams I found online I don't see anything in the electrical or starting system that could cause this. What could cause all electrical power to cut like someone threw a master switch. No amount of cycling the key and no amount of time spent sitting causes it to reset either.

 

You could have a battery cable that has green corrosion inside where the ends are crimped onto the cables. Or you could have a battery cable where it runs near the exhaust manifold and has been burnt. Either place could be down to where only a few strands of copper are sound. When you turn the key on these strands are capable of passing a minimal amount of current to maybe light the instrument cluster or what ever, but when you draw more power the few strands are not capable of passing more current, therefor the weak spot heats up, causing increased resistance, to the point that NO current can get through the bad spot so everything shuts down. After it cools down/you wiggle the wires/whatever, it repeats.

Time to get the DVOM out and run voltage drops on the circuits. (A test light won’t be much good as it doesn’t load the circuit enough to cause a bad spot to break down.)

 

pdqford, I've dealt with poorly routed battery cables in the past, 66 Cutlass with the battery cable less than an inch off the exhaust manifold, it would barley start when hot. I know more modern vehicles like our trucks are a bit more sensitive to voltage and current than my old Cutlass was. Why would a bad or corroded battery cable cause the entire electrical system to shut down like this? If I was dealing with bad cables I would expect it to attempt to turn over, maybe hear the starter click lights dim or show other signs of a weak/dead battery when trying to crank. I get what you are saying and understand the hotter the cable is the more resistance there is. Even if the damage or corrosion was severe enough to cause the electrical system to go dark, wouldn't current start to flow again after it cools sufficiently?

As far as the condition of the battery cables, I know the negative cable end was replaced at some point but I didn't see any sign of corrosion there. I'll inspect both positive and negative cables again. Then if nothing stands out check the battery's voltage and voltage at both the starter and the starter relay. Are there any other points I should check voltage at?


One additional question, is there any kind of under/over current or voltage protection on the electrical system of these trucks that would trip if the current wasn't just right to protect the electronics?