f150 won't start on just 1 battery

[DennisG01]

same as I posted in post #35

Here's your cookie



...Sorry, I got hungry while trying to figure out how to post it...

[99yam40]

that is only part of a cookie.

"when I crank the motor , voltage drops down to 8 volts"

this is when you need to be checking at all of the different places to see where the problem is. start at the battery and work your way through the circuit.

even the ground side can be a problem

[DennisG01]

Here's where I am . I had voltage at the nut on the back side of the battery switch at 13.1 . The voltage on the posts was around 8 on the #1 post , and 11 on the # 2 post . I replaced the battery switch , I now have 12.6 +/- at the posts and the nuts. I also replaced the power posts and voltage is 13.4 , although the main battery cables to the motor don't connect here , so it really has no bearing on the problem. I have 14 volts at the starter , when I crank the motor , voltage drops down to 8 volts . This is where I have stopped . I haven't done a starter draw to check amps yet , that will be my next step . Timing is good according to the yds . I don't quite understand why the voltage is lower at the battery switch than anywhere else

OK, so you did have a failed battery switch (or dirty connections, anyways). But now you have a new switch? Voltage is one thing, but even a thin piece of wire can transfer 13V, yet when the power is called upon to start the motor, that wire will burn up because of the amount of amperage required can't flow through it. A corroded battery wire can do the same thing - as in, not allow enough juice to flow.

Do the voltage drop test on your existing wires. If you want to do a quick "proof of concept" that the problem is most likely your wires, use jumper cables from battery to motor.

[DennisG01]

that is only part of a cookie.

Yeah, sorry. I got hungry!

[gsdanno1]

Existing wires to the motor , correct, as far as the voltage drop ? I have already verified 14 volts at the starter , so how else can I verify voltage drop

[99yam40]

you said voltage drops to 8, need to find out where that is happening

[pstephens46]

Existing wires to the motor , correct, as far as the voltage drop ? I have already verified 14 volts at the starter , so how else can I verify voltage drop

Here we go. These are Rodbolt's instructions for me as I had no clue awhile back.

ok
you don't understand how to use that meter.
its very simple.

your looking for a voltage DROP not battery voltage.

at the + battery TERMINAL not cable attach one lead using the DC scale. the other lead will go to the + terminal on the starter again the stud not the cable end.
turn on the key, you should see less than .5V,turn the key to start,should still see less than .7v.

now using the same autoranging DC volt scale attach one lead to the - stud at the battery the other to a good block ground.
turn on the key, you should see less than .5 V try starting it and you should still read less than .5v.

if the + wire reads more than .7V start backing up with the lead that was on the starter battery terminal IE battery switch's and such until it reads less than .7 while loaded.

if the - cable reads more than .5V start backing down the - cable looking for junctions and splices that may have failed.

what we are doing is finding the voltage DROP acrossed a resistance.

take the ground wire and visualize the circuit, barring any terminal junctions it is a straight piece of wire from point a (battery stud) to point B the ground wire attachment at the block.
the only resistance in a perfect circuit would be the resistance of the copper,very low.

now when we operate,or try to, the circuit we are using the electrical pressure,measured in volts, to force electrons(amps) down the wire.
if all is perfect the pressure drop from a to b will be a minimal loss maybe .5V or less.
now if we have a bad spot in the cable(a resistance) we may drop 5-8 or more volts ACROSS that resistor.

loose corroded cable to terminal connections can act as a resistor as can bad cables,bad cable to cable end or any junctions between point a and b.

[DennisG01]

You measured a drop in voltage at a particular point. But you're still not doing a "voltage drop test" - it's different. It's not complicated, but try googling that or searching on this site - I know there was some lengthy/good directions posted at some point.

PStephens - thanks for that - looks like were typing at the same time. And thanks for taking the time to do the search for him! I'm horrible at searching.

[gsdanno1]

Voltage drops to 8 at the starter while cranking .it's 14 with no load

[gsdanno1]

Got it , I'll print that and try tomorrow

[pstephens46]

You measured a drop in voltage at a particular point. But you're still not doing a "voltage drop test" - it's different. It's not complicated, but try googling that or searching on this site - I know there was some lengthy/good directions posted at some point.

PStephens - thanks for that - looks like were typing at the same time. And thanks for taking the time to do the search for him! I'm horrible at searching.

Didn't have to search much. Rodbolt was responding to my struggles with the same issue. Really helps to fabricate some nice long leads with alligator clips. With good solid connections of course.

[gsdanno1]

Part of the problem is the really poor cosole wiring that was done at the factory, makes it hard to isolate everything. Batteries are in the console . I'm going to take all the cables off again tomorrow and wire wheel all the connections I can get a drill to , then do the voltage drop test

[rodbolt17]

you do not need a load tester.
you have one.
it is called a starter.

simply hook your DVM to the battery, unclip the safty lanyard,crank it.
watch the voltage.

then do the voltage drop test.

in a perfect piece of wire from point a,the battery , to point b the starter or the block , there will be very little resistance thus no voltage loss.
any resistance will show on the meter as a voltage.
anything more than about .7 volts on the positive cable between the batt terminal and the starter terminal indicates an issue.
anything more than about .5v between the - batt terminal and the engine block indicates an issue.

yep it is that simple.

[gsdanno1]

ok , here is the latest info with PICS !! 12.7 volts at battery , cranked the motor and voltage dropped down to 10.4-10.8 after about 10 secs . Checked again about 5 mins later , voltage 12.5 . The first pic of is of the horrible wiring in the console , I've had this on the list of things to get fixed , but something else always comes up. I may have to do it in order to diagnose what's going on. Next pics are voltage drop results , starting with the positive side key off , then on , then following with negative side in the same order. Battery cables are a continuous run from the motor all the way into the console , where I can't see what happens when they come through the rigging tube due to what's going on in pic 1

[gsdanno1]

here's the rest