Voltage drop question, cranking engine over to start

[crispy'86]

So on cranking the engine with spark plugs out the battery voltage drops to about 10v which I find acceptable, with spark plugs in the voltage drops to 5,6v after 2-3 seconds of cranking. I'm thinking it's possibly the battery earth cable at fault, have used jumper leads etc onto the battery from block etc. What's the process for testing for voltage drop across the circuit?? Start at the block/starter on cranking? Starting at block working towards the battery while cranking?? I've tried this using a jump battery and jumping off another car

[Lloyd]

Where are you measuring it at ~5V? If it's measuring that at the battery and you know the starter to be good then your battery is toast/flat. If it's measuring that at the starter then start measuring drops.

If you're looking for drop on the positive side of things then get a multimeter and put one lead on the battery positive post and one on the starter main power terminal and get someone to crank it. That'll give you the drop over that side of things. For the earth find a good, clean point on the starter body and check between that and the battery negative post.

Check your terminals are actually clean and tight too, it makes a huge difference sometimes.

[rollaholic]

so it still drops to 5v with a jump on it?

[crispy'86]

Yip still drops to 5v when cranking jumping off a running car. Even running a jump wire to the starter solenoid and onto power it drops to 5v so maybe a dicky earth cable. I did get one second hand from pick apart. Resistance along the earth lead is good

[whynot]

Have you measured the volt drop over the earth lead while cranking? That would quickly point out if the lead is at fault

[Crucible]

Has the battery been charged properly and checked with a load tester? Is it calcium or lead acid type?

checking a large battery cable with an ohm meter is a waste of time. Voltage drop test is the only way, test with a multimeter block to battery negative like Lloyd suggests.

If you have vd, power will always take easiest path through meter, any voltage drop test must be checked with circuit loaded!

[crispy'86]

Had another look at it lastnight, voltage drop on negative side is 1v at the most, dropped to 4v on positive side using crap jumper leads. I used my multimeter from work to test for voltage drop, borrowed load tester to test batteries lying round and decent jumper leads. The battery in question is was using is absolutely shagged along with a few poor connections. Finally got the engine running last night dummying a few things for my igniter so I'll double check my findings tonight. It's looking like a relatively decent battery has done the trick.

Turns out i'll need to invest in a good battery load tester

[allencr]

...i'll need to invest in a good battery load tester

That's what the starter does, especially if the headlights are on.
Then check the ignition for what its got when cranking.

[crispy'86]

...i'll need to invest in a good battery load tester

That's what the starter does, especially if the headlights are on.
Then check the ignition for what its got when cranking.

Would be handy to have a load tester to check batteries before I install them so it saves a lot of hassle and time. (saying I have several batteries sitting round in the garage

[ezy10s]

Over the years I've come across this problem a few times and after much wringing of hands it has almost always been a sad starter motor.
Invite one of your fellow car mates round and swipe his starter out while he's opening the beer you gave him. You'll soon have your answer!

[crispy'86]

Over the years I've come across this problem a few times and after much wringing of hands it has almost always been a sad starter motor.
Invite one of your fellow car mates round and swipe his starter out while he's opening the beer you gave him. You'll soon have your answer!

Tried a couple other of my spare starter motors that had same end result, slow cranking. After borrowing some test equipment from work, came across a good battery, put that in and haven't had an issue since. Ended up being a crook battery I had thought was good.

Having refreshed my memory on testing voltage drop etc I now know not to get caught out again