VK 355 starter motor current draw.

[Ultravk]

Hi all,
Hopefully somebody out there is better with wiring than me and can shed some light.

Just finished stroked build, finishing touches going together and went to try crank it, nothing. Very slow anyway.

Battery is mounted in boot with welding cable to starter post then a section to connect alternator.
Accessories are connected to the permanent +ve on starter.

Ok so 12volts at starter while at rest,
Drops to around 7v while cranking.
.3 ohm resistance is wiring from starter to batt. .3 ohm through chassis to -ve post on batt.
Battery tests fine. If I connect jump pack to +ve on alt and -be to ground it cranks and starts fine.


I think I have a current draw somewhere as I have continuity to ground from the main post. But buggered if I can find it.

Any suggestions?
Thanks in advance

 

[Immortality]

What are your earths like? Need the same size earth cables connecting block to chassis and chassis to battery as the positive cable.

Make sure your chassis connections are to bare metal.

Those resistance measurements via a multimeter are taken at a low amperage, things can change drastically with high amperage loads.

What size battery are you running? with such a long cable you will see a voltage drop so a battery that works fine when in the engine bay will struggle when relocated in the boot.

 

[Ultravk]

What are your earths like? Need the same size earth cables connecting block to chassis and chassis to battery as the positive cable.

Make sure your chassis connections are to bare metal.

Those resistance measurements via a multimeter are taken at a low amperage, things can change drastically with high amperage loads.

What size battery are you running? with such a long cable you will see a voltage drop so a battery that works fine when in the engine bay will struggle when relocated in the boot.

The battery is a red top ultima. (800cca)
Originally I suspected the main earth connection but it is clean and tight.
Block to chassis is a a heavy gauge wire aswell.

So you think it's earth related?

 

[Immortality]

Battery most likely isn't the problem then so a voltage drop in the wiring. When you say welding cable do you mean the 32mm² or the 50mm² cable?

I'd still be looking at the earth connections.

 

[Ultravk]

Battery most likely isn't the problem then so a voltage drop in the wiring. When you say welding cable do you mean the 32mm² or the 50mm² cable?

I'd still be looking at the earth connections.

I'm not sure of the gauge rating of the cable, but it's spare earth cable from my transmig.
Diameter wise it's roughly the same diameter as the 1/2 inch braid fuel line.?

 

[harrop.senator]

Sounds like 0 gauge ive used welding leads before no issue. I had a weird one in my mrs ute battery in bay 3 starters 2 batteries later and replacing feed to starter and block to chassis to battery with 0 gauge. Put a relay to the solenoid on the starter motor and never had an issue four years later. Used to drop 6-7 bolts under crank and not kick the solenoid out correctly at all at times. Cant tell you why but common problems on cressidas mate told me to try it and it worked.

 

[gtrboyy]

Hook up a switch from 12v power to solenoid starter then flick it on..if turns over like it should it needs relay(most commodores have voltage drop on that wire)

If still slow then think check earths again battery to body & block to chassis again...use jumper leads as extra earths to test theory.

Some aftermarket 'hi-torque' starters are lemons straight out the box.

Does sound weird you said drops to 7v while cranking..usually if down around 10v factory hei won't spark *shrugs*

Found that out on my vh with 355..it would crank fine but just wouldn't want to fire with 10v showng on voltmeter with under charged battery & 55amp alternator.

 

[Ultravk]

Sounds like 0 gauge ive used welding leads before no issue. I had a weird one in my mrs ute battery in bay 3 starters 2 batteries later and replacing feed to starter and block to chassis to battery with 0 gauge. Put a relay to the solenoid on the starter motor and never had an issue four years later. Used to drop 6-7 bolts under crank and not kick the solenoid out correctly at all at times. Cant tell you why but common problems on cressidas mate told me to try it and it worked.

Yea I've heard of the starter relay issue the early commodores had.
Mainly where the neutral cut switch was still wired in? Mine has an aftermarket relay mounted under the fuse box. It's switched from the key, and takes permanent +ve from the main starter post. So the voltage is dropping to 7v while cranking like everything else is.?

 

[Ultravk]

Hook up a switch from 12v power to solenoid starter then flick it on..if turns over like it should it needs relay(most commodores have voltage drop on that wire)

If still slow then think check earths again battery to body & block to chassis again...use jumper leads as extra earths to test theory.

Some aftermarket 'hi-torque' starters are lemons straight out the box.

Does sound weird you said drops to 7v while cranking..usually if down around 10v factory hei won't spark *shrugs*

Found that out on my vh with 355..it would crank fine but just wouldn't want to fire with 10v showng on voltmeter with under charged battery & 55amp alternator.

If I hook a jump pack across the battery in boot it makes no difference but if I connect the jump pack at the alternator +ve and the earth the pack on the engine block, no worries fires straight up. Cranks at full speed.
When it's in the boot. It will engage starter just has no balls at all. Won't even turn over past 1 cyl compression

It's running MSD 6al2 and coil/dizzy.
Factory stuff is redundant.

 

[Ultravk]

One thing that's sticking out for me is there is continuity to ground through the +ve circuit.

I have disconnected EVERYTHING I've wired in such as fuel pump, And fans. Still has continuity.

So I kept checking further. On the factory harness just under the fuse box there is the main battery connector. This plug has 6 wires I think 3 reds 3 black and they originally
Joins together into 1x positive and 1x negative and joins to the battery.
They appear to be the main feeds into fuse box/under dash.

Because the batt is in the boot now. I've earthed the -ve on chassis and extended the +ve to the back of the alt.
This positive wire seems to be the cause of continuity to ground. It's around 240 ohm resistance

Any ideas why? Or if it should be like this? Seems wrong to me.