Low Voltage Cranking

[NomadE30]

I'll give you the details:

-M20 flywheel and starter (already changed the starter)
-M52 bottom end (that spins easily by hand)
-M50 wiring off an auto (98% sure that everything is right on it)

Is at 13.5 volts with key on, drops to 7-8 volts while cranking. Doesn't spin the engine fast enough to get it started (probably around 200 rpms if that). Trying to figure out why it's dropping voltage so bad, like I said I have changed the starter, I have unhooked all accessories just in case, and have unplugged every sensor to make sure.

Does anyone have a clue? I've about to tear into this auto wiring harness to attempt to eliminate the extra wires and hopefully find a problem, but I really don't want to create more problems than necessary.

Also there is no battery, it's hook directly up to a diesel truck that has two batteries so it shouldn't have any problem with CCA.

[jt223539]

I think your problem is that you need a battery in the car.

You can only run so much power thru the jumper cables at a time. You need to battery to store CCA so that the car can start.

Do your jumper cables get really hot when attempting this?

[NomadE30]

Jumper cables do not get hot. They're incredibly thick jumpers (around welding cable thick). I'm gonna pick up a battery after work and hopefully this will fix it...

[AndrewBird]

Sounds like a short somewhere in the starter power circuit. If the jumper cables are at least 2ga, they should be fine.

[M3PO]

Removing the auto wiring is easy but I highly doubt that's it.