1996 850 turbo

[Texasbrainstew]

I'm helping my friend with his headgasket job, we put everything back but when we try to crank the engine, it doing something funky.

We have good lights and power with a battery connected to jump the car.

When we try to crank it, it's slow and the power and lights almost immediately cut out. Then the positive power cable gets really hot.

What should we attack first??

[npexcept]

changing the head gasket is not as easy.
Before firing it up, you should have tried to turn it over by hand (did you?).
Is there much resistance?
Much can go wrong here. Wrong timing is the worst case.

[Texasbrainstew]

I hear ya! Something I've done quite a few times! I've never had this issue with engine swaps or rebuilds.

[erikv11]

I'm helping my friend with his headgasket job, we put everything back but when we try to crank the engine, it doing something funky.

We have good lights and power with a battery connected to jump the car.

When we try to crank it, it's slow and the power and lights almost immediately cut out. Then the positive power cable gets really hot.

What should we attack first??

This could be because like the cable is all cracked and corroded inside the lug where it connect to the positive battery (e.g. viewtopic.php?t=61747 , very common on 850 cars).

Where does the cable get hot, is it near the battery (see above) or somewhere else? Wherever the cable heats up, there is very little continuity leading to very high electrical resistance leading to heat.

The lack of proper voltage could be the whole issue. Try rigging together a new positive cable to bypass the existing one, see if that wolves the problem.

[scot850]

After you have checked the battery cables, check all the grounds are connected, and finally check the starter motor has not gone bad. My brother's 850 became slow turning over and I suggested swapping out the starter and now the car starts easily. A bad starter will make the battery leads hot, as will bad grounds make it hard to turn over.

Neil.

[npexcept]

Wherever the cable heats up, there is very little continuity leading to very high electrical resistance leading to heat.

Or the cable is fine and the engine is seized, leading to an overloaded starter motor.
First thing I would try is turning it over by hand! Making sure its free spinning.
If just the head gasket was replaced and everything was fine before (was it?), where would these electrical issues come from?

Dimming lights shows a high current draw.
A defective cable would not increase (total) current.
Something IS bad about the starter or the engine cranking.

The lack of proper voltage could be the whole issue. Try rigging together a new positive cable to bypass the existing one, see if that wolves the problem.

Best case here would just be a bad/weak battery. But also... before trying a new bat or jumpstarting, make sure its easily spinnable...

[erikv11]

Wherever the cable heats up, there is very little continuity leading to very high electrical resistance leading to heat.

Or the cable is fine and the engine is seized, leading to an overloaded starter motor.
First thing I would try is turning it over by hand! Making sure its free spinning.

Yes, you already said this. It's a good point, agreed, but maybe you didn't notice that OP already replied to you? OP says the engine isn't seized. And in reply to you says "yes, has done many engine swaps and rebuilds" etc.

If just the head gasket was replaced and everything was fine before (was it?), where would these electrical issues come from?

From, for example a corroded cable that was flexed during the job and broke. But yes, where did the issue come from? That's the question!

Dimming lights shows a high current draw.
A defective cable would not increase (total) current.

Dimming lights are due to insufficient amps i.e. low current to the headlights. Here's how a bad positive cable can cause that: A defective (broken, corroded, etc) positive cable can lead to high resistance at the break, which under constant voltage reduces amperage (current flow) everywhere: a defective cable can reduce current. That's why I asked where the heat is coming from: is it the whole cable or just on one spot? If it's the whole cable then check the grounds as per scot850's great suggestion, if the heart is in one spot then suspect a damaged cable at that spot.

Starter could be bad, sure, that's also possible.

Grounds and starter sound like best ideas so far. But I'd also check where the cable gets hot!