Starter motor heat soak.

[Eggbert]

Here's one so weird I doubt it could ever happen twice. My Ford starter has a separate winding to help throw the gear out. Sometimes it would crank the engine, and sometimes it would just spin. The "just spin" happened after it was hot.

I took it apart and found the ground for that winding was riveted to the starter frame. The frame had been painted (green, so likely zinc-chromate) at some point in the rebuild process so the contact was "iffy". Drilled the rivet out, tapped the hole for a machine screw (SS no less!) and put it back together. Cranked like it should every time after that.

Hopefully no one has a Toyota engine in their Cobra, but I've put new solenoid contacts in several Toyota starters and whether it will crank or not was always intermittent. I never did figure out exactly what was wrong, but I didn't tinker with any of them as they worked more often than not (meaning that's an excuse as I was lazy).

Bad grounds between starter and battery are also a common problem. Ground cable ought to go to engine block and paint should be removed from block where cable lug goes. It ought to get a ground from bolt threads and exposed underside of bolt, but it's wise to go a little further.

[Dek]

The starter on my 6.3 SBC AK used to crank slowly or sometimes not crank at all, usually around 5 to 10  minutes after I turned the engine off and the heat had built up under the bonnet. I cured this by swapping to a Tilton high torque geared starter and fitting a heat shield in between the starter and exhaust manifold.

Thanks Paul. I wonder what the mechanism of heat soak is? Do you think it increases resistance in the starter motor or solenoid?

[Dek]

Quite frankly, I've never had a starter suffer from a heat soak problem, however I have had multiple instances of a worn out solenoid. On the round solenoids typically found on top of the starter, the disc contact rotates a bit for every attempt. Eventually it aligns with a clean spot and the engine cranks. For firewall mounted solenoids, the pitting from arcing often causes the same thing.

It's been some time since I took a solenoid apart, so this is how I remember it, even if it's a bit off.

That sounds quite plausible John. I'll buy that and pass it on.

[Paul H]

The starter on my 6.3 SBC AK used to crank slowly or sometimes not crank at all, usually around 5 to 10  minutes after I turned the engine off and the heat had built up under the bonnet. I cured this by swapping to a Tilton high torque geared starter and fitting a heat shield in between the starter and exhaust manifold.

[Chris]

Quite frankly, I've never had a starter suffer from a heat soak problem, however I have had multiple instances of a worn out solenoid. On the round solenoids typically found on top of the starter, the disc contact rotates a bit for every attempt. Eventually it aligns with a clean spot and the engine cranks. For firewall mounted solenoids, the pitting from arcing often causes the same thing.

It's been some time since I took a solenoid apart, so this is how I remember it, even if it's a bit off.

Solenoid cap kits use to be reasonably cheap and easily available, all you needed to do was to remove
a couple of screws and be ok with a soldering iron .
As long as the actual solenoid wasn't cooked, you could usually tell by the smell or get your meters out to
check the resistance and insulation of the coils .
This kind of stuff was fairly easy work, some were a little more difficult due to fact they were riveted and needed
drilling .
Chris.

[Eggbert]

intermittent cranking failure. - heat soak problem?

Quite frankly, I've never had a starter suffer from a heat soak problem, however I have had multiple instances of a worn out solenoid. On the round solenoids typically found on top of the starter, the disc contact rotates a bit for every attempt. Eventually it aligns with a clean spot and the engine cranks. For firewall mounted solenoids, the pitting from arcing often causes the same thing.

It's been some time since I took a solenoid apart, so this is how I remember it, even if it's a bit off.

[ChrisP]

Just to add, my son has an LS engine and also had intermittent starter issues. He was using the Chevy starter. He tried everything including wiring and in the end, changed the starter (for a geared one). That cured it. Not saying yours is also faulty but just sharing his experience.....................

[Dek]

Thanks Chris and Jon. Between you, you have covered all the potential causes. As I said I was asking on behalf of a fellow Cobra owner. I suspected heat soak because the problem was intermittent but I may be wrong. I know when I replaced my battery I opted for one with higher cranking amps. Anyway thanks both for your advice which I've passed on.

[JON1D]

In addition to Chris post check all +ve connection points if an battery cut off isolation switch is in circuit. The engine earth straps are all good contact points with the chassis and engine.
Cover starter with heat reflective blanket if thermal barrier with a heat shield from headers.
If all fails go with a new high torque mini starter Powermaster. 

[Chris]

One of the first things I would do is to make sure that the starter cables that you are using are up to
spec, especially if they are on a long run from back to front .
If in doubt go up in size if you have room and your not concerned about the extra weight .
Vertually all modern vehicles only use the absolute minimum they can get away with and some use
aluminium which is ok but doesn't outperform copper .
The other thing would be, is your battery up to spec although I'm fairly sure you have already gone
down that route ?

Chris.

[Dek]

I'm asking this on behalf of an AK owner with LS3 who's getting intermittent cranking failure. I think this was discussed on the old forum but can't remember what the solution was. Am I right in thinking this was a heat soak problem?