G'Day lads, I have a problem that keeps happening, looking for ideas and suggestions.

Problem: Starter motors give up and let smoke out after a couple of cranks.

Vehicle Setup: 52 Ford F-5 with original flathead. Now on 12v battery (not a full 12v conversion at all) to negative ground. Coil is 12v. Starter Relay is still 6v and has a very loud 'click' when hit starter button.

It burns out the starter then just does the loud click from the relay.

It was fine up until now, starter numerous times no problem.

I have gone through 2 x starters so far.

My next step I can think of is to change out the relay for a 12v relay?

 

Have you checked all the wires and especially the cable from the starter to ground for good contact? When changing to 12V you’ll need to replace the starter relay as well.

 

The starter has no ground cable...it grounds through the block.

 

Of course you’re right - I meant the cable from the starter solenoid to ground

 

aha, yes of course. I will make double sure that is perfect in swapping out the relay.

 

If the relay were bad, it would be the one letting the smoke out. Is the face of the starter plate clean to bare metal, or painted? If the starter can't get good ground thru that, it won't crank well, but even that wouldn't make it smoke. I'm not sure what ground is being discussed on the solenoid? It grounds thru its mounting screws to the firewall.

 

I would check to remove any voltage regulator and horn relay wiring that still may be positive ground shorting your system.

 

It sounds like your starter relay is working perfectly. You obviously have:
1) Undersized starter cabling (high resistance).
2) Corroded starter cable connections (more high resistance).
3) Ground issues between starter/engine, engine/chassis, chassis/battery (even more higher resistance).
4) Battery is not fully charged (10 volts or less. Less voltage requires much higher amperage draw).

 

You haven't said it, but I think we're all assuming you've replaced the starter with a 12 volt starter, right? Original ones would be 6 volts for that truck. If you've converted to 12 volts, you're changing from positive to negative ground I assume, and an alternator instead of a generator (so no voltage regulator). So I'm assuming that the truck has the right starter and polarity to the starter. I know this is kind of basic stuff, but my experience is it's usually something simple that even experienced folks can overlook.

You say it's smoking, but is the smoke coming from the starter itself or from a wire? Is the starter wire hot after cranking? Is the starter casing hot? I'd assume that the only thing that would make an electric motor smoke is overloading it with power or some sort of short inside of it. It's possible to get a bad rebuilt starter, but not too likely two in a row. Still, I've seen folks get bad ones, and it's not as uncommon as it should be. Does the motor run? I suppose if the motor is bound it would smoke the starter some. Like Mixer Man says, having too low a gauge wire leading to the starter will cause some resistance, which will make the motor draw heavier amperage, but I'd assume it would smoke the wire first. 12 volts with alternators don't need the 00 gauge wire I need on my 6v starter and battery. I'm not experienced with 12v generators, so I don't know what the wiring needs are for that. Either way, if you're going to keep it 12 volts you probably want to use a 12 volt solenoid. They're cheap and take a few minutes to install.

 

Sorry for short reply.

All original wiring except coil.

Just replaced battery with 12v neg ground.

Does it need be positive ground?

 

The original 6V starter and start solenoid will work for years trouble free on 12V negative ground. No need to change either to 12V unless they are bad.
Mark

 

Is the clutch pushed in while trying to crank it?

 

Agreed. I still have the 6 volt starter and starter relay.

So, in re-reading the original post it says: "(not a full 12v conversion at all) to negative ground.". Are you still running the 6 volt positive ground generator and voltage regulator? What all have you converted to 12 volts besides the battery and coil? The starter is not polarity sensitive, it will only turn in one direction.
The original 6 volt wiring is more than adequate for 12 volts provided it's in good condition.

 

No other wiring or component changes besides a 12v coil.

 

It seems unlikely anything other than bad starters to begin with does it not?

Think of it like this, the only time the starter gets volts is when the relay is engaged. I checked the relay and it appears to be working correctly.

The starter does not care about polarity and 12v negative ground is fine to the opinion of the masses.

It has a good ground through the block presumably, is there a way to test this?

The only other thing is a stuck motor however I can turn it freely by hand and the ring gear looks intact?

Perhaps I just got unlucky with older starters?