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Slow cranking when hot


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1941 special series 40. When my car is warmed up it cranks over very slowly to restart. For instance, when I start it to leave home it cranks great, starts up on a dime, but if I stop to get some ice cream, say 20miles away it barely cranks the motor. No clicking just real slow to turn over. Sometimes as if the family and I are going to be stuck. 6v system still. I have new battery cables the diameter of your thumb. I tried a new battery last year and same problem. Is it possible the starter gets heat soaked? Very scary, like I don’t want to turn the car off from fear of a dead battery. This is the 3rd battery in 6 yrs. it’s the long 6v tractor battery. 
 

what does it take to convert to 12v?

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I had the same problem with my 39 Roadmaster. I removed the after market quick disconnect hardware between the negative battery post and the ground cable. (The one with the green dial.) No more slow turnover when the engine was warmed up.

Edited by BuickBob49 (see edit history)
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Switching to 12V is definitely not the answer, it's a band-aid for hack mechanics. My '41 starts within a few seconds, hot or cold, in any weather. You've got good battery cables, so now is the time to improve your grounds, maybe add a second ground strap between one of the starter mounting bolts and the frame near the battery tray, and check the condition of your battery with a load test. If it still cranks with difficulty after all that, it may be time to rebuild the starter. Worn bushings and brushes can make it difficult to start and generate a lot of resistance, so that would be your final step. And make sure both your power cable AND your ground cable are huge.

 

Think of electricity like water--it needs to travel in a loop to do its job, from battery to ground. A big hose feeding the starter will put plenty of pressure in, but if you only have a little hose on the outlet side (ground) the starter is going to be a bottleneck and resistance will skyrocket with no work getting done.

 

I can't post PDFs here, but I wrote an article about this very subject not too long ago. Do a search on this forum for the "Sorting a pre-war car" thread, where there are extensive posts about hard starts, cooling system, fuel system, and more. 

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I will take a look. I did bring the starter to a repair shop and they indicated it was good but maybe they didn’t disassemble it. Cables are huge. Ground directly to the block 

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Matt is correct.  Check the ground.  Also what gauge are the cables?   Modern battery cables are not big enough for 6 volt batteries.  You would be shocked how quick the motor will turn over with the proper gauge battery cables.

 

 

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1 hour ago, alsancle said:

Matt is correct.  Check the ground.  Also what gauge are the cables?   Modern battery cables are not big enough for 6 volt batteries.  You would be shocked how quick the motor will turn over with the proper gauge battery cables.

 

 

I can’t remember but they are .75” of copper. They are huge. Maybe 0000awg. They don’t need to be bigger than this, lol.

 

43 minutes ago, Morgan Wright said:

Get rid of the long tractor battery and get an optima 6 volt battery.

I may just do that. This battery is over $250 and not worth a dam. 

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Is the ground cable from the battery attached to the frame or engine/trans? If attached to frame, there should be another ground strap from frame to the engine. Where any ground attaches, make sure all paint is removed @ the mating surfaces. I agree with Ben, your cables should be the type with the battery connector as one piece. The way your cables are, it is creating 2 more places that could be a problem. Take them apart and clean also, along with the battery connections/posts. You could buy a new battery and still have the same problem if you don`t clean all connections. Good luck

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The ground is connected directly to the block. I did put these on 2 yrs ago and cleaned everything and that still didn’t help. I would change the connection type but I don’t think that is the problem since it seems to only present itself after long drives when engine compartment is hot. Starts on a dim all the time when the engine is not heat soaked. Do starters usually act like this? Maybe my battery just sucks again after 2 yrs old. Every 3rd yr I have had to replace the battery. Napa warranty is only 18mon and I seem to get 24-30mon. Car sits a lot especially over the winter but I keep it on a 6v tender in the winter

1 minute ago, pont35cpe said:

Is the ground cable from the battery attached to the frame or engine/trans? If attached to frame, there should be another ground strap from frame to the engine. Where any ground attaches, make sure all paint is removed @ the mating surfaces. I agree with Ben, your cables should be the type with the battery connector as one piece. The way your cables are, it is creating 2 more places that could be a problem. Take them apart and clean also, along with the battery connections/posts. You could buy a new battery and still have the same problem if you don`t clean all connections. Good luck

 

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I honestly don't think it's the battery. Grounds and then maybe cable connections, then starter itself. I've had the same long, skinny 6V battery in my car for 8 years and it never fails to kick it over. There's something else wrong. More battery may help, but again, it's a band-aid, not a solution.

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It can't be the ground strap. If it were, it would make no difference whether the engine is cold or hot. Why would the ground strap suddenly stop conducting just because the engine got warm when it was fine when it was cold?

 

 

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Check the voltage of your battery after the drive when the engine is hot, it's possible your not getting enough charge from  the generator/regulator. When the battery rests it recovers some what and hence spins the starter well.

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Next time it cranks hard, feel all the battery connections.  They will get hot if they are drawing amperage.  When I look at your engine bay, those bolt on connectors between the battery cables and the battery are my first suspect.  Those need to go.   This means making new cables.  I would call someone like Rhode Island wire to make them, or get all the parts from them.  Even new wires from their recommendation.  I would look for brass battery post connectors too. 

 

You could use a propane torch and remove the crimp on wire connectors at the battery and resolder new brass battery post connectors as an option. 

 

I just think entire cables made to the highest standard are your ticket.   

Hugh   

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I don’t mind redoing the cables but as someone stated above why only when engine bay is hot? That doesn’t seem to indicate a cable issue. Now the generator could be the culprit. That and the voltage regulator are brand new and I do see the meter showing charging but this is during the day and all my lights are led, well the brake lights are and headlights, which I didn’t use during this trip. Maybe the use of the brake lights is killing the charge and generator is not keeping up. The cable from the generator is very small and original. I will feel the cables next time the engine is hot and maybe even check the battery with my volt meter

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I had the same problem with a starter and after working on it for some time finally discovered that when hot, one of the starter bushings was worn just enough to allow the armature to drag .  Replaced the bushings and it has worked fine ever since.

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2 hours ago, Morgan Wright said:

It can't be the ground strap. If it were, it would make no difference whether the engine is cold or hot. Why would the ground strap suddenly stop conducting just because the engine got warm when it was fine when it was cold?

 

 

 

As temperature goes up, so does resistance. It's not resistance in the ground strap, per se, but it's resistance in the various parts that add up and prevent sufficient current from flowing through the starter to turn it. Starters are usually grounded through their housings to the block or bellhousing. But after decades of road use, there's dirt and grease and rust, not to mention paint on the block and the frame. Add in rubber engine mounts with rusty mounting bolts and more paint, and it's easy to see why that ground path might not be ideal. That's why I always add a ground strap directly from the starter to a ground point on the frame as close to the battery ground as possible. 

 

The ground strap is one part of it, but if your starter is the issue, then there's excessive resistance somewhere between the battery + terminal, through the system, and back to the - terminal. If you're not getting a good ground in the starter, then that's a bottleneck where the current can't flow easily. An additional ground from the starter opens it up. Heat in the starter increases resistance, not only in terms of current flow, but also in terms of how the parts fit--tolerances get tighter and if your starter is worn it may have to work harder to crank.

 

So while the cable might be OK, heat can change how easy it is for current to flow through the entire system.

 

 

Edited by Matt Harwood (see edit history)
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Thanks gentlemen. I am going to add a ground strap to the starter and I am pulling the starter and bringing it down to my local rebuild center who did my generator. I am going to tell him to rebuild it all with new and go from there. Rebuilt starter can’t hurt as well as the ground. I will report back

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The only other thought I just had was I have another thread going related to timing m. Is it possible the base timing is to advanced and the car will start cold since the engine has 60k miles on it but when it’s warmed up the timing is so far advanced that it’s causing my issue? Maybe Matt’s suggestion on the base timing set to zero may be the fix? Just thinking out load since I know on my other SBF I had if the timing was to far advanced past 10 deg base timing it would barley crank. That engine was rebuilt so that issue would present itself all the time, cold or hot. This baby is 80 yrs old and worn out, lol. 
thoughts?

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My guess is the reason everyone is steering you to cables and connections is that we have all been there, spent money on stuff we didn't need, and eventually found the trouble in the cables or connections.

 

Check the following with a multimeter set to a fairly low DC VOLTS scale. The freebie-with-coupon multimeter from harbor freight will do fine. All these readings are taken with the engine hot and starter dragging. All of these readings are taken while the engine is cranking, so you will need another person to help.

 

1) Battery voltage. Put your probes right in the center of the battery posts, not on a terminal or cable. How many volts?

 

2) Put one probe on the center of the positive battery post and the other on the copper bolt on the starter that the battery cable attaches to. What is the voltage?

 

3) Put one probe on the center of the positive battery post and the other on the starter case. What is the voltage?

 

This will either solve it or tell us where to look next. The measurements have to be made while cranking, and while the car is screwing up (hot).

 

Electric motors are less powerful when hot, so any non heat related problem could theoretically show up only when the car is hot.

 

Too much timing could do it. The engine will chug and nearly stop turning, as the firing impulse happens too early and it tries to push back against the starter.

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Id check the free things first. Timing is a common problem if set too far advanced. I have verified this during a hot, difficult start by simply pulling the coil wire before spinning the engine. If it spins fast with the coil wire disconnected  then timing is too far advanced. Next , battery cables and connections. I like making my battery cables out of welding lead with a fiberglass sleeve on the outside. I use crimp on ends like shown in the link. Most heavy truck parts stores with have these battery cable ends. I have also had a solenoid with eroded contacts between the two large lugs. You wont see it but a tired solenoid can really limit what a good starter can do , especially hot. I have also used a thermal hunting scope to trouble shoot hot spots and high resistance areas. It really works well in certain conditions.

https://www.remybattery.com/3-0-gauge-straight-battery-terminal-clamp-connector.html 

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I use this 00 AWG cable with the proper connectors already attached on my 1938 Buick. The drop across the cables during starting is less than 0.1V. 

 

https://www.ebay.com/itm/Positive-Battery-Cable-2-0-AWG-00-Gauge-Ga-Copper-Custom-Made-Auto-Truck-Marine/131905856172?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

 

I also found that the that cheap green knob type battery disconnect was dropping almost 0.5V during cranking. 

 

Steve D

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On 5/3/2020 at 8:09 PM, michealbernal said:

I had the same problem with a starter and after working on it for some time finally discovered that when hot, one of the starter bushings was worn just enough to allow the armature to drag .  Replaced the bushings and it has worked fine ever since.

I'll second this.  Tear your starter down and check brushes and armature wear as well.  Polish the armature, file the brushes if they aren't worn out.  Check for grease on the field coils.

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I had the same problem. Two major thungs fixed it. The first time it was a broken joint between the field stud on the outside of the case and the field coils inside. It takes a big soldering iron to re-solder the joint. The finale was to add a dedicated ground cable from the starter ground bolt directly to battery ground post.

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  • 3 weeks later...

Thanks for everyone’s help. I resolved the problem. Brought my starter to our local rebuild shop. Field was dragging on the armature. They cut the armature down to true it up. Rebuilt everything with new bushings, (brushes were good). Cost me $43.87 with tax. Cranks much faster now when cold and no more issues when hot.

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