Jump to content
nelmo

Starter motor feed

Recommended Posts

Is it possible, electrically, to be getting only a partial voltage feed to my starter motor?

I ask because, sometimes, my starter motor sounds great and the engine starts first time but other times, the motor sounds like it's running at half-speed and the engine takes ages to catch (the battery voltage is about the same on both occasions so I don't think the battery is the problem).

I always imagine that electricity is digital, you either get the full amount or none but this behaviour suggests otherwise?

I'm about to take my dash off and re-wire the ignition feed because at times, I can feel the wire getting warm when I'm cranking the engine, so I think my dodgy wiring may have a short/loose connection but just wondering if anyone agrees this is the cause of my starting issues?

  • Like 1

Share this post


Link to post
Share on other sites

I'd be looking at the power and earth connection to the starter first. Is the engine earth cable bolted solidly both ends to clean metal? Is the power lead from the battery to the starter also well connected both ends, both the bolted part and also the crimped / soldered connection between the cable itself and the terminal.

Share this post


Link to post
Share on other sites

To answer your question, and as Richy says - yes, you can get only "partial voltage" (to coin your phrase). What you're essentially finding is that if there's a bad connection, you'll get poor voltage. Bad connection means high resistance means low voltage (and warm/overheating conductors!) so you are right to jump on it and investigate :)

Share this post


Link to post
Share on other sites

Thank you both - good to know I'm not totally wasting my time. 

Share this post


Link to post
Share on other sites

I had an issue a few years ago where my starter motor was slow to turn but all my earths were fine. 
I replaced the starter and it fixed the problem. This was on my pinto and was an ancient starter. 

Share this post


Link to post
Share on other sites

When the starter is slugish imediatly check each connection to find if any are warm/hot or if the starter itself is hot.

Do not forget to check the pull in solonoid on the starter as this has switch contacts in it and might be hot.

Hot connections are bad connections.

It can also be worn brushes in the starter

Share this post


Link to post
Share on other sites

Most likely cause is pitting of the starter solenoid switch contacts, the switch moving contact "floats " on the end of the solenoid & so sometimes will be OK ( a pip of wear/burnt contact will align with a crater of wear ) & sometimes the opposite occurs ( pip of burnt contact to pip on fixed contact )

I have successfully stripped & cleaned several solenoids in the past ( Florin's is several years since strip/refurb )

Not difficult bit needs care & patience, otherwise the solenoid coil needs rewiring!!!  ( How do I know that? )

Share this post


Link to post
Share on other sites

The starter motor is only a few months old, so hopefully not that. 

Share this post


Link to post
Share on other sites

A systematic check of the earth cables then. 

Share this post


Link to post
Share on other sites

Another thing to check is that the ignition timing is not over advanced at cranking speed. The problem being the cylinder first firing too early slows the still upward travel of its piston. This happens on good engines when firing starts before the cranking speed is high, then all the others join in, keeping cranking speed too low. Simple test is to retard the dizzy a few degrees and see if the cranking speed improves.

With earths I have an overkill three!  Battery to car body, engine to battery and engine to car body,  so return circuit is doubled (in case of one connection being poor.)

  • Like 1

Share this post


Link to post
Share on other sites

Volts drop is calculatable, you can measure the resistance of the cable, crimps, etc and you will loose some voltage as current flows. The formula is expressed as millivolts per amp per meter, there are correction factors such as temperature, battery internal resistance (leading to resistance network calculations), power factor, all sorts, you even gain a little resistance to something called the skin effect, it’s all very quantifiable but boring. you can use ohms law, V/I.R,  and transpose it as you wish to get a few simple figures too. 
Think of resistance as you blowing water down a pipe. There is a pressure you apply to the end you blow in but at the other end the pressure is lower. That’s due to the water having mechanical drag as it passes the walls of the pipe, the longer the pipe the greater the differential. Eventually you can’t blow down it, not even a bit. The resistance of a long enough pipe will result in the water simply refusing to flow at the pressure you apply. Higher pressure is the same as higher voltage, it flows easier as it overcomes the resistance and can cover longer distances with lower drops. Current is like the volume of water that flows, expressed as amps in this case. That’s why the national grid distribute at 440kv, to save using cables 40 feet in diameter. 12v is very low voltage so current is always high, looking at the formula for volts drop low volts and/or high current result in increased voltage drop, so yes, cars are very susceptible to this phenomenon. 
there are tables and calculations to size appropriate cables, there are also rules of thumb and that’s all we need to use for a simple starter motor. 
 

sorry if that’s a long winded post. It’s what I do for a living I’m afraid 🙄

Share this post


Link to post
Share on other sites

Man, I have been such a cowboy in the past 😳. I took my ignition plug off and found that I had tried to squeeze 4 neutral wires into one crimped plug! 😜. As soon as I took the insulation tape off, half the wires dropped out immediately - jees...

I still cannot work out/remember why so many wires need to go into one plug but I tidied it up with a complex of linked butt connectors so that I only had one wire to go into the plug itself (hope that is ok?). Connected up and all seems to be good again (for now).

Thanks for all the help and suggestions...🙂

  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...