? for the engine guru's

[Todd B]

I have a question that has stumped me for years. Forgot about it until today when my 53 semi wouldn't start. How come when you can spin a motor until the batterie is down and it won't start, you hook a tow strap to a truck (in my case today a Mach quad axle dump truck) and pull it for 50' they will start every time.

Please explain.

Todd B

[bkahler]

I have a question that has stumped me for years. Forgot about it until today when my 53 semi wouldn't start. How come when you can spin a motor until the batterie is down and it won't start' date=' you hook a tow strap to a truck (in my case today a Mach quad axle dump truck) and pull it for 50' they will start every time.

Please explain.

Todd B[/quote']

Todd,

If I was to hazard a guess I would say when you're cranking with the battery and its not spinning fast enough you're also drawing down the voltage to the ignition system which means your spark is too week to ignite the air/fuel misture. Whereas if you're towing the battery voltage is up somewhat and not being pulled down by the starter. Higher voltage at the coil means better spark.

Anyone else have any theories?!

Brad

[Merle Coggins]

I was kind of thinking the same thing, Brad.

[grey beard]

Personally, I think it's just serendipidy . . . let me explain. I have experienced what you describe, as I'm sure have others. Having said that, I have also experienced pulling a cold diesel around a trucking company lot quite a while, while hanging out the door spraying starting fluid into the air intake. Please trust me when I say they don't always pop off when you start to drag them.

Then again, maybe you just quit thirty cranking revolutions too soon - had you hung onto the button just a little more, she might have started. I also agree with the logic expressed above that system voltage is really low when cranking, and this would logically have a negative effect on the volts available to the ignition system.

[Vintrader]

Well I am no engine guru, But it feels more like combustion pressure, than a electrical thing when you POP THE CLUTCH. I have started many a old ride by pushing it by myself with the drivers door open, jumping in, shifting to 2nd, and letting off the clutch. When the battery had little or no charge. Dutch

[Todd B]

I agree with Dutch, I have started (gas) tractors, cars, trucks & many times my Harley with the batterie so dead it does nothing. I am leaning towards Brad's answer but I don't know the technical side of the engine. I can tear anything apart and put it back together with only a few extra parts, but I am lost internally. I just realized I have a friend who has been building motors his whole life for high performance cars all over the US. I will call him tomorrow and get his 2 cents.

Thanks for the imput so far.

[Plymouthy Adams]

Would suggest you investigate the condition of the starter circuit..do you have a voltage drop on your cables? Is the rear bushing enlongated due to age and allowing the armature to drag a bit? Both these conditions will cause you to draw excessive amps during cranking and thus limit the amount of current available to the ignition circuits..are your battery cable suffficient size and are they getting very warm after just a few attempts to start? Quick check would be to set a second battery in and jumper (independent) to the ignition..

PS..sometimes this condition can be fixed by replacing a weak coil...have seen that a couple times also..

[Todd B]

My trucks usally start within a few spins, I am not sure exactly what happened this time. I think it was a combination of very wet/ damp conditions, sat for 2 weeks, I forgot I had a electric fuel pump and forgot to give the pump a moment to work and after all of that I think I flooded it.

We kind of got side tracked from my original question. Why when a vehichle won't seem to start by cranking, jumping or even with a charger on, but they pop right off when you dump the clutch.

Todd

[Plymouthy Adams]

no we did not..still think you need to investigate for current drop/insufficient size cable or dragging starter right on to possible bad coil...

[Don Coatney]

I agree with the theory on the starter dropping voltage and current to the ignition system when attempting to start the engine with the starter. And as has been mentioned clean, tight, and well grounded electrical connections do address this issue.

I have converted my car to 12 volts. When doing so I added a ballast resistor that drops the voltage to the ignition coil and points. I also replaced the starter solenoid switch with a furd 12 volt 4 pole solenoid switch. The extra pole on this solenoid switch is hot with 12 volts when ever the starter motor is running. I connected this 12 volt supply to the coil side of my ballast resistor there by giving my ignition system a full 12 volts each and every time I hit the starter button. There is one slight drawback to connecting the solenoid switch this way. If using a remote starter button under the hood the engine will start even if the ignition key is turned off. Of course the engine will quit running as soon as the remote starter switch is released.

[Todd B]

I asked my freind who builds motors my question and his answer was "bottom line, you pulled power from your ignition when you turn the starter over. when you pull start a motor there is absolutly no ignition power loss." he totally agreed with all of you and my mistery is solved.

Thanks for the input.

Todd B