The Stovebolt Site Forums The Shop Area The Electrical Bay Should my 1951 chevy 1 ton be positive ground?

Hi. I have a Canadian 1430 (3800) 1 ton. I've been trying to get it started. It turns over fine but has no spark. The previous owned has a 6v battery installed with positive as ground. I thought only the GMC trucks came with positive ground. Is this a Canadian truck thing?

What size engine is in your truck?

If it is a Chevrolet 216 or 235, my guess is it would have been 6v negative ground.

If it is a GMC 228, my guess is that it would have been 6v positive ground.

Did you ask the previous owner if it started/ran with that battery cable arrangement?

All of the above is speculation - wait to hear from a Canadian.

The previous owner died. His family tried but couldn't get it started. It's a chevy 235 in the truck.

no, Chevys are neg ground, find a wiring diagram here - if it's hooked up + ground then you'll need to re-polarize the gen/reg when you hook it up right, find the 48-51 shop manual here

Bill

Incorrect ground polarity will not keep an ignition system from having a spark, but it can reduce the firing voltage about 20%. You need to find the no spark problem somewhere else. Chances are the points are dirty or burned, or there's a ground where the primary wire goes through the side of the distributor. If it's got a Chevy engine, the ground polarity needs to be negative.
Jerry

Jerry --

You write that reversed polarity "can reduce the firing voltage about 20 %." I was wondering if you could say more about the physics of how that occurs. I've read about a polarity reverser that switches the polarity across the points every time the starter is depressed but I've never seen one in real life -- any thoughts on that?

Just curious.

Thanks

Pete P.
Harborcreek, Penna.

Pete, I believe the polarity switching setup was to lengthen the life of the points, didn't last long .... and maybe on GMCs only?

Bill

A polarity reversing switch on the starter was used on Chevrolet starters from 1941-1948 (except for COEs).

Reversing coil polarity makes the current travel through the primary (low voltage) coil winding backwards to its normal direction of flow, which effectively puts the primary in series with the secondary winding when the magnetic impulse generates the high voltage spike to fire the spark plug. The inductive reactance, which is the equivalent to resistance induced when current flows through a coil winding, reduces the available voltage seen at the spark plug. I've seen that indication many times on an oscilloscope when diagnosing poor performance complaints.

On distributorless ignition systems where two spark plugs fire from opposite ends of a single coil secondary winding, half the firing voltages will be noticeably less on a scope pattern when the spark originates at the "cold" ground electrode and jumps to the insulated center part of the plug.
Jerry

So then are there + ground and - ground coils I wonder? Could a larger capacitance condenser absorb the additional back EMF and compensate?

I also dabble in antique tractors and although I'm a bit of a purist and tend to oppose 12v conversions and electronic ignitions I have done them if a customer insists. Post-conversion, spark is OK but on the front mount model N Fords in particular it's just not quite perfect. I was blaming the ballast resistors but perhaps polarity reversal is partly to blame.

Pete P.
Harborcreek, Penna.

Unless you're dealing with an integral-coil distributor like the old banjo-housing Ford distributors with the coil mounted directly on top, just reverse the primary leads on the coil when changing polarity. The problem is caused by the secondary winding being connected to the wrong end of the primary which causes a reverse EMF being generated as the secondary current looks for a ground. The secondary actually uses the battery and the primary wiring, ignition switch, etc. as a path for the high voltage to find its way back to the end of the circuit. If the coil is labeled "ign" and "dist" at the primary terminals, just connect them backwards to the labeling when reversing the polarity. (+) and (-) labeled coil primary terminals are connected with the ground polarity toward the distributor.
Jerry

Yup, it's the front mount dist. with the coil that just sits on top. In the stock configuration, 6V- is applied to the terminal on top of the coil and the primary is grounded via the points through a contact on the bottom of the coil which touches the points/ condenser mounting screw. With a Pertronix 6V electronic ignition conversion or a 12 V conversion (with or without Pertronix) one must reverse polarity.

Even backwards it works OK but not as good as stock, in my opinion. I don't have a scope but it would be an interesting experiment to quantify the decrease in spark intensity.

Thanks for your insight.

Pete P.

A scope pattern with a reversed coil polarity will show the pattern upside down, at least on the primary circuit.
Jerry

It is not true that ALL Chevrolet trucks are negative ground. First run 1941 Military trucks are 25 amp positive ground.

[img]http://www.surfacezero.com/g503/data/3764/medium/2014-07-12_09_21_34.jpg[/img]

What make G503 was that device used in?
What engine?

Tim,

It is not a Ford or Willys G-503, it is a Chevrolet G-4112. These trucks had the first 235's produced, and YP series and early ZP series trucks had 25 amp positive ground electrical systems.

http://books.google.com/books?id=52...onepage&q=chevy%20g-4112&f=false