Positive ground conversion

1970 cortina GT · Post by **1970 cortina GT** » Mon Aug 21, 2017 12:30 pm

After studying the wiring diagram of my 66 MK1 Cortina for extended periods trying to sort out my pile of ' spaghetti like' wires I see that it is wired as positive ground, I have heard of people converting to negative ground but wasn't sure what all was involved. Anyone had any experience with this?

JAN · Post by **JAN** » Mon Aug 21, 2017 2:08 pm

Physically turn the battery around and reconnect the leads. Depending on circumstances, you might need new leads. If it has posts rather than lugs, the positive post is a larger diameter than the negative post, and the leads might be too short in the new position.

Reconnect the low tension leads on the coil the other way around. This will almost certainly mean removing the coil. loosening the clamp screw and turning it 180 degrees within its mounting bracket.

Take a wire form the live (now positive) battery terminal and touch it briefly, about a second is enough, to the small terminal on the dynamo. This will repolarise the dynamo and allow it to charge as before.

Job done!

Sideways · Post by **Sideways** » Mon Aug 21, 2017 2:42 pm

JAN wrote:Physically turn the battery around and reconnect the leads. Depending on circumstances, you might need new leads. If it has posts rather than lugs, the positive post is a larger diameter than the negative post, and the leads might be too short in the new position.

Reconnect the low tension leads on the coil the other way around. This will almost certainly mean removing the coil. loosening the clamp screw and turning it 180 degrees within its mounting bracket.

Take a wire form the live (now positive) battery terminal and touch it briefly, about a second is enough, to the small terminal on the dynamo. This will repolarise the dynamo and allow it to charge as before.

Job done!

All that.

Verify that your radio/antenna is isolated from the car as well.

Post by **peteleo** » Mon Aug 21, 2017 7:38 pm

GT model the tach circuitry will need to be modified for - grd. If you're
using a coil with cb / sw terminals which is meant for + grd. Purchase a new coil with - and + terminals.

1970 cortina GT · Post by **1970 cortina GT** » Tue Aug 22, 2017 7:40 pm

Thanks for the info gentlemen, very much appreciated. Another quick enquiry, what does anyone have for an original motor for this 66 GT ? Haven't decided which way to go for a motor as yet. I have my 1600 out of the mk2 GT but not completely sold on that yet. Just checking my options and prices, thanks.

1970 cortina GT · Post by **1970 cortina GT** » Tue Aug 22, 2017 7:50 pm

I should add that in an ideal world I'll find another twin cam to put in the car

Brett Wilkie · Post by **Brett Wilkie** » Tue Aug 22, 2017 10:16 pm

Car looks to have great potential, have a read here regarding the polarity swap on the tachometer.
http://www.lotus-cortina.com/electric/convert.htm

Post by **peteleo** » Tue Aug 22, 2017 11:45 pm

Section on : 'Ignition coil wiring reversal' i don't agree with. Coils with cb / sw are meant for positive grd cars. sw is the primary. cb secondary. Reversing polarity will cause a voltage drop.

Brett Wilkie · Post by **Brett Wilkie** » Wed Aug 23, 2017 12:43 am

That is interesting, I have never put it to the test myself so I can't really comment. They did talk about it further though if this is of any significance, I will paste this in.

Testing Ignition Coil Wiring Reversal.

You REALLY need to test this. With the ignition off, engine off, pull a spark plug wire loose from a spark plug. Place it loosely back in place.

Now start the engine. Hold a no.2 lead pencil carefully in one hand. With the other hand, pull the loosened spark plug wire slowly away from the plug. You should see a good size spark jumping from the wire to the plug. Now... CAREFULLY place the sharpened lead point of the pencil into the path of the spark... you should see a "flare" of particles flying from the pencil lead TOWARDS the spark plug. If you see a "flare" of particles flying from the pencil lead TOWARDS the wire... you have the coil hooked up backwards. Remove the pencil lead from the spark path. Replace the wire. Shut the engine off, and re-reverse the coil low-tension wires and re-test. Get it right!

Why does this matter? If you got this wrong, you will be losing about 50% of your voltage at the spark plugs! Weak spark! Essentially, if the ignition low tension coil wiring is reversed, the coil will... "pull" spark, rather than... "push" the spark. Electrons prefer to leave a high-temperature (high energy) surface (like the center electrode of the spark plug) rather than a low-temperature (low energy) surface (like the arm of the spark plug). More voltage (more spark) is available if the electrons move in the correct direction, in the direction they prefer, from the high-temperature center electrode of the spark plug to the (relatively) low-temperature arm of the spark plug. Test it! Get it right!

Post by **peteleo** » Wed Aug 23, 2017 8:09 am

You can't just switch wires on a cb / sw coil after reversing polarity because there is a 100 to 1 wire wound ratio between the secondary and primary. The same goes for a + / - terminal coil used as a replacement for a + grd car.

I've done an article on this: https://englishfordparts.blogspot.com/2 ... stems.html

enfoprefect · Post by **enfoprefect** » Wed Aug 23, 2017 8:40 am

Peteleo I have to disagree with you on coil reversal. When I replaced the original engine in my '48 Prefect with one from a '58, I upgraded all the electricals to 12v and switched to negative ground. The 12v coil was labeled sw/cb and I just switched the wires. This was in 1965 and we put 30,000 miles on it as a second car over the next 5 years. Not so many miles since but it still runs perfectly well today. Starts instantly and runs like a champ. Or at least it did a few months ago. Currently up on blocks for some underbody work.

JAN · Post by **JAN** » Wed Aug 23, 2017 9:21 am

enfoprefect wrote:Peteleo I have to disagree with you on coil reversal. When I replaced the original engine in my '48 Prefect with one from a '58, I upgraded all the electricals to 12v and switched to negative ground. The 12v coil was labeled sw/cb and I just switched the wires. This was in 1965 and we put 30,000 miles on it as a second car over the next 5 years. Not so many miles since but it still runs perfectly well today. Starts instantly and runs like a champ. Or at least it did a few months ago. Currently up on blocks for some underbody work.

I have to agree. Swapping the LT connections does not involve the HT coils, but simply allows the HT spark to flow in the right direction. I too have run coils with SW / CB marked terminals in the reversed position after converting to negative earth, and never had a problem.

Post by **peteleo** » Wed Aug 23, 2017 9:41 am

I've tested the output of a coil marked cb / sw on a + grd car. Then testing the same coil on a - grd car by switching terminal wires. The output is approx. 10% less. Yes, of coarse, the engine will run anyways.

Not reversing the the terminals on a cb/sw coil for a - grd car will also work
but will result in a lower negative voltage output.

1970 cortina GT · Post by **1970 cortina GT** » Wed Oct 11, 2017 1:00 pm

Old, topic but I have another question. Once I have converted my tacho to negative earth is there anything else I need to do to it if I'm using a Petronix electronic ignition? There is a place in NY called Nisonger that suggests there are some other changes necessary? They are looking at $275 US to do the conversion, appreciate your thoughts anyone. BTW I ran an Accuspark electronic ignition in my MK2 and whilst it is negative earth I didn't change anything else in the tacho.

Sideways · Post by **Sideways** » Thu Oct 12, 2017 5:52 pm

peteleo wrote:I've tested the output of a coil marked cb / sw on a + grd car. Then testing the same coil on a - grd car by switching terminal wires. The output is approx. 10% less. Yes, of coarse, the engine will run anyways.

Not reversing the the terminals on a cb/sw coil for a - grd car will also work
but will result in a lower negative voltage output.

Pete, that 10% "loss" does not mean a 10% reduction in 'spark', engines of this era are fine with 15,000 volts or so and reversing a coil still provides more than that.

That said, I typically replace any/all coils, distributor parts (points, condenser etc) with solid state items, Pertronix, MSD et al do the job just fine and do not require constant monitoring and adjustment.

This http://www.lotus-cortina.com/electric/convert.htm may help the OP - never tried it myself but looks fine at first glance.