Category: ignition coil

Tech Tip: Ignition Coil Polarity Check on Early Model Vehicles – Randy Rundle @Underhood Service

Tech Tip: Ignition Coil Polarity Check on Early Model Vehicles – Randy Rundle @Underhood Service


Rundle’s Rules: Ignition Coil Polarity Check

Prior to 1956, the year when most all of the domestic auto manufacturers upgraded to 12-volt electrical systems using standardized negative ground designs, some of the electrical systems on cars and trucks were positive ground and some were negative ground.

Ford used positive ground, while General Motors used both.

Chevy trucks were negative ground, while GMC trucks used positive ground. There was and still is a lot of confusion concerning the polarity of electrical systems and how to properly connect the ignition coils, as well as the battery.

On classic and antique vehicles, you can test for correct polarity of the ignition coil by using a voltmeter.

Connect the negative lead to the (-) negative terminal and the positive lead to the engine block.
Set the meter on the highest volt range (these connections are the same whether you have a positive ground or negative ground electrical system).

The secondary winding’s polarity, which you are testing, is determined by the combined hookup of the battery and primary windings.

Crank the engine over (do not start it) and the needle of the voltmeter should show an upward swing to the plus or positive side (don’t worry about taking a reading).

If the needle swings down to the negative side and gives a negative reading, your coil is hooked up backward. To correct the polarity, simply reverse the coil primary leads.

A coil with reversed polarity will have about a 20% lower output, which may not show up at idle and low rpms, but can cause an engine to miss or stumble under load and at higher engine rpms.

This is why a technician who changes the points, condenser and other electrical components will still detect an engine miss.

The lesson here is, too often, it is “assumed” that the wiring is correct.

— Randy Rundle

Randy Rundle is the owner of Fifth Avenue Antique Auto Parts, Clay Center, KS, and services antique and classic vehicles. An author of six automotive technical books, Rundle has spent 20-plus years solving electrical, cooling and fuel-related problems on all types of antique and classic vehicles.

2000 Chevrolet S10 Xtreme Ignition Coil Change Part 1


If you’ve been reading the blog you’ll know that the S10 has been having some breakdown issues which appear to have been corrected by the replacement of the ignition coil. Now the coil I have is an aftermarket unit from NAPA.(kindly bought in the States by a relative :))

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To remove the coil you need to remove the throttle cable bracket and then remove the bracket that contains the coil and the ignition module. As you can see the original coil is riveted to the bracket. The NAPA coil comes with a little fitting kit. To remove the original coil I think the rivets will need to be ground or drilled out.

More next time!

Triumph Herald Positive to Negative Earth Conversion (originally published in 2008)


This is how I converted my Herald to negative earth.

1. Disconnect the battery

2. Remove the battery from the tray
3. Refit the battery around the “other” way

4. Connect the terminals in the correct
configuration for negative earth

5. Swop the LT connections on the ignition coil
to also reflect negative earth configuration
(you may have starting problems if you don’t)

Please go to this link where there is a basic
but very clever way of testing if the polarity
is correct. here is the test

6. Remove the large terminal from the dynamo

7. Take a piece of wire connected to the live
terminal of the battery and “flash” it on the
dynamo terminal a couple of times(it will spark)
The dynamo should now be repolarised
Refit the large terminal to the dynamo

8. Start the engine and check the charge rate
of the dynamo

In my case the dynamo charged prefectly first time, you may however need to flash it again if the charge rate is not sufficient.

My fuel gauge and wiper motor appear to work OK, I will therefore be leaving them alone!

The heater blower also appears OK, however I will swop the wiring over in the near future to see if there is any difference in the operation pos to neg.

Please don’t take this method as the same in all cases, follow these steps at your own risk 🙂

Coming soon “The Alternator”

Herald Update (originally published in 2008)


I have purchased an alternator for the old girl, and will shortly be fitting it and converting to negative earth at the same time.

Got a very good deal on the alternator, eBay of course!

I’ll publish the trials and tribulations of the conversion here in the future.

It looks as if only the heater blower, tank sender, battery and ignition coil will need reversing during the process, I think???