Dwell Meter Connections for a 6V Positive Ground System

[Jocko_51_B3B]

After doing some looking into it, there seems to be some doubt and confusion about how to hook up a dwell meter to a 6V positive ground system. That was true for me too.

 

I had bought a used dwell meter on eBay a long time ago but only last week decided to try it out on my B3B. I searched around for how to hook up the red and black leads to a 6 Volt positive ground electrical system but I kept reading answers like "just reverse the leads" without understanding anything about the connection or why the leads need to be reversed. So I tried to figure it out based on common sense (which isn't always trustworthy I know) and then hoped to find explicit confirmation somewhere as to whether I was right or wrong. 

 

After staring at the circuit, drawing pictures, and scratching my head for a while I came up with this:

RED lead must go to BATTERY POSITIVE

BLACK lead must go to POSITIVE TERMINAL OF THE COIL (the coil terminal that has the wire connected to the distributor.)

 

If I'm wrong can someone please correct me. But earlier today I found a comment by busycoupe in the following p15-d24 thread that backs me up:

 

 

If you don't care how something works but only care about making something work by following step by step instructions, then the following might not interest you. For me,  I like knowing the "why" (if possible) not just the "how to". True understanding goes a long way IMO. Let me know if my understanding below is flawed. That's how we all learn.

 

My logic was that the RED lead must go to the most positive point in the circuit (which is BATTERY POSITIVE) just because RED usually means positive. The BLACK lead must go to the most negative possible point that can provide an ON-OFF signal for the meter to interpret as dwell which must be the POSITIVE terminal of the coil because that is where the coil connects to the points. It seems ironic that the POSITIVE terminal of the coil actually gets hit with a large NEGATIVE spike when the points open but that's what I think happens. As I see it, when the points close electron flow goes out from the (-) terminal of the battery into the (-) terminal of the coil, through the coil, out the (+) terminal of the coil, through the closed points, and finally to positive ground back to battery positive. But when the points open, electron flow through the coil cannot stop immediately because the magnetic field in the coil primary takes time to collapse. So for an instant the electrons must keep flowing out of the (+) coil terminal and continue piling onto the condenser causing it to spike highly negative until the points close again. Somewhere I read that in later 12V systems the condenser voltage can go as high as 600 Volts. Since this is electron flow, the condenser voltage must go hundreds of volts negative.

 

 

 

 

 

Edited May 2, 2022 by Jocko_51_B3B

[tom'sB2B]

It will be interesting to hear from someone with knowledge. I've always hooked the leads up to the coil.

[Jocko_51_B3B]

Tom,

Out of interest and to further my understanding, how exactly did you connect the RED and BLACK leads to the coil?

Jocko

Edited May 2, 2022 by Jocko_51_B3B

[Merle Coggins]

You are correct. With a Positive Ground system the Positive (Red) lead of the meter would go to the ground side and the Negative (Black) lead would connect to the points wire from the coil. You're not really "reversing" the leads as Red is still going to Positive and Black to Negative. Some people refer to this as "reversing" the leads as they are used to putting the Red on the coil-points wire and Black to ground, which is correct on a Negative Ground system. Either way you connect Red to the Positive side and Black to the Negative side, so you are not "reversing" the leads.  

[JBNeal]

 

This is very similar to the diagram that accompanied the 70s vintage Hawk Dwell-Tach that Dad had in his old toolbox...whatever paper those instructions were printed on, the silverfish just chewed to pieces, but this is a generic version of that diagram.  Having used it several times over the years on 12V neg.grd and 6V pos.grd, these test lead directions are accurate.  This diagram assumes the user understands the complete function of each component, so it just shows basic test lead connections for modern negative ground electrical systems.  The note to reverse the connections for positive ground electrical systems would mean that the tester black lead would be on the coil negative post, and the tester red lead would be on chassis ground.

 

My understanding of what is going on here is that the dwell meter is measuring a signal each time the ignition coil discharges.  The coil + post is battery voltage from the keyswitch, and this voltage feeds the primary and secondary windings for what is a step-up transformer that takes battery voltage and multiplies it greatly to be distributed to each spark plug.  The primary winding exits the coil at the - post and heads to the distributor condensor and breaker points.  When those points open, the magnetic field of the primary winding collapses, discharging the secondary windings through the high tension lead to the distributor cap and rotor then to the indexed spark plug wire.  The - post is used to monitor the current change when the breaker points open because typically this is easier to physically access than on the distributor terminal as shown in the diagram as the alternate connection.  However, if the distributor is installed on a testing stand such as shown in the linked video, then the dwell lead connection is made at the alternate connection.

 

So why do they say reverse the connections for positive ground systems?  When ground polarity is changed, so is the direction of current and ultimately electron flow.  The dwell meter is made so that it only registers electron flow in one direction, so negative ground systems have current flowing from the battery positive post into the red test lead and out the black test lead to ground to register needle movement on the graduated scale.  With positive ground systems, the current flows from ground to the battery negative post, so the dwell meter leads are reversed since current flow is reverse of negative ground systems.

 

additional information - testing the distributor 

additional information - ignition system servicing

[wagoneer]

Here is a short video I did showing the wiring connection