Dwell meter question...

[TodFitch]

Does anyone here know how a dwell meter actually works?

 

Reason I ask is that I setup a distributor on the work bench using the method Don Coatney has photographically documented using an electric drill to spin the shaft. I had a 6v coil but used a 12v battery for the supply with no dropping/ballast resistor. (Figured that the over voltage wouldn't hurt things for the short period of time I was doing the adjustment.) Dwell showed steady at 36 degrees (specified value for that distributor).

 

Mounted the distributor in the car and in re-checking noticed that the dwell was steady at 40 degrees. Only difference I can think of is the voltage everything was running at. But I was under the impression that the dwell meter would be insensitive to that.

 

Did a quick Internet search to see how the internals are actually put together on it so I could see if it might be voltage sensitive but everything I find are on how to use it, not how it works. . . Must be pretty simple but I'd rather not dissect mine just to see how it works.

[Alshere59]

Sounds like it should have worked. A long shot, but how did you power your dwell meter as it may be voltage sensitive? 12 volts then 6 and how is it powered per the factory??? Or is that your question?

[Don Coatney]

This might or might not help but it sure makes for interesting reading.

 

http://books.google.com/books?id=rC0DAAAAMBAJ&pg=PA94&lpg=PA94&dq=how+to+build+a+dwell+meter&source=bl&ots=OB_k9qAWKV&sig=Cy0bc21PAIvFKlsD8AMQyfesvMo&hl=en&sa=X&ei=ubUPUa2ODJDo9gTZlIDgCg&ved=0CE0Q6AEwBQ#v=onepage&q=how%20to%20build%20a%20dwell%20meter&f=false

 

 

 

[Plymouthy Adams]

it reads the time the points are closed expressed in what we call the duty cycle

[TodFitch]

This might or might not help but it sure makes for interesting reading.

 

http://books.google.com/books?id=rC0DAAAAMBAJ&pg=PA94&lpg=PA94&dq=how+to+build+a+dwell+meter&source=bl&ots=OB_k9qAWKV&sig=Cy0bc21PAIvFKlsD8AMQyfesvMo&hl=en&sa=X&ei=ubUPUa2ODJDo9gTZlIDgCg&ved=0CE0Q6AEwBQ#v=onepage&q=how%20to%20build%20a%20dwell%20meter&f=false

 

Thanks! That has a circuit diagram which will give me something to think about.

 

Seems like the fact I set the points and tested with 12v on the work bench and then re-checked on the running engine with 6v should not have made any difference.

[1941Rick]

This might or might not help but it sure makes for interesting reading.

 

http://books.google.com/books?id=rC0DAAAAMBAJ&pg=PA94&lpg=PA94&dq=how+to+build+a+dwell+meter&source=bl&ots=OB_k9qAWKV&sig=Cy0bc21PAIvFKlsD8AMQyfesvMo&hl=en&sa=X&ei=ubUPUa2ODJDo9gTZlIDgCg&ved=0CE0Q6AEwBQ#v=onepage&q=how%20to%20build%20a%20dwell%20meter&f=false

This whole book has some interesting reading......

[ptwothree]

The difference in the two dwell readings might be from a worn shaft bushing.  When tested with a drill motor, there is little or no side loading of the dizzy shaft.  When the dizzy is installed, side loading occures when the engine is running.  A worn bushing might account for the difference in the two readings.

 

You can check this by reving the engine with the dwell meter hooked up. A variance of more than two or so degrees would indicate worn shaft bushings.

 

Most dwell meters I've used aren't voltage sensitive so 6 or 12 volt should work.

[TodFitch]

The difference in the two dwell readings might be from a worn shaft bushing.  When tested with a drill motor, there is little or no side loading of the dizzy shaft.  When the dizzy is installed, side loading occures when the engine is running.  A worn bushing might account for the difference in the two readings.

 

You can check this by reving the engine with the dwell meter hooked up. A variance of more than two or so degrees would indicate worn shaft bushings.

 

Most dwell meters I've used aren't voltage sensitive so 6 or 12 volt should work.

I wondered about that. The drill I used was low speed so not much information there. But on the engine the dwell reading was rock solid from idle up through 2500 RPM (RPM measure from the same tach/dwell meter).

 

From schematic for a dwell meter on the article linked to by Don that the dwell reading could be affected by supply voltage. On that schematic the supply was batteries in the tester itself. But my little tach/dwell meter is powered by the wire from the points. If they use a similar circuit and supply the "battery" for it from a rectifier and regulator from the coil wire then I guess it is possible they cheaped out on the circuit and it might have some voltage sensitivity. Since all points type ignition systems I am aware of run on 6v (via a ballast resistor on a 12v system), with the possible exception of when the starter is running, it could be that they did not actually design it for a full 12v on the 6v coil.