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MG TD TF 1500 - Moss Tech Tip -- Dwell Angle

Moss states in a tech tip that on a positive earth car the dwell angle can not be checked with a timing light that has the dwell checking feature because of the reverse polarity.The dwell wire hooks to the point side of the coil. I tried it on my TF and got erroneous readings,like 25 degrees and should be around 60, sometimes none at all. How can we check the dwell angle this being the case? What test instrument will work on our cars? PJ
PJ Jennings

P J

I have the Actron CP7677 digital multimeter. About $40 on Amazon. It does all the usual multimeter things but also has an RPM and Dwell setting. It also has little clips that slip on to the probes after pulling off little plastic sleves.

If you are looking for something inexpensive I would recommend this one. It works on my positive ground TD for the dwell and RPM modes. Just reverse the leads. It does not have a timing function.

Stay away from the Blue Driver brand on Amazon. I had 2 bad units in a row and gave up and got the Actron.

Stuart



S Grimm

Thanks Stuart that will work. I'll pick one up on the net. PJ
PJ Jennings

Now I hear a bell ringing...

I "think" on a positive ground car that you get to see the "undwell" angle ......

So instead of 60+/- 3, you would be looking at 30+/- 3 deg.

Anybody else heard this?

It makes sense as you are looking at the period that the voltage is zero as the points are closed, in the positive ground car the voltage would be "positive", so the points open period would appear as negative.

Now I have myself confused.

Thoughts?

Peter

PS: Just found this!

https://www.mgexp.com/forum/t-series-and-prewar-forum.46/measuring-dwell-angle-with-positive-ground.3733797/#msg-3733797
P G Gilvarry

Now I hear a bell ringing...

I "think" on a positive ground car that you get to see the "undwell" angle ......

So instead of 60+/- 3, you would be looking at 30+/- 3 deg.

Anybody else heard this?

It makes sense as you are looking at the period that the voltage is zero as the points are closed, in the positive ground car the voltage would be "positive", so the points open period would appear as negative.

Now I have myself confused.

Thoughts?

Peter
P G Gilvarry

I have the same meter, Stuart, and like you I'm pleased with it.
Kevin McLemore

P G.

According to Actron the device will read correctly for positive ground vehicles, just hook it up backwards. Really all it is doing is measuring the time the breaker points are closed so it doesn't matter which way the current is flowing.

Stuart
S Grimm

I would be interested to see if that worked, I looked at the instructions and didn't see anything on dwell, but it appears to be obvious.

It is plastic so connection would not be an issue.

Will await the results.

Thanks,

Peter
P G Gilvarry

Peter,
It could happen with a meter that senses only the rising and falling edges of the signal.
But I guess that most of them are level sensitive.
So the negative ground meter is sensing for 0V and + 12 V (with many transiant due to LC circuit)while the positive ground ignition delivers 0 and -12V (+ many transiant as well)

Laurent.

LC Laurent31

Well if I put the Crane electronic system in the car, dwell is no longer an issue, but I need a new meter anyway, so the Actron is on the list. PJ
PJ Jennings

I figured out what it does, just trouble explaining it.

It has something to do with the direction of change it is looking for and the duration of that change.

Negative ground car the voltage goes negative by 12V when the points close.

On a positive ground car the voltage goes Positive by 12V when the points close.

If the meter is sending the change and duration of the negative trend it will be seeing the reverse of the points closure on a positive car.

That is the other part of 90deg on a 40 cylinder car.

If you look at the link I provided the guy messed with his points and verified the theory. Also confirmed by an engine guy from experience.

Peter
P G Gilvarry

I got fed up with a digital multimeter measuring dwell bouncing readings up and down and trying to work out the mean.
So I looked on eBay for an analogue version and got an old school Lumen analogue dwell and revs tester for 8.50 UKP.
When I asked the guy if it worked, his reply was, 'it did when my grandfather last used it ten years ago'. I nearly cried.
And it does work perfectly.

David

David Wardell

I am using one I bought from Halfords over 30 years ago along with a timing light.

Kept them in an old briefcase all that time, they are in great shape.

Have a couple of Craftsman that I picked up on Ebay, all work.

All are analog.

Peter
P G Gilvarry

This thread was discussed between 18/09/2019 and 23/09/2019

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