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MG TD TF 1500 - Dwell angle

I need advice on how to use a gunsons testune diagnostic multimeter. I have the instrument and leads, but no instructions. (Any copies for sale?)
I wish to check the dwell angle on my refurbished distributor.
It seems Gunsons no longer stock them. Can anyone help?

Has Oldham TD16143
HJ Oldham

Has - I'll check to see if I have the instructions around some place and make a copy for you if I do. Checking dwell with the unit is just a matter of connecting the red and black clips to power and ground (red to positive and black to negative) and the blue wire to the primary connection on the side of the distributor (I hook up to an unused spade lug on the terminal of the coil that is attached to the distributor). Turn the knob on the meter to dwell and start the engine (I have not tried reading the dwell while just cranking the engine, so I don't know if there is enough damping in the meter to be used this way). If your car is negative ground, the reading is directly from the scale. If you have a positive ground, you will need to take the complement of the angle displayed on the meter to get the correct dwell angle.
I hope that this helps somewhat. Good luck - Dave
David DuBois

Dave,

Interesting. I never could get close to the correct dwell on my TD using my meter. I finally gave up and set the points with a feeler guage. I did not know the meter would display the complement on a positive ground car. To get the accurate number do you just subtract from 100?

Thanks for clearing this up!

Evan
Evan Ford - TD 27621

Dave,

Thank you for your advice, it most certainly helps!
(Yes! + ground.) I will go through the sequence tomorrow and see how it works out.

Absolutly standard and original TD. I am forever trying to tune the engine, running gear and ancillaries to perfection - no smoke, no rattles, no leaks... It's very good but., Reality is owning a TD methinks!

Has
HJ Oldham

Evan,

As I remember my junior high math, a complement of an angle is found by subtracting the angle from 90. i.e. the complement of 50 is 40.

warmly,
dave
Dave Braun

No more calls please, we have a winner - Dave is correct, for the complement, subtract the angle read on the meter from 90. Of course, Dave is not as far away from his junior high math course as some of us are.

If we had all gone out and purchased the cheap, 2 wire dwell meters, we would not have this problem. Most, if not all of them read directly. On the plus side Has, the Gunson Autoranger multimeter is extremely accurate. I took mine into work before I retired and checket the calibration using a function generator set against a frequency counter and found that the Autoranger tachometer function was right exactly on. Cheers - Dave
David DuBois

Fellows. I hate to be the wet blanket here, but dwell angle is not related to 90 degrees. I'm not sure "compliment of the angle" part. On my meter, I just reverse the leads on positive ground.

"Dwell angle is a measure of the duration of time that the primary circuit of the ignition system is closed to energize the primary windings of the coil. It expressed (and measured) in degrees of rotation of the distributor rotor, hence the use of the term "angle".

In actual operation, as the distributor mechanism rotates, the points (or electronic module in electronic ignition systems) are closed for a certain number of degrees of rotation, and open between these points. Simply as a matter of interest, this means that the total number of degrees during which the points are closed, plus the total number of degrees that they are open, will equal 360 degrees."
Dallas Congleton

Thanks Dave & Dave! I didn't think about the dwell "angle." I assumed the meter scale was 0-100.

Dallas, I haven't tried reversing the power leads on my meter. I know a few of it's settings will not like that (especially voltage.) Also, since the engine is a 4 cylinder, wouldn't you divide the 360/4 = 90?

Evan Ford - TD 27621

Hi, Dallas - I think you're both saying the same thing, just from a different perspective. The "dwell angle" definition you give is absolutely right, but on a four cylinder car it has to be referenced on the basis of one cylinder, which consumes 90 degrees of the distributor's time (and 180 degrees of the crankshaf's time, but that's another matter).

So, the dwell on a four cylinder car can never exceed 90 degrees (or 60 degrees on a six cylinder, or 45 degrees on an eight, or 30 degrees on a twelve). On our fours, the dwell is usually about 57 to 60 degrees, and on an eight it runs about 27 to 30, or about two-thirds of the maximum. That's why most twelve cylinder cars have two distributors and two coils, as the 18 to 20 degrees of dwell available with a single dizzy in a twelve cylinder car doesn't allow enough time for maximum coil saturation at higher RPM's, so they split the duties between two full units.

The complement thing is just because of the way the three wire dwell meters (normally set up for negative ground systems) measure the dwell setting. Think of the meter reading as the percentage of the time the probe voltage is low (close to -12 Volts, and when the coil is charging) versus the maximum time it "could" be low (90 degrees on a four cylinder car). In a positive ground system, the standards are reversed, and you must subtract the "meter" reading from 90 degrees to get your "true" reading. Dave is right (as usual) that the old two-wire meters are a lot easier to use.

Hope I haven't confused the issue even more - - Alec

Alec Darnall

Thanks for a clear definition Alec. I missed the connection with a four cylinder only.I still think of this in degrees rotation,or portion of the arc of a circle.
The other analogy seemed to be for a right angle. My meter is a two wire Fluke and apparently more idiot proof.
I apologize for earlier broken sentences and typos. Typos are a given with my typing, but I also use a spell checker. I am having a little problem with cataracts at the moment, and the challenge of proofing the spell checker corrected text in the little box has bested me.
Dallas
Dallas Congleton

I have a copy of the testune manual which I can try and scan and email, or post.
Henry

Hi Dallas,

Dave DuBois said complement, and knowing that we were dealing with our little positive ground four cylinder engines, I accepted that definition of what needed to be done with the angle to get the true angle of dwell. My first real car out of college was a 1979 Mazda RX-7, and except for my MG I have not owned a car with points and a condenser since. I guess I've forgotten that one might need to check the dwell on any other type of engine! I found your explanation very enlightning. Thanks to you and Alec for explaining it so clearly.

warmly,
dave
Dave Braun

To all - I can't take credit for coming up with the "complement" idea on my own. The instruction booklet that came with the Gunson Autoranger multimeter has a neat conversion chart on the back page that gives actual values vs meter readings for RPM on 8, 6, 5, 3, 2 and 1 cylinder engines (the meter is designed for 4 cylinder engines) and for dwell on 4 cylinder, positive ground as well as on the above engines. Looking at the relationship between meter reading and actual dwell for psoitive ground, I recognized that it was the complement (I too remember junior high math in my more lucid moments), plus it said earlier in the booklet that the meter measured dwell angle from 0 to 90 degrees. Cheers - Dave
David DuBois

This thread was discussed between 02/11/2005 and 04/11/2005

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