Tach/Dwell meter

[jackathy]

The NorthEast has magnificent driving weather this weekend. So, in the course of driving around I picked up a tach-dwell points tester at an estate sale. I can't find instructions anywhere on its use. All I find is that is useful in timing. There is a switch on it that moves between 'tach' and 'PT. Res.'. It seems to measure RPM and Dwell degrees for an 8 and 6 cyl engine and the tag stating that for 4 cyl double the 8 cyl reading. It has a positive and a negative clip, what do I clip them to?

 

[Paul W.]

Jack, here is an excerpt from a very old Dwell-Tach tester instruction book I own:

1. For negative ground vehicles, connect RED lead to the distributor terminal of the ignition coil and the BLACK lead to any good engine ground.

2. For positive ground vehicles, connect the BLACK lead to the distributor teminal of the ignition coil and the RED lead to any good engine ground.

On my TR4, I was surprised to find out that my actual idle according to the meter was 1300-1500. My stock cable driven tach showed the idle to be about 1000. I guess the tach cable stretches over time, causing inaccurate readings. I will have to rebush/rebuild the strombergs and see if I can get that idle down a bit.

Good luck

Paul W.
Torrance, CA

 

[aeronca65t]

jk:

You are right about the weather in the NE....I've even been driving my StepVan with the doors slid open....good flying weather too.

On the meter, hook it up as Paul says and switch it to "Pts". Start the car with the meter set at 8 cylinder. You should get a reading close to "30" (which must be doubled to read "60"....."60" is the correct points dwell for you car). If you do not get the correct reading, you should re-gap the distributor points and re-check the dwell. If you set the points gap at 0.016" (when they are rotated to be fully open), you will be pretty close, but the dwell meter is more accurate.

 

[jackathy]

Thanks both for the quick replies. I finally got around to using it today. The needle on the dwell reading fluctuated very rapidly, but always between 0 and 10! No time to do anything about it. I had replaced the points in the spring but never quite figured out how to set the gap correctly. I was so thankful that it ran, I put that job off to another time. The engine only exhibits problems when accelerating up to 4000 rpm shifting through the gears. Taken slowly ,not exceeding 3000 or so, everything runs quite smooth.

I returned to the estate sale today and got a Timing light, enough sockets to handle any job on any thing. and 5 tool chests! The guy who died worked on Corvettes and he had more tools than any garage I've ever been in. The light has the positive and negative clips and one bare wire. How do I use this?

 

[willy_1959]

Hey I am totaly, totaly new to the idea of "dwell" setting, I am just exploring learning that, so much so that i went to a tech session today at the chicago british car union show, and they "just told" me that the dwell for a four cyl should be 90 ie 1/4 (4cyl) of a circle or 360. and that on a 6cyl it would be one sixth or 60. and on 8cyl it would be 45.

Let me know if i just learned the wrong thing, or I misinterpreted what I heard??

 

[Dave Russell]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by willy_1959:
"just told" me that the dwell for a four cyl should be 90 ie 1/4 (4cyl) of a circle or 360. and that on a 6cyl it would be one sixth or 60. and on 8cyl it would be 45.
Let me know if i just learned the wrong thing, or I misinterpreted what I heard??<hr></blockquote>
Willy,

This may be more than you wanted to know.

The point closed time is the dwell time. In the case of a four cylinder with four cam lobes, there is a total of 90 degrees available for each lobe. Four times 90 equals 360 or one revolution. The 90 degrees is split up into 60 degrees point closed time (dwell) & 30 degrees open time for each lobe.

For a six cylinder cam there are only 60 degrees per lobe available & it is split into 35 degrees closed (dwell) & 25 degrees open.

An eight cylinder would have only 45 degrees total available & so on.

The dwell (amount of time that the points are closed) controls how much time is available to charge the coil. The longer the time the more charge & hotter spark. The spark fires when the points open & allow the built up magnetic force in the coil to collapse & generate a spark in the coil secondary.

As the rpm increases the dwell in degrees presents a shorter coil build up time & eventually there is not time enough to get the coil charged in time for its next discharge. The long dwell time of a four cylinder will provide adequate spark voltage up to eight or nine thousand rpm. At the other end of the spectrum, an eight cylinder engine will only have enough dwell time to charge the coil to maybe 5000 rpm.

This defficiency (short dwell) in eight cylinders used to be overcome by using dual points which were arranged in a semi overlapping configuration to provide more dwell time. One set of points opens the circuit & the other set closes the circuit. Some 12 cylinder engines used dual distributors to solve this problem.

If the cam were designed to give long closed times with very short open times the open/close cycle would require a very quick opening rate & due to the inertia of the movable point The point would not follow the cam.(point float) so there is a limit to how much dwell you can get with point ignition.

Many modern engines now use one coil per cylinder & electronically switch the low voltage to the coils. This eliminates all concerns of adequate dwell time, & rotor, cap & wire problems.

A point setting of .015 will usually do the job just fine. As the point rubbing block wears (points close up) the ignition timing will be retarded.

When the coil primary is charged through the closed points, as the points open the coil discharge current has to go somewhere. All it can do is arc across the points which rapidly burns the points. The condenser provides an alternate path fror the discharge current (bypasses the points). It also has a secondary function of electrically tuning the coil by allowing more rapid current discharge than just arcing across the points would.
D

[ 09-07-2003: Message edited by: Dave Russell ]</p>

 

[willy_1959]

thanks Dave, that makes more sense,

The guys putting on the seminar knew their stuff, so I just misinterpreted what I heard. I think I will re read what you wrote a few more times

maybe tommorow lol