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Measuring dwell - having difficulties :-(
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Dangermouse
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PostPosted: Sun Sep 17, 2006 4:25 pm    Post subject: Measuring dwell - having difficulties :-( Reply with quote

I have an electronic, automotive multimeter, like this one...

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I want to use it to measure my dwell angle, so I hook up the red lead to the #1, negative post on the coil, ground the black lead and start the car. The meter just seems to go nuts, screen flashes on and off, displays gibberish, seems to stablilise for a few seconds and then blanks again. Not at all happy.

I don't think the meter's faulty as the other functions seem to work ok.

But - the coil is 6 volt; is this causing the problem?
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glutamodo Premium Member
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PostPosted: Sun Sep 17, 2006 4:30 pm    Post subject: Reply with quote

I've got one of those meters too. Never have been happy with its dwell (or tachometer) readings. I found that keeping the probe wires as far away from the coil and the spark plug wires helped a little but, but I don't even bother trying to use it for tach/dwell anymore, I've got a couple of other digital meters that work OK, plus an old analog meter that I think works the best of them all.

Oh, and the 6V part shouldn't really make a difference to a self-powered or unpowered meter.
-Andy
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atye
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PostPosted: Mon Sep 18, 2006 6:40 am    Post subject: Reply with quote

I have one of the old analog type dwell meters. Found it in a buddy's tool box a few years ago, he had no idea what it even was. I've used it many, many times and the car runs noticibly different when the points were set by the dwell meter vs a feeler gauge. Not sure if a DMM would work right on 6v or not, good question there.
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PostPosted: Mon Sep 18, 2006 7:03 am    Post subject: Reply with quote

I have and old analog meter with the needle type. It is for 6 cyl and 8cyl and says double the reading for 4 cyl. it works pretty good for my aplication.
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PostPosted: Mon Sep 18, 2006 7:48 pm    Post subject: Reply with quote

atye wrote:
I... runs noticibly different when the points were set by the dwell meter vs a feeler gauge ...


Which was better for you? I have been setting mine by dwell lately, but really do not know.

Thanks,
-Rick
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PostPosted: Mon Sep 18, 2006 9:15 pm    Post subject: Reply with quote

Quote:
Not sure if a DMM would work right on 6v or not, good question there.


Oh yeah, if its some weird problem that noone's ever encountered before; that'd be me all right Rolling Eyes

Anyone wanna buy a multimeter? One careful owner, used once...
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atye
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PostPosted: Tue Sep 19, 2006 5:53 am    Post subject: Reply with quote

rsperko wrote:
atye wrote:
I... runs noticibly different when the points were set by the dwell meter vs a feeler gauge ...


Which was better for you? I have been setting mine by dwell lately, but really do not know.

Thanks,
-Rick


I noticed that the car ran LOTS better after setting the points with a dwell meter than a feeler gauge.

The dwell (to my understanding) in simple terms, is a measurement of the amount of spark it takes to jump the air gap of the open points. If you use a feeler gauge on worn points, there may be small ridges that will affect the measurement, but the dwell meter will not have the same problem because it does not physically touch the worn surfaces.

I would put the meter on the coil, remove the cap, and have someone spin the motor over while I tweaked the adjustment on the points.
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doug
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PostPosted: Tue Sep 19, 2006 8:42 am    Post subject: dwell Reply with quote

dwell or dwell angle is how many degrees your points are closed in relation to the rotation your rotor. This determine how much time your coil has to energize.
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atye
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PostPosted: Tue Sep 19, 2006 9:04 am    Post subject: Reply with quote

Embarassed
I was close, right?
LOL

I'd swear I read that somewhere (about the air gap)... maybe that was too many brain cells ago. Rolling Eyes

Regardless, dwell is the way to go!
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Buggeroff
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PostPosted: Tue Sep 19, 2006 10:54 am    Post subject: Reply with quote

Why don't you just use a six volt bulb and a couple of wires across the coil feed and ground ?
Set your points, set your timing marks, switch on ignition, rotate distributor untill the bulb lights up and voila, dwell is done.

BTW, that big black clamp looks like it is for measuring current draw.
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PostPosted: Tue Sep 19, 2006 11:48 am    Post subject: Reply with quote

crtclff wrote:
Why don't you just use a six volt bulb and a couple of wires across the coil feed and ground ?
Set your points, set your timing marks, switch on ignition, rotate distributor untill the bulb lights up and voila, dwell is done.


Because that is 'Static Timing' and not checking Dwell Angle?
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PostPosted: Tue Sep 19, 2006 11:59 am    Post subject: Reply with quote

You can mark your pulley wither with a protractor or by checking it after a meter has set it dead on, then set dwell statically like you can do timing.

http://www.desperado.scvnet.com/~philh/images/dwell.pdf
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glutamodo Premium Member
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PostPosted: Tue Sep 19, 2006 1:17 pm    Post subject: Reply with quote

Maybe it is a 6V problem - although I've had some problems in the past checking the dwell with my meter, I just threw my meter on my 12V cars and it was working fine. (I do know the inductive tachometer sensor is useless on my cars though, no matter what wire or where I move the adjuster thingy on the inductive pickup)

Now that I think back again, I'm not sure if I ever used this particular meter on a 6V car before, I got it only a few months before I quit being a mechanic and I may not have got the chance to then. Could be that 6V isn't strong enough for this meter to get a good enough signal to read reliably.

NOTE: I just realized there's something else you might be able to try with this meter: Dwell is a duty cycle measurement, transformed into the traditional "dwell angle" range depending on how many cylinders you have. Now, if your meter is the same type as mine, there will be a separate duty cycle setting that might work for you. On mine, it's 3 clicks to the left of OFF, and says %duty. This gives duty cycle based on a 100% scale. Use the same connections, the positive/red lead to +1 on the coil, negative/black lead to a good ground on the car, (I often clip mine to my rear bumper and keep that wire out of the engine compartment) And if yours will read a steady duty cycle, then all you have to do is multiply your reading by 9/10 (0.9) to get the dwell. Or, go in reverse, and make it even simpler, VW speciifies dwell to be 44 to 50 degrees, and to translate that 44-50 degree range for a duty cycle, you'd multiply by 10/9ths (1.11) and get 48.8 to 55.5 degrees, aiming for the middle of that range. I'd try that and see if it works.

-Andy
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Dangermouse
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PostPosted: Sun Sep 24, 2006 4:46 pm    Post subject: Reply with quote

Thanks for the suggestions folks, just for the record, my digital multimeter couldn't read duty cycle either - I guess they're just calibrated to measure 12V outputs.

I'll have a look at setting dwell statically as KTPhil suggests, or try and find myself an OLD meter.

(Keywords for future searchers: 6 volt, 6V, dwell, digital multimeter, crappy).
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PostPosted: Mon Sep 25, 2006 3:27 am    Post subject: Reply with quote

I tried mine on the duty cylce measurement the other day and it worked, but it's 12V, so maybe 6V isn't strong enough after all. At least it was worth a try.

FYI - whenever I've set my dwell to the 47 degrees the factory recommends, and have double checked it with a feeler gauge, I've found the gap to be .017-.018, that's within the .016-.020 range the Bentley states too.
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Dangermouse
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PostPosted: Thu Oct 12, 2006 3:21 pm    Post subject: Reply with quote

So, I acquired an old, analogue-type dwell meter and hooked that up and it seemed to function fine so I guess at this stage I'm prepared to resign myself to the fact that 6 volt systems bring some 6 volt problems. I still wouldn't be tempted to change it over though, I like it running 6 volts.

Image may have been reduced in size. Click image to view fullscreen.


BUT - in order to get a dwell reading of approx. 50 degrees (25 degrees on an 8-cylinder scale), I had to gap my points to .035 rather than .016; should I be concerned about this and what does it say about the condition of my distributor?

After I did this the engine wasn't running too well and I returned the point gap to .016 (but I have a nagging suspicion that that might be because I timed the engine on cylinder #3 instead of #1 'cos I forgot to check the position of the rotor . I'll have to go back and repeat the exercise).
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KTPhil Premium Member
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PostPosted: Thu Oct 12, 2006 3:41 pm    Post subject: Reply with quote

Something ain't right, and it seems everything is off by a factor of 2...
6V vs. 12V
0.016' vs. 0.035'
50 degrees vs. 25 degrees (normal for the 8 cyl scale)

Do yourself a favor, use my static method (only requires a light bulb) and then see what the dwell meter reads.
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PostPosted: Wed Jan 27, 2021 10:00 pm    Post subject: Re: Measuring dwell - having difficulties :-( Reply with quote

I learned something today that I wish I had known when I first posted this thread and thought I'd share for those as quick on the uptake as myself Laughing

I know, from searching, that many folks have struggled with getting an accurate dwell reading on 6V cars running points. Feeler gauges will get you close but correctly points gap should be set using a dwell meter. Old, analog equipment is exactly that and fancy digital meters often don't appear to work correctly with the 6V signal, or so I long thought...

Maybe everyone already knows this and I just didn't pick up on it correctly.

I have a nice digital meter that I have used for tach, timing advance, voltage etc, everything except dwell reading as when I tried it it just bounced numbers around which didn't make any sense and I put it down to 6V oddness as I have before. On my 6V car, I have powered it directly from a 12V battery source and it works fine like that for everything, except dwell...

Image may have been reduced in size. Click image to view fullscreen.


BUT, if like I did today, I add an additional ground wire from the 12V battery source negative terminal to a ground point on the car itself - it works perfectly!

Who knew! Maybe everybody except me Laughing
Anyway, I'm posting this because it would have been useful information to have at the time.
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Wayne S. Johnson
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PostPosted: Wed Jan 27, 2021 11:40 pm    Post subject: Reply with quote

Print a pic of a degree pulley and taped it to the pulley.
Connect a small lamp from the points to ground and turn on ignition sw.
Rotate the pulley, when the light goes off record the position, when the light goes on record the position.
Subtract the two numbers (Correction: "the values shall be divided by 2 to have the real dwell angle"). Thanks Herby 1200.

Useful to test the accuracy of your dwell meter.


Image may have been reduced in size. Click image to view fullscreen.

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Last edited by Wayne S. Johnson on Sun Jan 31, 2021 11:41 pm; edited 4 times in total
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PostPosted: Thu Jan 28, 2021 4:16 am    Post subject: Reply with quote

glutamodo wrote:

NOTE: I just realized there's something else you might be able to try with this meter: Dwell is a duty cycle measurement, transformed into the traditional "dwell angle" range depending on how many cylinders you have. Now, if your meter is the same type as mine, there will be a separate duty cycle setting that might work for you. On mine, it's 3 clicks to the left of OFF, and says %duty. This gives duty cycle based on a 100% scale. Use the same connections, the positive/red lead to +1 on the coil, negative/black lead to a good ground on the car, (I often clip mine to my rear bumper and keep that wire out of the engine compartment) And if yours will read a steady duty cycle, then all you have to do is multiply your reading by 9/10 (0.9) to get the dwell. Or, go in reverse, and make it even simpler, VW speciifies dwell to be 44 to 50 degrees, and to translate that 44-50 degree range for a duty cycle, you'd multiply by 10/9ths (1.11) and get 48.8 to 55.5 degrees, aiming for the middle of that range. I'd try that and see if it works.

-Andy


This is correct, let me explain why.
Dwell angle is related to distributor rotation (not the crankshaft rotation!).
Distributor, in a complete revolution (360°), has to spark 4 cylinders, so it has 90° available for each cylinder.

So the idea is, to have the coil energized for about one half the time available, to set dwell at the half of degree available, i.e. 90°/2=45°.

If it was an 8cyl engine, the degrees available for each cylinder are 360/8= 45° so the dwell for 8cyl engine is about 20-30°.

The dutycycle is a percentage of a square wave: how many time it is "on" respect to the full period.

So 100% duty cycle corresponds to 90° dwell in a 4 cyl engine. And 50% duty cycle corresponds to 45° dwell. This is why 9/10 is the factor between dwell and duty cycle.

Pay attention that duty cycle is the ON time of the square wave, so the multimetr shall be connected on +15 and 1 of the coil.

If connected on ground and the coil it measures ON when the coil is OFF so you have the opposite of the dwell angle.

A dwell meter is good to check the variable-dwell distrubutors (accuspark, etc.) to chek how the dwell angle is adapted to the engine speed.
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