[DML] Re: Dwell meter setting?? More than you might want to know... ? too LONG?

[doctorDHD@xxxxxxx]

Guys, 
 
The  "dwell meter" was originally designed to measure the time, in  degrees 
that the "points" of the ignition system stayed (dwelled) closed.   That is 
dependant on how many cylinders the engine has.  If it was a 4  cylinder engine 
the maximum dwell would be 360 degrees divided by 4 (cylinders)  which is 90 
degrees, meaning that if the points were closed all the time (ie.  100 percent 
of the time) they would be closed for 90 degrees for each  cylinder.  Thus for 
a six cylinder car the maximum would be 360 divided by  6  = 60 degrees and 
for an eight cylinder car it would be 360 divided by 8  = 45 degrees.  Thus the 
dwell meter was used to measure and set the amount  of time, as a measure of 
the rotational angle of the distributors 360  degree rotation, that the points 
were close for each cylinder. Modern ignition  system don't use points any 
more to "trigger" the ignition coil. (Factoid,  the spark of the old ignition 
systems is generated when the points opened, not  closed, and this is when the 
primary current stops flowing through the coil,  allowing the magnetic field to 
collapse rapidly thus inducing the high voltage  in the secondary winding of 
the ignition coil...)
 
Back then, the dwell meter displayed the amount of time the point were  
closed versus the full cycle, be it 90, 60 or 45 degrees for a 4, 6, or 8  cylinder 
engine.  The dwell meter was in essence measuring what is know as  the duty 
cycle, which is the percentage of time current is flowing  during a cycle.  ie. 
if on 6 cylinder engine the dwell is 30 degrees then  the duty cycle is 30 
divided by 60 or 50%.  Back then they called it dwell  and measured it in 
degrees.
 
Getting back to the future, or should I say the present, our CIS fuel  
injection system relies on a signal from the Lambda ECU that tells the frequency  
valve to repeatedly open and close, which controls the pressures in the fuel  
distributor and in the end, the amount of fuel sprayed out of the fuel  
injectors.  This is how it keeps a constant air fuel ratio over the varied  amount of 
air being sucked into the engine at different loads and speeds.
 
The Lambda ECU is constantly sending a signal to the frequency valve, at 70  
cycles per second, per page D:04:08 in the DeLorean Workshop Manual.  In  
order to change the fuel pressures, the frequency valve depends on the duty  cycle 
of this signal powering it, meaning during what percentage of the signal  
cycle is current flowing to the frequency valve versus 100% of the signal  cycle. 
 If the duty cycle were 100% then the valve would be open all the  time, if 
the duty cycle was 50% then the valve would be open for 50% or half of  the 
time in each of the 70 cycles ever second...
 
So theoretically we need to know the duty cycle in percent not the  dwell in 
degrees, but since dwell meters are easy to come by and duty cycle  meters are 
not, they were nice enough to convert duty cycle to dwell degrees for  us.  
Note that the manual calls this duty cycle / dwell degrees a  pulse ratio, but 
states it as degrees in the specifications.
 
So how does one convert from dwell to duty cycle?  Again it is the  
percentage of the maximum dwell for a particular cylinder setting or scale on  the 
meter.  For instance, the manual states (please rise and turn to  page D:04:15) 
that in normal operation the "Pulse Ratio" should read 35 -  45 degrees.  Now 
what setting and scale should be used for that since 45  degrees on an 8 
cylinder setting would mean the valve is open 100 percent of the  time (45 / (360/8) 
= 1) where as 45 degrees on the 4 cylinder setting would mean  the valve would 
be open during only 50% of each cycle (45 / (360/4) = .5   THUS the setting 
and scale you use is critical.
 
What we really want to measure is duty cycle.  The convenient way is  to use 
a dwell meter set to the proper range.  The DMC Technical Manual  specifies 
setting the dwell meter to the 4 cylinder scale (I have not seen this  important 
piece of info in the workshop manual)  (...and of course you need  to read 
from the 4 cylinder scale as well.) On many dwell meters there is no 4  cylinder 
scale, therefore you need to set it to 8 cylinders and multiply your  reading 
by two.  Now in reality you could use the 6 cylinder setting and  scale but 
then the numbers in the manual would not be the correct ones to use,  so why 
bother...
 
Remember you will be setting the "dwell" to a range as it will be pulsating  
/ varying up and down a bit on a properly warmed up and operating engine.
 
Also very important is to recheck it after you reseal the access hole for  
the 3mm allen as this is a vacuum leak that can change the settings.
 
Does any of this make sense?  Is anyone out there still awake??
 
BTW, Don't do what I did the first time, which was to connect one wire of  my 
dwell meter to the "test point" and ground the other wire. (This is how  
you'd measure dwell on a car with points and is wrong, wrong, wrong for  us.)  The 
other wire actually connects to another terminal in the  diagnostic socket 
which goes to plus 12 volts and not ground...
 
When in doubt, RTFM (Read The F...ing Manual)
 
Dave Delman
D² & 6530
 
Message: 15        
Date: Sat, 4 Feb  2006 18:27:47 -0800
From: Ryan Wright  <ryanpwright@xxxxxxxxx>
Subject: Re: Dwell meter  setting??

Dave,

I'm following this procedure:  http://mysite.verizon.net/vze7u0gn/id6.html

I don't know why it says the  use the 4 cylinder setting... My meter
doesn't have such settings. Just a  "dwell" option and no choice of
cylinders.

-Ryan

On 2/4/06,  David Hudgins <painterdave72@xxxxxxxxx> wrote:
> Does anyone have a  procedure for setting the dwell meter for the co?? I 
have
> one but it  says to use the setting for a four cylinder??? Why is that??
>  thanks..Dave..and where do most of you all set your co at on the dwell  
meter..
> meaniing what does it read when u are  done???
>
>







Thank  you,

Dave



"Just Say  NO, to the COUCH POTATO!"®


To see how go to _www.tvpedaler.com_ (http://www.tvpedaler.com/)   



[Non-text portions of this message have been removed]






To address comments privately to the moderating team, please address:
moderators@xxxxxxxxxxx

For more info on the list, tech articles, cars for sale see www.dmcnews.com

To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/dmcnews/

<*> To unsubscribe from this group, send an email to:
    dmcnews-unsubscribe@xxxxxxxxxxxxxxx

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/