Ignition warning, is it true or false? - Off-Road Forums & Discussion Groups
Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

 
LinkBack Thread Tools Display Modes
post #1 of (permalink) Old 12-06-2005, 05:09 AM Thread Starter
 
Join Date: Feb 2004
Posts: 946
Thanks: 0
Thanked 0 Times in 0 Posts
 
Ignition warning, is it true or false?

I just recalled interesting artical warning about leaving a spark plug wire off.

I was always courious as to why many FSM states that you should never pull a spark plug wire off an engine to test if the plug is not firing. I always though this was because your dumping raw fuel into the catalytic converter and it would overheat. I always though "well heck, if it's not firing anyway you already have a problem".

Appearently what this person says happens is that since the current is not discharging thru the spark plug it finds some other path to ground. Usually inside the distributor. It's worse in some positions than others because of the layout of the distributor. They actually list the positions prone to problems.

Maybe this leads to carbon tracking or burn thru of the rotor. Possibly damaging the electronics if the spark find a path thru the controller.?

I never seen this type of problem before but there have been many mysterious ignition problems that upon reflection may be caused by such conditions.

What do you guys think? Is there a greater tendency for the spark to discharge inside the distributor if a spark plug wire is left disconnected ??

I'll see if I can find the artical.
John_Strenk is offline  
Sponsored Links
Advertisement
 
post #2 of (permalink) Old 12-06-2005, 05:26 AM
Addict
 
84_CJ7's Avatar
 
Join Date: Jun 2003
Location: Valdosta, Georgia
Posts: 413
Thanks: 3
Thanked 0 Times in 0 Posts
 
Re: Ignition warning, is it true or false?

When I was converting to the Stealth HEI Module (which many of you helped with) I read that pulling the plug wire would cause the very problem you described. I came across this tool -

It's an HEI ignition tester.

Accurately check for spark in ignition systems without removing spark plug from engine. Testers verify that energy is sufficient for spark plug to fire. Attach spark plug wire to tester end. Clamp tester to engine as ground. Crank engine and check for spark.
84_CJ7 is offline  
post #3 of (permalink) Old 12-06-2005, 06:33 AM Thread Starter
 
Join Date: Feb 2004
Posts: 946
Thanks: 0
Thanked 0 Times in 0 Posts
 
Re: Ignition warning, is it true or false?

[ QUOTE ]
..It's an HEI ignition tester.

Accurately check for spark in ignition systems without removing spark plug from engine. Testers verify that energy is sufficient for spark plug to fire. Attach spark plug wire to tester end. Clamp tester to engine as ground. Crank engine and check for spark.

[/ QUOTE ]

HEI ignition tester? [img]/ubbthreads/images/graemlins/smile.gif[/img] I wonder what the difference is ?? It's just like the one I got to test the spark on my lawn motor;
John_Strenk is offline  
Sponsored Links
Advertisement
 
post #4 of (permalink) Old 12-06-2005, 06:51 AM
Way Outta Control
 
Join Date: Sep 1999
Location: usa
Posts: 10,815
Thanks: 0
Thanked 1 Time in 1 Post
 
Re: Ignition warning, is it true or false?

It's true.
The voltage from the coil rises and rises. Normally the energy finds it's way to ground through the wire to the plug, but it will go the easiest path.
Disconnected it rises much further than normal - to it's highest potential.

The distributor insides is designed to let it go that high without jumping anywhere. Notice the better caps have barriers or ridges on the inside - helping increase that resistance.

But - a little moisture, a little dirt etc allows that energy to follow along it - to wherever! Moisture and dirt is conductive. If it finds it's way to the module circuit - usually by hitting the pick-up coil - poof!

(One way to prevent that - when you pull a plug wire stick your finger in the hole - it will safely go to ground through you. NO!!!!)

As far as pulling the wires to find one that's not firing - if the wire was broken it was already going high - no problem. If it wasn't firing because the plug was shorted/founded/grounded, then pulling the wire allows the voltage to rise.

ne thing to remember - when pulling wires to find a not firing cylinder - always pull it at the cap end of the wire.
1 - it's easier, 2- it's much safer. When you pull the wire off at the plug, you are now holding onto a "hot" wire with lots of voltage in it - coyuld get exciting. If you pull the wire at the distributor end - you are holding onto a dead wire - safe.

Whenever you are touching the ignition wires, always use only one hand. Try not to lean on the fender while doing it.
If you have one hand touching ground, and the spark hits your other hand, the energy passes up one arm and down the other - right through your heart! Not good! It can stop your heart - yes, it's happened.

If you are leaning on the fender the energy goes up your arm, down your body and jumps to ground through the front of your pants!
UH - there are better ways to get a thrill!

Pulling wires is standard practice, but like most everything, it has it's risks.
RRich is offline  
post #5 of (permalink) Old 12-06-2005, 10:52 AM
**DONOTDELETE**
Guest
 
Posts: n/a
Re: Ignition warning, is it true or false?

That is where a Oscope comes in handy. Hook it to your tach lead, connect the trigger to cyl #1 and you can see what is going on. Bad plugs and wires show up real fast.
post #6 of (permalink) Old 12-07-2005, 12:32 AM
Addict
 
jeepnut_nh's Avatar
 
Join Date: Mar 2002
Location: near Nashua, NH
Posts: 484
Thanks: 0
Thanked 0 Times in 0 Posts
 
Re: Ignition warning, is it true or false?

said it before--i use my timing light--connect it up and just put inductive pickup on each wire and see if the light pulses--hard to get shocked this way, and no need to disconnect wires
jeepnut_nh is offline  
post #7 of (permalink) Old 12-07-2005, 09:07 AM
Way Outta Control
 
Join Date: Sep 1999
Location: usa
Posts: 10,815
Thanks: 0
Thanked 1 Time in 1 Post
 
Re: Ignition warning, is it true or false?

The problem with the timing light trick -
If the wire is partially broken the light will still flash.
If the plug's fouled or shorted, the light still flashes too.

All it's telling is there's high voltage in the wire, but not how much or how it's firing in the chamber.
Same way with the MM neon light testers.

The phoney plug testers in the pictures - the ones made specifically for HEI have a longer gap - since the HEI can fire a bigger gap. They look the same, but the center electrode is back inside further, giving that longer gap.
A separate plug will work too. Just ground the shell and look at the spark - set the gap to about normal.
Should be a bright blue thin spark with a definate "snap" to it.
Not bright, intermittant, or a red spark indicates troubles.


But - like was mentioned - a good scope will tell you much more at a glance. Even then, sometimes all the ignition voltages are correct - because the ingition system is fine.


But a dead cylinder, or partly dead, still can be detected by pulling wires. That's why any decent automotive scope has the ability to kill cylinders one at a time and in groups. They are safer, they don't allow that cylinder to fire by shorting the primary, there's no high voltage generated that can destroy something like pulling wires could. But ya gotta do what ya gotta do.

Remember Factory service manuals are constantly calling out special "factory" tools and reasons you need to take it to the dealer - and pay him. Most factories have their own "authorized" scope. I know, I spent most of my life involved with supplying those dealers with their test equipment - and training them.

A dead or partially dead cylinder (misfire) can be caused by many things other than ignition - most common is lean mix, usually due to a vacuum leak, low compression, flat cam, EGR leak, weak spring, valve action, injector, etc. - lots of things.
RRich is offline  
post #8 of (permalink) Old 12-09-2005, 10:15 PM
 
Join Date: Sep 1999
Posts: 5,383
Thanks: 0
Thanked 1 Time in 1 Post
 
Re: Ignition warning, is it true or false?



Why in the world do you think I'm always writing 'Test Plug', and 'Unhook the coil', Ect, Ect, Ect....

It's hard as hell on not only the rotor and cap, but the ignition coil and module when you...

1. Pull a plug wire off while the engine is running.
2. Leave the ignition 'On' instead of turning the key off or over to 'Accessory'.
3. Don't run a dedicated ground to the head(s).
That spark has to ground somewhere, and rusty, greasy or chemical treated head bolts don't do the job correctly.
That crappy little ground strap that comes from the body (the body that probably isn't properly grounded either!) usually goes to the intake manifold, NOT the heads...
4. Don't run a dedicated ground to the ignition module.
All of the problems with grounding listed above apply to the ignition module also...
Junk Yard Genius is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Off-Road Forums & Discussion Groups forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome