How to test distributor modules

B

Bruce Hart

Active member
Oct 18, 2011
1,414
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I took four modules to O'Reilly's to have them checked out. Only thing they did was to check for resistance.
Is this really a good way to verify if the modules are any good.
--
Bruce Hart
1976 Palm Beach
La Grange, Wyoming

Genetically Modified Chevy
 
We have an instrument for that, but it does not put an load on it so it
does not show what happens when it has been running for long period.
Some will assemble the distributor with the cap , rotor and coil and hook
up the positive and ground and rotate the shaft or the distributor and
watch it arc from the posts.
Not good to do but once.

> I took four modules to O'Reilly's to have them checked out. Only thing
> they did was to check for resistance.
> Is this really a good way to verify if the modules are any good.
> --
> Bruce Hart
> 1976 Palm Beach
> La Grange, Wyoming
>
> Genetically Modified Chevy
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>

--
Jim Kanomata ASE
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502
 
Here's a pretty good procedure for an installed module, as well as the coil
and pickup:
https://www.jalopyjournal.com/forum/threads/is-there-a-way-to-test-an-hei-module.706237/

And this leads to a more complete, complex and entertaining one:
https://www.gmcmi.com/wp-content/uploads/2014/05/HEI-Testing.pdf

Here's a pretty complete review of different HEI modules which will show
you that the part store's use of a multimeter is essentially useless --
module models vary widely in the resistances that are expected:
http://gpzweb.s3-website-us-east-1.amazonaws.com/Ignition/HeiModules/HeiModules.html

Frankly, I think testing modules alone is pretty much a waste of time,
considering that they will sometimes allow the engine to start and run OK
but later cause problems in performance. There are numerous Google
articles about how to test one to determine whether missing spark is due to
module failure -- those are valuable for getting the engine running but
also cannot predict whether the engine will run well.

One other comment: Any time you have module problems, check the coil and
its connections -- the coil CAN and often does cause module failure.

Ken H.

> I took four modules to O'Reilly's to have them checked out. Only thing
> they did was to check for resistance.
> Is this really a good way to verify if the modules are any good.
> --
> Bruce Hart
> 1976 Palm Beach
> La Grange, Wyoming
>
> Genetically Modified Chevy
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
Sure will. Particularly if the screws that hold the coil inflate are even a
tiny bit loose. One of those 4 screws has a ground terminal under the head.
If loose, it will toast modules regularly. I fixed one at our last rally
for an owner that had failed 4 modules in about 2500 miles. Tightened those
screws and no more failures, yet.
Jim Hupy
Salem, Oregon

> Here's a pretty good procedure for an installed module, as well as the coil
> and pickup:
>
> https://www.jalopyjournal.com/forum/threads/is-there-a-way-to-test-an-hei-module.706237/
>
> And this leads to a more complete, complex and entertaining one:
> https://www.gmcmi.com/wp-content/uploads/2014/05/HEI-Testing.pdf
>
> Here's a pretty complete review of different HEI modules which will show
> you that the part store's use of a multimeter is essentially useless --
> module models vary widely in the resistances that are expected:
>
> http://gpzweb.s3-website-us-east-1.amazonaws.com/Ignition/HeiModules/HeiModules.html
>
> Frankly, I think testing modules alone is pretty much a waste of time,
> considering that they will sometimes allow the engine to start and run OK
> but later cause problems in performance. There are numerous Google
> articles about how to test one to determine whether missing spark is due to
> module failure -- those are valuable for getting the engine running but
> also cannot predict whether the engine will run well.
>
> One other comment: Any time you have module problems, check the coil and
> its connections -- the coil CAN and often does cause module failure.
>
> Ken H.
>
>
>
>
>
>
>

>
> > I took four modules to O'Reilly's to have them checked out. Only thing
> > they did was to check for resistance.
> > Is this really a good way to verify if the modules are any good.
> > --
> > Bruce Hart
> > 1976 Palm Beach
> > La Grange, Wyoming
> >
> > Genetically Modified Chevy
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> >
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
!0 or 15 years ago I went through 3 modules at one or two thousand miles each time. At Dick Paterson's recommendation I re-gapped the plugs down to
.040" and have not blown another one since.

They always quit in the most inopportune time. One on an interstate in heavy traffic. Another on a narrow 2 lane country road with ditches on both
sides. One an unfriendly cop called a wrecker and by the time the wrecker got there I was long gone.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
The fact that those dual thread ramp coil screws thread into plastic presents an added challenge. What tajes out modules is excessive Voltage rise on
the coil secondary. This causes excessive (proportional) Voltage rise on the coil primary at the moment the module is shutting off current to the
primary and the field collapses. If you keep all secondary parts in good shape and connections good, the spark event completes with less coil Voltage
rise. I’ve mentioned before I’ve been driving GM HEI since 76 and never lost a module. Also it might be good to clean and replace the white heat
sink compound under the module every 10 years or so as it shrinks as it dries out. Do not use dielectric grease which is not heat sink compound. Most
better tool companies make a briefcase type tester. This will give a go/no go test to confirm the semiconductors are switching. I don’t think they
stress the module compared to a true high road load situation that might be encountered, but will weed out dead units.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Yes, care is needed when tightening any fastener in plastic. Plastics vary
widely in strength, but unless they are reinforced with carbon fibre or
other similar products, they are nowhere close to the strength of the
fastener itself. Dick Paterson is my go-to guy when it comes to "tricks"
that can be used to insure that those fasteners don't loosen up due to
heat/cool cycling and/or engine vibrations. He blames loose fasteners as
the #1 cause of module failure. I tend to agree with him.
Jim Hupy
Salem, Oregon

On Thu, Dec 23, 2021, 6:04 AM John R. Lebetski
wrote:

> The fact that those dual thread ramp coil screws thread into plastic
> presents an added challenge. What tajes out modules is excessive Voltage
> rise on
> the coil secondary. This causes excessive (proportional) Voltage rise on
> the coil primary at the moment the module is shutting off current to the
> primary and the field collapses. If you keep all secondary parts in good
> shape and connections good, the spark event completes with less coil Voltage
> rise. I’ve mentioned before I’ve been driving GM HEI since 76 and never
> lost a module. Also it might be good to clean and replace the white heat
> sink compound under the module every 10 years or so as it shrinks as it
> dries out. Do not use dielectric grease which is not heat sink compound.
> Most
> better tool companies make a briefcase type tester. This will give a go/no
> go test to confirm the semiconductors are switching. I don’t think they
> stress the module compared to a true high road load situation that might
> be encountered, but will weed out dead units.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
> Yes, care is needed when tightening any fastener in plastic. Plastics vary
> widely in strength, but unless they are reinforced with carbon fibre or
> other similar products, they are nowhere close to the strength of the
> fastener itself. Dick Paterson is my go-to guy when it comes to "tricks"
> that can be used to insure that those fasteners don't loosen up due to
> heat/cool cycling and/or engine vibrations. He blames loose fasteners as
> the #1 cause of module failure. I tend to agree with him.
> Jim Hupy
> Salem, Oregon

I had a module fail on my way to the Fall 2006 GMCMI convention held in Berrien Springs, Michigan. "The motorhome" stopped running Westbound on I94
within coasting distance of a closed rest area. I was able to coast in and parked well away from the traffic. While in that safe place I figured it
might be a module so I replaced it, as well as a distributor cap and rotor that I had as spares. It started right up and away I went.

While at the rally I tested the module I replaced with a module tester that Paul Bartz carried in his coach. Yes it tested bad. Upon examination of
the rotor I noticed a burned area and a tiny hole. Also, the cap was missing one spark plug terminal which I found in the distributor. The missing
spark plug terminal explained why for thousands of miles why I couldn't get a smooth idle. Every time that spark plug was supposed to fire, instead
of going to the spark plug it jumped from the terminal to the rotor eventuality burning a hole which then killed the module. The distributor cap was
from Spring Field Ignition and Dick Patterson was at the rally. I showed it to him and he promptly gave me a new one he had for sale.

I now carry a module tester with me in "the Motorhome" and several brand name modules.

--
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com


Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water
heaters.
 
More likely it failed because the missing terminal caused the coil voltage rise to go to maximum as no reachable path to ground. That spark may jump
to the next available terminal as it nears. That gives that cylinder a grossly advanced spark. This can hammer out ring lands or head gasket. I think
the rotor/button fail is simple wear item. They do that over time or if contact is bad. The rotor could have initially whacked the cap if not
perfectly homed or poorly made. I have seen caps where the terminal(s) were not cut and the rotor would collide.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
I have only one thing to say about caps and rotors. Genuine GM parts are a
cut above the aftermarket stuff. Particularly when it comes to primary
circuit contacts. Frequently the aftermarket stuff will move instead of
interconnect with the plugs. That will fry a module quickly.
Goes without saying about the secondary circuit. Make all plug wires
connect where they should, and keep the plug gaps no greater than .045".
You know that funny looking plastic ring that you couldn't figure out where
it went? It secures the secondary conductors to the cap. Use it. Good
insurance policy.
Jim Hupy
Salem, Oregon

On Thu, Dec 23, 2021, 12:31 PM John R. Lebetski
wrote:

> More likely it failed because the missing terminal caused the coil voltage
> rise to go to maximum as no reachable path to ground. That spark may jump
> to the next available terminal as it nears. That gives that cylinder a
> grossly advanced spark. This can hammer out ring lands or head gasket. I
> think
> the rotor/button fail is simple wear item. They do that over time or if
> contact is bad. The rotor could have initially whacked the cap if not
> perfectly homed or poorly made. I have seen caps where the terminal(s)
> were not cut and the rotor would collide.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
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