MSD coil testing
Well, I went out to crank my trucl today and discovered that I have no spark.
So I did a little testing...
Tested battery voltage at battery: 12.6 volts.
Tested power lead to HEI: 12 volts
Pulled off dist. cap and thoroughly cleaned it (lightly sanded contacts throughout). Reinstalled it.
Still no spark
Pulled coil out of cap and did some more testing...
With a DVOM, I tested for resistance between the red and black leads for the coil . Got a reading of OL.
Tested between the yellow lead and black lead. Reading of OL
tested between the yellow and red leads. Low resistance (less than 1 ohm).
I have tried to find an exact testing sequence for the MAD coil and have yet to find one.
I fear the coil has died.
Shouldn't there be a reading between the red and black other than OL?
What about the yellow and black?
shouldn't the reading between the yellow and red leads be OL instead of less than 1 ohm?
also, if anyone happens to know NAPA's part # for the MSD in cap coil for a GM hei it would be greatly appreciated. There is a NAPA warehouse 5 minutes away and I bet they probably have it, though te guy answering the phones seems to have no clue.
I would think the module or pickup would go out before the coil.
Doc here, :pimp:
To measure the Coil Resistance on the Stock General Mystery Coil, Set your DVOM to R X 1 or autorange, and Calibrate the scale, (It should read 000 , or close to it, if not recal, change the battery or note the difference)
Place one probe on the "BATT" Terminal, The other On the "TACH". Observe the reading..It SHOULD read LESS than an Ohm, but MORE than 000. Outside of that range, replace the coil...
On the Secondary Side, Set your DVOM for R X 10k, or autorange, Place one probe on the "BATT" Terminal, the other on the Rotor button pickup carbon element..note the reading, then move the "BATT" probe over to the "TACH" terminal, In Both Cases, your reading should be between 6,000 and 30,000 ohms ..out side that range, the coil is bad..
If that checks out OK, I'd suspect the module..You can take that to Auto Zombie and have it tested for free..do it several times to get it hot..If it is bad, and you get a replacement, HAVE it TESTED too, before you leave the store! The fail rate out of the box is like 1 in 3 or 5...
If it is bad you'll spend weeks scratching your head saying.."No it's new..can't be bad.."
IN either Case, replacing the old module back in the Dizzy, or the New, DO not forget the heatsink compound, or you'll do it again very soon..you can get a tube for the old one at the Auto parts store while your there..The new one comes with a tube..
If the Module does check good, and it's a no spark issue, then the next component to fail would be the Magnetic pickup in side the dizzy, you can look up the Specs for your truck in ohms..(two are given If I remember for year boarders..before year xxx was xxx, after year xxx was xxx type thing)
Or a quick and dirty test is just put the star in between high lobes, and pass a mailable screw driver in front of the pick up, If the ohms reading jumps up then down, It's doing it's job. If not ..
You need to pull the Dizzy, remove the drive gear, and disassemble the dizzy to get the unit out.. :pain: :smash:
That should get ya runnin' mon..
Coil in cap GM HEI coils are NOTORIOUS for failing, as they are epoxy filled and do not leach heat from their cores, and run in a covered atmosphere. Their failures are usually enough to kill the HEI module if severe enough.
The failure is usually a layer short in the primary windings, and also usually causing failure of the module by resistance change and trigger side overloading, BUT, it usually isn't readable with an ohm meter.
I suggest the coil and module be removed from the distributor and taken to a parts store with the new style run testers. Most Auto-Zone's have these new type testers, and they actually rely on a running test, NOT just resistance checking.
You also need to make sure the contact points from the small black wire to the coil yoke, and the yoke to the brounding buss bar underneath it are clean and in good contact. In cap HEi coils don't use a floating ground like a regular coil does, they use a dedicated grounding of their outer yoke, and if the ground path isn't clean and tight, no spark. The ground buss bar connects the coil yoke to the module wire hold down screw in the distributor body, via the center wire in the three wire loom and connector to the cap, grounding the coil.
Best thing to do with that epoxy coil is remove it, add an MSD 8401 cap adapter, a section of coil wire, and either NAPA IC12 or MSD 8200 OIL FILLED coil instead of the epoxy filled coil, or epoxy filled "performance" coil. In short, get that coil out of the cap.
Doc here, :pimp:
Maybe you can tell me, I have not got a straight answer yet..
WHAT is exactly the make up of the HEI module? Is it a High speed switching transistor on a wafer subsrait..or it it a Switching transistor with a gain amp on a wafer substrate or what..
And do you have an internal diagram of one? If so please post it for me..I'd really like to know..
And question number 2.. Is there available aftermarket, for standard General Mystery HEI, a magnetic pickup in a "Clamshell arrangement" so as after initial installation, pulling the Dizzy won't be necessary..
If anybody would know, I figure it would be you, I can't get straight answers on either.
Ok, an update....
tested the coil and it was bad. replaced the coil. (Went with an 8225 MSD)
Still no spark.
tested the module. Module was bad. replaced module.
fired right up. i also put die-electric grease on all of the connectors...just in case.
took it for a test run...
now the damned vacuum advance isn't working properly...but while changing the module I noticed the mechanical weights weren't in great shape. probably stuck closed.
So... I guess I'll upgrade the weights, get a new cap and pickup this weekend. Hell, I figure if I replaced half of it, mine as well go all the way!
Thanks for the info guys. i really appreciate it. once I actually thought about how the coil and module work, i figured out how to test them properly. big DUH on my part.
First time I have ever seen a coild AND a module go bad together.
Guess there is a first time for eveything.
Doc, I haven't a clue as to the makeup of the HEI, all I know is, it is the ONLY OEM inductance type module that actually extends dwell time to make more spark, and when coil'd right, are downright indestructible. It is better than the TCBI systyems of the PerTronix and Unilite and other drop in systems by far, as they have NO drivers for dwell extension.
As far as the pickup, no-one I am aware of has a two piece easy change pickup setup, just the full circle ones. I don't use the stock HEI pickups or reluctor/armatures.
joe, It isn't the first time I have seen it, as I posted above. It is VERY common for the coil to layer short and take the module out from overworking the resistance to it in the trigger wire circuits. I see it all the time in coil in cap HEI's, and it is more common than the large cap gurus would have a person believe.
I would also think that if the mechanical advance was sluggish, so is the shaft to reluctor movement, which requires the dist to be completely disassembled and cleaned, then regreased. It would be the stiffened dead grease holding the shaft ftrom turning free inside the reluctor. HEI's have the tendency to NEVER get looked at until they are darned near dead. So the greases emulsify, lock up the mechanical advance, and make the thing the real nightmare it really is.
I'd also be very careful with replacing the weights and center for that dist. I'd suggest the weights marked 41, center marked 375. These parts are common to late 70's/early 80's Chevy Suburban with 350 engines, EVERY new crate engine and the entire ZZ crate carburetted engine family, small and big block.
Do you have these numbers on the weights and center?? If so, I'd keep 'em and use a Crane adjustable vacuum advance, p/n 99600-1. This kit has a vacuum advance degrees stop plate that needs to be mounted differently than the Crane instructions to work correctly, not hard to do. And, it comes with three sets of mechanical advance springs.
The Crane kit and the correct number weights and center will make that dist come alive, if you want it to work right. I have a series of 6 pics that can help, give me a regular e/mail address that I can send pix to, and I will do so, and help you through it. I do it all the time for everyone.
Well, it seems I am back for more....
Ok, replaced the weights and springs in the dist.
Ran GREAT...better than ever before.
Wrong. When attempting to leave the house this morning I encountered the same problem once again.
At first, the truck cranked normally. It's cold natured so cranking it is an art of sorts. While trying to crank it the battery got weak....so I busted out the jumper cables. Hooked up to my atepfather's jeep and tried again...
Hmmm, acts like it isn't getting spark? that's odd. everything is brand new and worked last night.
So i pull the #1 plug and guess what...no spark AGAIN! already late for class I had to leave it here.
I guess I'll pull the distributor today and see what I find.
Maybe the coil I used is the problem...killing modules.
maybe the pick-up inside the dist is crap...weak magnets.
this is beginning to be frustrating. :mad:
Doc here, :pimp:
You mentioned you put heatsink Compound on all the Connections? Do you mean wires?
The Compound is supposed to go on the Bottom of the module, Between the advance plate and the module..after you clean off the old compound.
If you just put it on the wires, and put the module in dry, It may have shorted, overheated, and burned out. Pull it and have it checked again.
If you need a new one:
I'd type all this out instead of quotes, but today I have a major migrane..that won't give me a break..sorry about that!
Let us know what you find, perhaps Ignition man has some ideas for you too..
I thoroughly cleaned the base plate where the new module mounted and applied the packet of grease supplied with it to the bottom of the module. I ensured that the bottom of the new module had a nice even coating of grease.
I then applied my own electric grease to the connecting terminals for the wiring (both ends of the module and the connections to the dist. cap).
I am beginning to suspect that the coil I used is the culprit. As soon as the rain and snow stop here I plan on testing the coil I used and testing the new module just to be sure.
A friend who works at advance can get me an accel dist. for under $130...which is about what it would cost to replace the coil, module, and pick-up...so that may be the next option.
I appreciate all the help you and ignition man have given.
I think a brand new dist. would be the best next step if the old one doesn't test out well.
Thanks again guys.
I'd strongly suggest you avoid trhe Acceel distributors and coils, they aren't, in my opinion, what they used to be when they were still made here in the U.S.A.
I'd say to have your stocker redone. HEI's aren't that hard to work on and even a novice can dial them in with the right wrecking yard parts, a Crane adjustable vacuum advance and a little coaching from the site here.
Ok, finally found the true problem!
After various tests I decided to take the entire dist. to the college and put it on our machine.
before doing so I completely disassembled it and thoroughly tested everyhting.
Coil tested fine.
Module tested fine.
Pickup coil tested fine.
magnets are still good and strong.
Dist cap has no apparent craks or carbon tracking.
treassembled the dist and hooked it to the dist testing machine...
Pulled the cap off and jumpered to it. coil fired fine.
With the cap off, pulled off the rotor and tested it. rotor tested fine...until we pressed the tang down to simlulate the cap being on...
Apparently the rotor had developed a small crack/hole beneath the tang that connects it to the dist cap/coil. when fully depressed the spark was jumping through that small hole and going straight to ground through one of the mounting bolts. It had also been the culprit that killed the module and original coil. the apparent defect could not be detected with the naked eye...
So, after all that testing and retesting...buying new parts...etc...
It all boiled down to a bad rotor. The one part I thought was OK.
But, at least I know that everyhting else is functional...and I learned not to overlook the simple things.
Thanks once again for all the replies.
I'm going to buy a new cap and rotor tonight and get my beloved truck up and running again.
Just in time to race a friend from the college in the 73 vette I just fixed for him :D
Doc here, :pimp:
Thanks for posting back with the Fix! :thumbup: May help someone else down the line!
That's a rare one...haven't heard of that happing since my 55, 6 & 7 Chevy days with a 283 and a broken motor mount..The engine would rise and the Dizzy would hit the firewall and Stretch but not actually damage the cap, but snap the rotor tang off..KAPOW! No run no more.. :sweat:
Glad to hear you got it going though..Good Job! :thumbup:
Its actually fairly common. The material under the tang starts to carbon track burns through and shorts to the distributor shaft. It usually starts with a skip and gets progressivly worse. This is fairly common especially with cheap after market Auto Zone 3 dollar rotors.
Yup, I should have posted that as well, good catch on the rotor, and as stated above, another very common problem with the large cap HEI's. So common, I tend to forget there are others out there that haven't heard of it yet.
I'd suggest the Wells rotors only, A-Z has them for a reasonable price. Wells stuff is actually OEM GM in different packaging, and is of exellent quality.
Another failure issue is the carbon post/rotor. Might make sure the carbon post is not damaged, and isn't broken off, is installed correctly, and protrudes past the end of its guide on the rotor side of the cap. If the post is installed above the insulator under the coil, there will be a gap between it and the bar on the rotor, and that will cause severe overloading of the coil/module, failure to follow.
Proper sequence is...post (spring end up), insulator, coil.
HEI's are really finicky on little stuff like this, and will find any opportunity to fail/kill electrical parts they possibly can.
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