#1  
Old 09-17-2019, 02:22 PM
Formula jg Formula jg is offline
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Default HEI Pickup and Ignition Coil Testing

I have another thread going in the Street section that started as a carb issue but may now potentially be a distributor problem.

When I stab the throttle at light cruise I get a bad stutter and when I stab throttle enough to open secondaries the engine will literally cut out if I keep the throttle in the stabbed position.

Coil is an old Accel (1990) with red/white leads. Using Ohm meter I touch both these wires and get a reading of 0.5 ohms and when I connect 1 meter lead to either the red or white and other lead to underside of coil it reads 8950 ohms.

Looks like coil maybe ok unless there is something else going on that can't be tested with ohm meter, let's not forget the coil is almost 30 yrs old. Anything else I can do to test this igintion coil?

Now for the pickup coil it reads 830 ohms but when I wiggle the green/white wires the meter numbers fluctuate a bit. I have 4 other new and used pickup coils in my tool box that I tested and they all act the same when moving the 2 wires around, is this normal?

Thanks for any direction you guys can provide.

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Old 09-18-2019, 04:37 AM
Geoff Geoff is offline
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Provided your meter is making a solid connection with the p/up wires, you should see NO change in the ohms reading. Changing/flickering reading indicates a bad/broken connection.

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Old 09-18-2019, 05:48 AM
"QUICK-SILVER" "QUICK-SILVER" is offline
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Default I'll 2nd that

Quote:
Originally Posted by Geoff View Post
Provided your meter is making a solid connection with the p/up wires, you should see NO change in the ohms reading. Changing/flickering reading indicates a bad/broken connection.
I'd retest and move the wires slow and see what happens. The green and white wires usually fray and break right where they go in the pickup. If reading goes way high or open the wires are definately coming apart inside the insulation.

Clay

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Old 09-18-2019, 10:20 AM
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Just a thought if you suspect pickup wires are the cause. Assuming you are using vacuum advance, when you stab the throttle, you're loosing vacuum, causing the vacuum can to pull timing out flexing those pickup wires. Maybe disconnect the vacuum can and take it for a drive and see if the problem still exists.

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  #5  
Old 09-18-2019, 11:00 AM
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Sure sounds like the pickup coil.......if confirmed, change it when it's in the garage...not easy on the road...need to take the dist apart. It's not xpensive, just some time.

George

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Old 09-18-2019, 09:15 PM
Formula jg Formula jg is offline
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jonmachota78, why can't I ever come up with these simple and logical tests, I'm always looking to complicate my life and have less money in my pocket...need to hang out with you guys a little more.

I bought a new AC ignition coil earlier this week because I wanted one as a backup to the 30 yr old thats in there now. Retested all my spare pickup coils and each one has the same issue when wiggling the wires the ohms jump around, so I ordered a new one and it better have solid wire connections.

I will update once further testing is done this weekend.

  #7  
Old 09-18-2019, 09:35 PM
70gtojosh 70gtojosh is offline
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When you get your new pick up coil pay attention to the metal piece the coil sets in. If it is not stamped or machined to the correct thickness/height it may be difficult to reassemble the distributor. I've been down that road. Had to reuse that bottom metal piece in my experience. Just because the part is new does not make it good! Good luck.

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Old 09-19-2019, 04:56 AM
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The most important part of replacing the p/up coil is to ensure it is centered, such that none of the reluctor tips make contact with the pole piece tips. There must be an airgap.

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Old 09-19-2019, 05:50 AM
Formula jg Formula jg is offline
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70gtojosh, I’m trying to determine exactly which part of the pickup coil your referring to.?

Geoff, thank you for the air gap tip.

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Old 09-19-2019, 09:52 PM
70gtojosh 70gtojosh is offline
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It would be the part that has the little "arm" that the vacuum advance can attaches to. Hope that helps.

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Old 09-21-2019, 10:05 PM
Formula jg Formula jg is offline
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Update.
The new pickup coil has not arrived yet but I wanted to eliminate other possible culprets in the meantime.

First thing I noticed was the ignition coil screw holes in the distributor cap were stripped and the coil was not held firmly in place so I replaced the cap. Retested the pickup coil with ohm meter and the numbers definitely jump around when wiggling the wires.

Put it all back, road test with vac. adv. distconnect as suggested by 70gtojosh and there's no stumble or stutter, awesome.
Reconnect vac. adv. and still no stutter or stumble when stabbing the throttle. The only thing that was changed was the dist. cap so I guess some wierd movement with the lose ignition coil was occurring when stabbing the throttle.

I went ahead and replaced the 30yr old Accel ignition coil with the new ACDelco coil just to see if there was any difference. First noticed the new coil needs longer cranking to start the engine, the Accel would fire it up instantly. Short road test is inconclusive at this time, need to review the data logger to see if anything jumps out at me.

I'll test the new pickup coil when I recieve it and if the ohms are steady when moving the wires around then I'll install it and report back.

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Old 09-23-2019, 04:39 PM
Formula jg Formula jg is offline
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I rec'd the new pickup coil and sure enough the ohms reading jumps around when wiggling the green and white wires, now I have 5 of these things all acting the same.

Can someone reconfirm that there should not be any change in ohms reading at all when moving the wires or is a little fluctuation in numbers acceptable?

thx all.

  #13  
Old 09-23-2019, 06:35 PM
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Given that all 5 do it, I'm willing to bet that an analog meter's needle would be fairly steady to not moving at all under the same test. Reading a slight change is hard to do with a digital meter.

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Old 09-24-2019, 05:50 AM
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Hard to believe 5 coils, one new, have an intermittent connection. Try a different meter....

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Old 09-24-2019, 11:46 AM
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Try a recheck: put the terminals on like a wood surface and put the probes forcibly on the terminal ends to make sure the electrical contact is solid, then move the wires.

Don't have the pickup coil near anything steel (or ferrous (iron)) when moving the coil. The coil is actually an electrical generator when the magnetic field around it changes. That can confuse a multimeter.

George

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  #16  
Old 10-03-2019, 12:13 PM
Formula jg Formula jg is offline
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Ohms still fluctuate after checking it on wood and isolating from other metals.

I went ahead and installed the new pickup coil anyway. Road test and no change to the stumble when stabbing throttle. Last thing left to replace in the HEI was the GM 990 module, so I figured might as well replace it. Short road test and so far so good, no stumble but don't quote me on this yet I need more seat time to properly evaluate the result of module change.

  #17  
Old 10-03-2019, 12:41 PM
Formula jg Formula jg is offline
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Another question I have is regarding advance weight springs and how to check their stiffness? The light springs and stiff springs are obviously easier to identify but anything in between is difficult.

I'm in the process of re curving the HEI because of to fast a curve causing detonation at light and heavy loads. After testing several weight/cam combos it appears that when the cam is #'s up the mechanical is general 20* or greater and the rate is much faster than when the same cam is #'s down and using the same springs.

My engine likes a lower initial (9*-12*) + manifold vac. adv. to idle at approx. 18* however the only way I can find to achieve this is with cam #' up but then the curve is to fast and I don't seem to have a stiff enough spring to slow it down.

Attached is a pic of springs I have.

Any other way to slow down the curve?
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  #18  
Old 10-05-2019, 02:27 PM
David Ray David Ray is offline
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New here, but, I was one of the persons that was on the GM team that designed and developed the large coil in cap HEI's. I also am well versed in both large HEI's, and small-body HEI conversions, 40 plus years of them.

Easiest way to get 21/22 degrees of timing for mechanical advance for the large HEI is to use 41 weights, 375 center scroll plate. These are what comes in the GM ZZ crate engine HEI's, and a lot of later Suburban distributors, and has mostly been adopted for aftermarket HEI's. These ARE NOT the parts used in the after market curve kits sold by most, those kits eliminate the vacuum advance, and set the curve degrees to 16, for DRAG RACING use only, NOT for street use.

There are no lists, nor info on what curve is given by combination, but remember, EVERY stock curve, except the ZZ, is an EMISSIONS curve. Some of the their combinations may well work for a special performance application, but, it will take years to go through all 437 of them to find the right one.

As far as replacing a magnetic pickup in a large HEI, DO NOT JUST GET THE COIL FOR THE PICKUP, BUY THE FULL AND COMPLETELY ASSEMBLED PICKUP, their outer reluctor is already centered, and do not take it apart.. The difference in parts costs is only about a dollar, the frustration level in getting the outer reluctor reset to not hit the inner, is well worth that extra dollar. If the pickup isn't stable when its wires are moved, the pickup is defective, and when new ones do that, they are new defective, and should not be used. The issue is, where the single windings wires are attached to the stranded wires to the connector are bonded together, the bond is very weak, and failure will ensue, not long into the future. If the readings are constantly changing when the pickup and wires are let sitting with no movement, the insulation around the actual coil inside the pickup is compromised,and allowing the windings to touch each other inside the coil area, new defective pickup coil.

In testing an ignition coil, resistance values are relatively useless. They rely on a cold coil, when most large HEI coils fail when they are at operating temperatures. So, to properly test a large HEI ignition coil, take it out of the distributor, and find a good auto parts store that has an off vehicle electrical tester that will actually run the coil. Let it run to get it to operating temperatures, and see how it goes. Replace as needed.

Be aware, the epoxy filled coils in the large HEI caps do have significant failure issues, such as not leaching coil heat out of their jackets, epoxy does not conduct heat away from its source, it rejects it back into the source. When this happens, the insulation between coils degrades, the windings touch each other, causing a "layer short", which changes the loading on things like an HEI module, or MSD box internals, to failure. Ever hear this "That HEI module failed FOR NO APPARENT REASON". Well, NOTHING evr fails for no apparent reason. Lots of people have had numerous HEI module failures, and were told that it simply was not the coil that caused it. then, after washing time and money on every other part in the distributor, they finally change the last thing left, the thing they were told had no problems, ever, and, the distributor stopped killing HEI modules. Amazing.

There are simple parts available to move the coil out of the cap of the large HEI, allows the use of a remote mounted round oil filled coil away from the distributor, stops module failure issues once and for all. Also reduces the under cap temperature of the large HEI by as much as 55 deg/F. on average. MSD makes the parts needed.

As far as off idle leanness, lack of transition, both INITIAL and IDLE timing has a lot to do with it. If these settings are not correct, off idle transition and idle transfer settings will be adversely effected. One thing is to get the INITIAL timing worked out, conservatively, and supplement the IDLE timing with the full manifold vacuum sourced added timing correct, then, get the carburetor set up right. It isn't that hard to do.

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Old 10-06-2019, 12:43 AM
Formula jg Formula jg is offline
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David, thank you for all that insider information very useful. I spent a lot of time trying to line up the reluctor, didn't even think they sold the complete assembly.

The issue I'm having is a stutter-stumble when I stab the throttle, at first I thought it was carb related and after going through the carb and testing many changes without any improvement I moved on to the distributor. Sometimes it stumbles and sometimes it stutters but either way it feels like spark is cutting out.

Everything in the distributor is now replaced and today I thought I found the issue when testing the plugs and wires.. One wire was dead and a plug was acting weird when checking the ohms. Replaced these but the issue continues.

I also switched out the advance weights now have #60 weight and #402 up cam. Engine seems to really like this with 10* initial, 22* mech and manifold vac. is at 9*. It's all in just under 2000rpm a little to fast but can't find any springs to slow it down, any suggestions?

Only managed to get 1 road test in with this new curve and the stutter stumble seems to have improved, need more seat time to assess this setup better.

Really want to sort this out before storing the car soon and time is running out.

  #20  
Old 10-06-2019, 12:25 PM
David Ray David Ray is offline
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Before I realized the after market sold the entire pickup preassembled, I used to take 'em apart, and replace the coil, then, fight to reset the centering on the outer reluctor. Then, I got the wild idea to take a section of an old main shaft and clear both ends, drill and tap one end for a countersunk head screw. I then took a piece of clear thick plastic, and cut it to center on the shaft, and have the dimension to have the outer reluctor fit over it, centering the reluctor in the center of the main shaft bore in the bushings. I don't do many large HEI's any more, but I have used that tool for a great many large HEi pickup centering jobs people have only replaced the coil on, and didn't get the reluctor centered.

If I am reading you correctly, you now have 10 degrees INITIAL, and 24 mechanical, for 34 TOTAL, that sounds really good, but, too fast. Any aftermarket spring kit for either GM points, or HEI should have the springs to get it right. You could more than likely tolerate 36 TOTAL, should you wish to do the changes in the last paragraph in this answer.

Now, for the rest if the INITIAL/IDLE setup. Engine vacuum is 9 in/hg, is that right? and, how many degrees of timing does the vacuum advance give? What port do you run the vac adv off of?

Realize that almost every engine likes ti IDLE between 22 and 24 degrees of timing. Most engines won't stat with INITIAL timings that high, so, we have to find a better way, and it is not hard.

We can keep an easy start, conservative INITIAL setting, like your 10 degrees, and supplement the IDLE timing when the engine starts and builds vacuum, to pull the rest of the degrees in by actuating the vacuum advance.

Only things we need to do are to make sure the vacuum advance is the right vacuum pull to match the engine vacuum, and an adjustable works, so does any of the 20 or so stock vacuum advances, for each, points or HEI.

Next, a simple drill and tap, add the Crane stand alone degrees stop, so we can do a simple dial in of the degrees the vac adv gives, separately from the vacuum pull on the diaphragm, and plug it into full manifold vacuum, go.

BTW, the ONLY adjustable vacuum advance to even consider is the Crane, now owned by FAST.

Now, this is NOT an ad, but I put together a complete set of instructions and pictures package on how to do a proper degrees stop for the vacuum advance, using either a home made, or Crane 99619-1 scroll stop. They are NO COST, and are not a part of my other entity with distributors, if you want a set, ask for them. No cost, NO strings, NO ads.

Ask for the email address in a PM, please, as I do not think posting it here would be allowed.

Once the stop is done correctly on the vacuum advance, I think you could move the INITIAL to 12 degrees, and set the vacuum advance to 10 to 12 degrees, and plug it into a full manifold vacuum source, for a total IDLE timing of 22 to 24 degrees, just what the engine will like.

With the vac adv sourced from full manifold vacuum, we also have the advantage of pulling in extra timing when engine loading is lower, or, with minimum load, such as no load in gear cruise. Please remember, the actual proper name for a vacuum advance has always been "Load Compensator". Ford used to set their distributors up with only vacuum advances, they didn't have mechanical advances, and their name for that series of distributor is "Load-A-Matic".


Last edited by David Ray; 10-06-2019 at 12:36 PM.
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