Hey guys got a question on an hei I am trouble shooting. I have it out and on the table. Hooked to a power supply and a spark tester. If I spin the gear at a quick revolution it sparks perfect. However if I spin it slower say about a turn per 2 sec it does not spark. Is this normal is there a minimum speed it needs to turn to create spark? Thanks!!
Hamers
The starter turns the engine at about 200 rpm to start.
That means the dizzy turns at about 100 rpms
That comes down to 1.66 turns per second
You said it wont spark when turned 1 rev per 2 seconds.
Doesn't seem to be enough of a difference to me to cause an issue, but it does seem according to the math that there is.
If it wont spark at a slow RPM it will make starting the engine hard.
2 things that come to mind are that
1: The pickup coil is a PM generator. The faster they are excited or turned, the more voltage is produced. This MAY be the trouble. If the coil is not up to par, or the magnet is cracked, it could be a problem as far as producing a clean voltage spike strong enough to fire the module.
2: The other thing is a circuit in the module that is called "active dwell"
The ignition coil is never grounded with the Ignition on and the engine off, UNTIL the engine is turning over (spinning). This is done to keep the coil from overheating and burning out in case the ignition is left on.
The active dwell keeps the coil on time sufficient to charge the coil to provide ample discharge voltage without staying on too long at slow RPMs which would in turn build excessive heat in the coil.
It is likely the module needs to "see" a certain amount of voltage spikes in a given amount of time to "Wake up " the module, to activate the ground on off pulse to the coil. Kinda like priming a pump.
Part one is all fact. Part 2 is mostly fact but I can only speculate on the precise operation of the "active dwell" circuit and its functionality. One of Motorola's little secrets (Original designer)
Other than the fact that the active dwell does control the on off time of the coil primary to prevent a meltdown, how it initializes upon startup is a guess for me. An educated one, but a guess just the same.
You can check the voltage output of the pickup coil with a voltmeter ,it will give a good enough reading to give you a POV reading (Peak output voltage) .
IIRC it takes about 1.25 volts at minimum to fire the module.
Were you having trouble with the engine it was in?
That means the dizzy turns at about 100 rpms
That comes down to 1.66 turns per second
You said it wont spark when turned 1 rev per 2 seconds.
Doesn't seem to be enough of a difference to me to cause an issue, but it does seem according to the math that there is.
If it wont spark at a slow RPM it will make starting the engine hard.
2 things that come to mind are that
1: The pickup coil is a PM generator. The faster they are excited or turned, the more voltage is produced. This MAY be the trouble. If the coil is not up to par, or the magnet is cracked, it could be a problem as far as producing a clean voltage spike strong enough to fire the module.
2: The other thing is a circuit in the module that is called "active dwell"
The ignition coil is never grounded with the Ignition on and the engine off, UNTIL the engine is turning over (spinning). This is done to keep the coil from overheating and burning out in case the ignition is left on.
The active dwell keeps the coil on time sufficient to charge the coil to provide ample discharge voltage without staying on too long at slow RPMs which would in turn build excessive heat in the coil.
It is likely the module needs to "see" a certain amount of voltage spikes in a given amount of time to "Wake up " the module, to activate the ground on off pulse to the coil. Kinda like priming a pump.
Part one is all fact. Part 2 is mostly fact but I can only speculate on the precise operation of the "active dwell" circuit and its functionality. One of Motorola's little secrets (Original designer)
Other than the fact that the active dwell does control the on off time of the coil primary to prevent a meltdown, how it initializes upon startup is a guess for me. An educated one, but a guess just the same.
You can check the voltage output of the pickup coil with a voltmeter ,it will give a good enough reading to give you a POV reading (Peak output voltage) .
IIRC it takes about 1.25 volts at minimum to fire the module.
Were you having trouble with the engine it was in?
You can Ohm out the pickup coil .It should read between 500-1500 ohms.
Ohming it out proves the winding is good, but weak or broken magnets (pole piece ) can be checked by looking at it for cracks. Otherwise you need an Ocsilloscope to check the wave pattern, as it will be able to display anomalies such as shorted windings or ghost patterns caused by cracks in the magnet
Ohming it out proves the winding is good, but weak or broken magnets (pole piece ) can be checked by looking at it for cracks. Otherwise you need an Ocsilloscope to check the wave pattern, as it will be able to display anomalies such as shorted windings or ghost patterns caused by cracks in the magnet
Hey thank you both for the info its much appreciated!!
The reason I am testing this HEI is I have a no start condition. On top of that I found I also have an intermittent and weak spark. I have been chasing issues with this HEI for a while now. At first I had no spark while cranking but as soon as I released the key it would sputter on occasion. If I did get it going it would have a nasty miss under load.
I found a bad connection at the starter and burn marks on the carbon button.. thought I had it. Fixed the connection and replaced the button with a low resistance msd button. Still the same.
So then I went deeper. It diagnosed to be a pick up coil that had "failed in an uncommon way" as the HEI help guide said. I had no spark while cranking but I had spark if I put the big soldering gun next to the pick up or probed the G tab on the module with my test light connected to power.
So I got a new pick up and installed it. This seemed to be the ticket as it started and ran good. Then on my way home the miss came back ever so slightly. Next morning no start. So I did the same tests and got the same results.
So I took it out and now here it sits on the table. Only time it does anything abnormal is if I spin it slowly. That is the cliff notes on how I got here. I feel I should also mention all of this started when it got super cold in my area.
The HEI is a MSD Billiet but I have since converted it to have stock electronic parts in it as the MSD stuff burned up. So stock module stock condenser and stock pick up. The only thing that looks different inside it is the spiked wheel that works with the pick up coil. I believe its the reluctor?? It seems to have a larger gap between it and the spikes on the pick up and is a different shape.
So I also have a closer to stock Accel HEI on the table to play with too and its reluctor(??) is different. It looks like all the stock ones do. I also notice I can feel the magnet engage with the spikes on the pick up when I spin the gear on the Accel HEI. Funny thing is I can spin this one as slow as I want and it sparks on the tester as the spikes line up.
My only issue running this one is the mechanical advance is garbage. The MSD has way nicer timing. Also the MSD reluctor is one unit with the shaft so I cant change it. On top of that I am not sure running the Accel would solve the miss either but it may since I really do not understand that part of this issue. Again thank you for the help and any further insight you can give!!
The reason I am testing this HEI is I have a no start condition. On top of that I found I also have an intermittent and weak spark. I have been chasing issues with this HEI for a while now. At first I had no spark while cranking but as soon as I released the key it would sputter on occasion. If I did get it going it would have a nasty miss under load.
I found a bad connection at the starter and burn marks on the carbon button.. thought I had it. Fixed the connection and replaced the button with a low resistance msd button. Still the same.
So then I went deeper. It diagnosed to be a pick up coil that had "failed in an uncommon way" as the HEI help guide said. I had no spark while cranking but I had spark if I put the big soldering gun next to the pick up or probed the G tab on the module with my test light connected to power.
So I got a new pick up and installed it. This seemed to be the ticket as it started and ran good. Then on my way home the miss came back ever so slightly. Next morning no start. So I did the same tests and got the same results.
So I took it out and now here it sits on the table. Only time it does anything abnormal is if I spin it slowly. That is the cliff notes on how I got here. I feel I should also mention all of this started when it got super cold in my area.
The HEI is a MSD Billiet but I have since converted it to have stock electronic parts in it as the MSD stuff burned up. So stock module stock condenser and stock pick up. The only thing that looks different inside it is the spiked wheel that works with the pick up coil. I believe its the reluctor?? It seems to have a larger gap between it and the spikes on the pick up and is a different shape.
So I also have a closer to stock Accel HEI on the table to play with too and its reluctor(??) is different. It looks like all the stock ones do. I also notice I can feel the magnet engage with the spikes on the pick up when I spin the gear on the Accel HEI. Funny thing is I can spin this one as slow as I want and it sparks on the tester as the spikes line up.
My only issue running this one is the mechanical advance is garbage. The MSD has way nicer timing. Also the MSD reluctor is one unit with the shaft so I cant change it. On top of that I am not sure running the Accel would solve the miss either but it may since I really do not understand that part of this issue. Again thank you for the help and any further insight you can give!!
Here is an Wiki article on HEI distributors. Lot of good info . http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor
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