Distributor pick-up coil

Posted by: Tech junkie

Distributor pick-up coil - 12/05/17 11:28 AM

Rebuilding a 70's-80's electronic dist. for my 318. I've got two factory Chrysler shop manuals. One is a '74 and the other is '83. Both have specs for the resistance across the pick-up coil. The old one sais 350-550 ohms is acceptable and the newer one sais 150-900 ohms is ok. I dug out 8 old ones kicking around and got these results...257, 265, 268, 270, 275, 278, 283,and 290 ohms. These coils came out of working units so I know they were at least usable. They fall within the newer spec., but what's better? More resistance or less?
Posted by: jwb123

Re: Distributor pick-up coil - 12/05/17 11:34 AM

I seldom use an ohm meter to check pickup coils anymore, main thing you will find is an open or shorted coil, with an ohm meter. I use a scope because it will give you the quality of the signal the pickup is making. Main thing is to get the air gap right when installed in the distributor. Go with your newer spec.
Posted by: dogdays

Re: Distributor pick-up coil - 12/05/17 02:04 PM

This question Illustrates a valid point. The difference between "good" and "good enough"

If you handed me a pickup coil and asked me to test it I'd put an ohmmeter on it and verify that the coil did indeed conduct electricity. Then I'd hand you back the coil and say it works.

The reluctor flying past the pickup coil is very close in concept to a string vibrating near an electric guitar pickup coil. It generates a voltage across the pickup coil. As the tooth approaches the pickup the voltage increases. When the tooth travels away from the pickup the voltage changes polarity. The voltage curve passes through zero when the tooth is exactly lined up with the pickup coil. The electronic ignition reads the induced voltage across the pickup coil and fires the box as the waveform passes through zero.

The guys who were designing the ignition system determined an optimum range of pickup coil resistance. Within this range they can be extremely certain will work.

As years go by the designers develop more confidence in the sensitivity of their box. The allowable resistance range of the coil grows. Or, as years go by the box is made more sensitive by part substitution.
Either scenario results in a wider "good" range for the pickup.
Maybe the pickup coil manufacturer offers a discount if a wider range of coil resistances is allowed. So they test their box and low and behold it will work over a wider range.

Broadening the acceptable resistance band for whatever reason decreases the unit cost of the pickup coil.

So as the years go by the published acceptable values change.

Here's a similar example: The date stamped on a milk carton. It is supposed to indicate there are seven days after the stamped date before the milk turns, as long as it is properly refrigerated.
Yet when there are milk drives for the hungry they won't accept milk older than the stamped on date. The same for canned food, except that this is even more stupid. Some of the warnings say use before this date for best flavor. Exactly no warning exists that says eat beyond the posted date and experience sudden death. My wife collects for a food bank. The food bank will not accept food past the "best by" date. They throw that food away. Better that it go in a dumpster than feed someone, having substandard flavor.

I eat that stuff all the time; saves on the food bill. I have also eaten yogurt several months past its "best by" date. Never a problem. Of course it had been refrigerated and the container wasn't bulging.

If you like conspiracy theories, look at the best by date as a mechanism for manufacturers to sell more product.

I know, we're far afield, but I was trying to illustrate the shifting values of "good enough".


PS: Saying use an oscilloscope shows how far from reality some people are. Who has one lying around?
Posted by: Supercuda

Re: Distributor pick-up coil - 12/05/17 03:09 PM

Originally Posted By dogdays

PS: Saying use an oscilloscope shows how far from reality some people are. Who has one lying around?

I do, it's even calibrated.

My torque wrenches are calibrated too.

There are two ways to do things, the right way and everything else.
Posted by: moparx

Re: Distributor pick-up coil - 12/06/17 09:48 AM

Originally Posted By Supercuda

There are two ways to do things, the right way and everything else.

remember now, "everything else" is 98% of the planet. biggrin
Posted by: Cab_Burge

Re: Distributor pick-up coil - 12/06/17 12:28 PM

When it comes to testing a device that generates a magnetic pulse, as long as the pick coil is not open, broken wire, then I would test those distributors with all the ignition hooked up, grounded well, and see if the coil sparks when you spin the distributor by hand scope work up
Did you calibrate your ohm meter before use? If not try that thumbs
If your ohm meter is not the type that you can calibrate the Ohm scale then see if you can beg, borrow or get one some how and retest those pickups thumbs
Posted by: Tech junkie

Re: Distributor pick-up coil - 12/06/17 02:02 PM

As usual, thanks for the info everybody. At least I have a bit more direction now. For lack of better testing methods, I'm going to install the best looking coil and evaluate its performance by the density of tire smoke it helps produce.
Posted by: Dcuda69

Re: Distributor pick-up coil - 12/06/17 02:04 PM

They are magnetic pulse generators. As stated by Dogdays they create an AC voltage as the tooth from the trigger wheel passes by. I'd be more concerned with that than resistance. Stick one in the dist,connect a voltmeter to the leads,set the scale on ac volts and give the dist a spin. The number I've always been told is 300mv ac minimum. If it makes that it should be enough to trigger the box which in turn controls coil primary current flow.(I'm guessing any of those listed will work) Good luck.
Posted by: 383man

Re: Distributor pick-up coil - 12/07/17 08:11 PM

I usually do like Cab says. I just ohm it first to make sure its not open or shorted and then I test it with my AC voltmeter. I found most that read between 150 to 900 ohms are usually good and put out a strong enough AC signal. Course make sure the magnet is not missing or part of it broken off. Ron