Distributor Pickup test

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Dartndude73

Dartndude73

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Hi, I have a 73 dart 340 with a mopar performance Distributor, msd 6al. I want to test the pickup. Engine won't fire I have done extensive work on the entire ignition system. Tested and retested everything. Looks like the pickup is no good. I have a 73 Chrysler service manual. Basically they want you to use an old Chrysler test tool that I have no access too.
The only thing I've been able to come up with in my searches online is something from Dave's place - it has instructions on testing the entire ignition system including the pickup. According to the the article the pickup can be tested with an ohm meter and should read 350 to 500 ohms.
I had already installed a new standard electric # lx102 pickup from O'Riley auto parts. I checked it online it seems to be the correct part. When tested it was only 280 ohms. Ok I figured I just got a new junk part happens a lot lately - Chineseium.
I got a replacement lx102 today. Before I installed it I checked and it only tested at 300 ohms. Makes me wonder if it's another junk part, incorrect part or maybe Dave's info could be wrong. Can anyone verify that the Dave's place test is correct? The only other part I could find was at Napa an Echlin part mp881. Special order of course.
Thanks
 
There is no possible way, without expensive test equipment, to "truly" test such things "for certain." Without specific engineering data on the component such as inductance, etc, even that might not be practical, possible, or of much accuracy--especially with questional quality and specs of import parts!!!

In other words, try a replacement. and THAT is coming from a guy who hates to "throw money and parts" at a problem

An example is something that would cause an internal partial short. IE a few turns of the coil have corroded, somehow been overheated by current that is not supposed to be there, or had some defect in the wire when new.........some way that turns of wire are shorted.

You might think this would not make much difference, but what happens is, that these shorted turns make what is called "a shorted turn" or "shorted winding" and this acts sort of like in a transformer and degrades the performance of the coil It just won't generate the pulse power that is needed. This same fault applies to other components--the rotor of alternators, or either the primary or secondary of an ignition coil.
 
I just had a pickup fail on my 340 a couple weeks ago. I ohmed out several good used ones and they were in the 280 Ω range if I remember correctly. You can test everything by pulling the distributor and spinning it by hand. Remove the coil wire from the cap and put it close to ground. You can also momentarily short the two distributor plug wires on the harness side to trigger the ignition box.
First thing is make sure your gap is set right on the pickup.
I should note that the failed pickup did Ω out correctly but was in fact bad.
 
I have had Mopar picks up read as low as 140 ohms. And they work fine. Cars that have VA connected flex the p/up wires during operation. Do enough miles, & the wires break, no spark.
I doubt the new p/up is faulty. Check that the air gap is 0.006-8". Another test you can do: disconnect the p/up leads & connect a volt meter [ AC range ] to the p/up leads. As the dist rotates, a small voltage should be generated as the reluctor tip passes the pole piece; anywhere from a few millivolts to volts, depending on the rotational speed of the dist shaft.
 
I have the factory tester for the "new" electronic ignition (big blue box) and I can assure you that it is highly inaccurate. I also hate to admit it but if I have eliminated all other possibilities then I would replace it.
 
Pull the distributor, verify the gap is good and chuck it up in a hand drill. Connect an ac meter to the two leads, you should see 4-5 VAC. Test the MSD by temp grounding the white lead to ground.
 
Basically 250-800 ohms is showing the coil part has goocd continuity. That is at Room temp. The issue with these usually is that when they get hot they open up so test it hot also.
 
Another way is take it out and spin it with a drill. The out put from the pickup wires should show a AC voltage in millivolts, that how they work. That signal is then sent to the ECU which basically amplifies it and tells the coil when to fire.
 
Murray said:
I have the factory tester for the "new" electronic ignition (big blue box) and I can assure you that it is highly inaccurate. I also hate to admit it but if I have eliminated all other possibilities then I would replace it.
Click to expand...
They are hit or miss. There are a few other ones out there that are pretty accurate.
 
Thank you for the replies. As usual alot of good advice. I've done a lot of work on this cars electrical system. It was a total trainwreck before I started. Looked like an explosion in a spaghetti factory under the hood and dash. I relocated the battery to the trunk. I had some brand new 2/0 welding cable I used for the leads +/- to the front. All the grounds are good. To the block, frame, fenders, firewall. I deleted the original fuse block. Installed a new box for strait 12 volt on the front fender. Any switched connection now comes from the new box in the glove box. I did the amp meter delete and re routed new wiring bypassing the firewall connection. "Found that one on a link from this site." I got rid of any wiring that looked even a little bit questionable. All my connections are solder and shrink tubing. All the wires are sized correctly and tested. New blaster coil, new cap, rotor, reluctor and pickup. also lightened the springs on the mechanical advance. Tested the MSD by grounding out the leads - it thru a beautiful blue spark about a 1/2 inch across the tester gap. I did re check the distributor pulled it and used my brass feeler gauge. It's at .008. I'm getting good voltage to the MSD unit from the 12v and the switched side while cranking.
So I'm going to install the new pickup tomorrow give the spin it with a drill to see what I get for voltage as suggested above.
 
If you spin the dist shaft with a drill you will probably get volts [ ok ], not millivolts.
 
A question for halifaxhops. Can I use my 150M storage scope with the right probd, or will i destroy (another) lead? just curious.
 
For the pickup? Prob it is just a AC sine wave out. I have to re check what the volts are out again. Was seriously thinking MV. That might be under load of the ecu though.
 
Ok I wanted to update this thread. I pulled the distributor first thing I found was the pickup would touch the reluctor with just light pressure against it. Once I broke the connection it was set correctly at .008. I lightly pushed on it again and it stuck to the reluctor again. I took out the pickup and replaced it with the new unit checking the 3 leg clip / retainer. It was fine. After installing the new pickup the fit was a lot tighter and I could not move it towards the reluctor after setting the gap. looking the old pickup over I could see the base was warped also looked like the pin that holds it in place was not made correctly. I could have very easily bumped it with the the cap when installing it and knocked it against the reluctor. I re tested the ohms. 320 like. Used a cordless drill and multi meter to test it per instructions above. I got about 6vac.
I reinstalled the distributor. Double checked the MSD unit. After a little coaxing it fired up. Got it timed and did a little adjustment on the carb. Was running reasonable. Shut it off and restarted it 4 or 5 times. The engine seemed pretty hot and after about 30 minutes I shut it off and would not restart. I look into that tomorrow. Check cooling. I'll check the fuel return line and regulator. I don't think it was vapor lock though.
 
[1] There is a magnetic [ pull ] every time a reluctor tip goes past the pole piece, which contains a magnet. If there is any slop in the system [ dist shaft worn, pivot holes worn etc ] then contact might be made.....& then no spark.
I do not think your problem is p/up related....& never did from post #1. They are extremely reliable until the wires break from vac adv flexing them.
I would try another coil.
 
No magnets in the picup coil, thete is one on ethe "pole" piece the coil wraps around. It works like a AC generator and when the reluctor passes it it generates a small AC sine wave. One of the Mopar myths for sure. Mattax you have that diagram from that TSB still?

mag25.jpg
 
i also think the thing still works if the pole and reluctor tip collide. i have in my collection a dizzy with a reluctor that has worn away due to the pole and pickup/magnet not being rivited properly to the base.... it still worked. not very well, timing was a mess, but the car started and ran. i have taken to punching the rivet and making sure i use a decent screw with good head to mount and adjust.

i guess in this day and age you could pull one apart and use a magent designed for a BLDC hub motor if you had an application that needed a bigger voltage to trigger. modern magnet some kind of super alloy with stronger magnetic field. AliExpress if full of em in all shapes and sizes.
 
Yes it will still trigger at almost no clearance. If the gap gets too wide is when you will have issues. Also, and slop in your bushings or vac advance mech will require more clearance to keep it from hitting. I just got a new proform distributor and it had more slop than my 20 year old MP one.
 
Replace it. If it works, the old one was bad.
 
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