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With the ignition on, the positive side of the coil reads about 1 volt below the battery after going through the ballast resistor. OK.
Touching/lifting the neg coil terminal with a grounding wire drew a tiny micro spark. Dropped in a new coil and got a nice fat 1/4” spark. But still, I get no spark when rolling over.
I pop the distributor cap and both the iron reluctor wheel and coil assy have some corrosion. Replaced both as they’re cheap. Gapped at 0.006 and still no fire when rolling.
I pulled the header off the ECU and measured 350 ohms down the new pigtail to the pickup coil and back (and no shorts to gnd). The voltage of the ECU header connection from the ignition switch was very close to the battery voltage. OK.
The pigtail had one of those male-female connectors built in for the reluctor pickup coil so the polarity should be correct.
The wire between the ECU header and the negative coil terminal showed a proper continuity.
The last test in the Service Manual was to check the ECU itself for the ground pin to be connected to its metal case. Being good, the manual says ECU has failed. I pickup a MasterPro ECU and when turn the key on and then at the moment I turn it off I get a nice spark from the coil. HOWEVER, rolling over I STILL get nothing!
Since I get a good spark when I cycle the key, the ECU output circuitry has to be pulling current through the coil and then opening up for the spark to perform. So the coil is def good.
Since the distributor pickup is just a simple magnet and coil of wire, I thought simply I had bought a faulty ECU. I bought another and it did exactly the same thing. No spark when rolling!
To troubleshoot further, it appears I need to get an oscilloscope on the pickup leads to find out what the pulse voltages are … but the problem is I have no specifications to base a good/no good judgement on.
Does anyone have knowledge about what voltage the pulses need to be to make the ECU fire?
Or any other ideas/suggestions as to what I should try to get the old van back on the road?
Great job with the diagnostics, it seems to me you've covered all bases but one. My suggestion is to look at the outputs of the ignition switch, specifically when the key is in the Start position. The ignition system gets power in both the Run and Start positions, and you seem to be testing with the key in the Run position. But when in the start (cranking) position you seem to be losing power to the ignition system so the output from the ignition wire while in the Start position could be bad. FYI, having ignition power in the Run position but not the Start position is also a typical symptom of a bad ballast resistor, but you seem to think the resistor is good which leaves the ignition switch.
The 12v supply to the ECU WAS DISAPPEARING when the key went from Run to Start. Fixed the wire and the engine fired right up!
I had power there in the Run position, got spark with turning the key (assuming the ECU had to be pulling current through the coil and opening up to let the coil current collapse). My unthinking assumption stumped me LOL! 🤣
This engine is 36 years old and with $75 of MasterPro parts from Oreilly’s I bought the other day, I’ve fully rebuilt the entire ignition system and it roared to life sounding just like the 1970 Dodge Charger 318 I bought just out of high school LOL! (Beats the heck outta the wife’s 2014 Santa Fe Sport whose engine blew completely … Hyundai did fix it free, but it took 5 months.)
I really appreciate your great help!!!!!! 👍🏻👍🏻👍🏻
For anyone curious about the voltage output of the reluctor coil and learning an easy way to disconnect the coil and test trigger the ECU to spark, I recco the YT video from Lakeland Marine and Auto that gives two really direct and practical trouble shooting methods. This guy also knows his stuff working on Chrysler Marine engines!
Reluctor Coil Voltage should read 200 mv with a voltmeter set on an AC scale.
With the ignition on you can touch a 12v test light to the male pin of the unplugged reluctor cable coming from the ECU (gray wire) and it will trigger spark. (Don’t use just a jumper as it might draw too much current from the ECU and damage it).
The 12v supply to the ECU WAS DISAPPEARING when the key went from Run to Start. Fixed the wire and the engine fired right up!
I had power there in the Run position, got spark with turning the key (assuming the ECU had to be pulling current through the coil and opening up to let the coil current collapse). My unthinking assumption stumped me LOL!
It happens to all of us, that's why a second pairs of eyes is always a good idea!
11-30-2022, 07:59 PM
Weird Ignition Pbm: 1986 B250 318 Engine
