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300zx

520 ignition problem

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300zx

I call the 520 I am currently trying to recover "Rusty". Rusty sat outside for 10 to 15 years before I bought it to either part or restore. I started with the engine. After taking it apart, cleaning it up and repairing whatever was needed, I bench tested it and it ran fine. Started it two or three days in a row. All was good. The last time I ran out of gas and mistakenly left the ignition powered. I realized this later when I found the battery run down. I moved on to getting the tractor ready to power the hydro. I sat the engine on the tractor and hot wired it like I had bench tested it so I could eliminate any problems from the tractor interlock system. It would not fire. I had checked the coil when reassembling the engine and the resistances were good, 3 ohms on the primary side and 17.5k ohms on the secondary. When I checked it this time the primary side was less than 1 ohm which is too low (maybe a weak coil and had failed when I left the ignition on). So, I changed the coil with one I knew was good. Still no spark. I was now suspicious of the module. The book says to rotate the fly wheel by hand with the ignition powered and to check the voltage to ground on the negative side of the coil. It should run 1 to 1.5 volts until it fires and then should be battery voltage, about 12 volt. When I checked the voltage, it was running 11 to 11.5 volts all the way arround, never 1 to 1.5. So I suspected the module may have failed when the coil failed. I was prepared to take the module from the good engine, but decided to run the same test on the module to cofirm it was a good test. The good engine tested the same, 10 to 10.5 volts all of the way arround, never 1 to 1.5. Based on this, the Onan diagnostic test for the module would say the good engine's module is failed, which I know it isn't because that engine runs. At this point, I am lost! Has anybody been down this road or have any suggestions? The only thing I can think of to do is to swap the module from the good engine to Rusty's and see what happens.

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WH nut

Is the motor plugged into the harness when you hot wire it. If so unplug and see what happens, you could have something in the cicuit grounding out the ignition

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Save Old Iron

John, a few possibilities here.

Poor continuity thru the "engine connector" (9pin molex connector)

The 12 volts from the battery to the ignition coil goes thru the 9 pin connector. A high resistance in the connector pins will starve the ignition module from the current it needs to pull the negative side of the coil to ground. To eliminate the possibility of the contacts in the 9 pin connector causing this issue,

jumper the + terminal of the coil directly to the battery + terminal and repeat your test again. If you get the 12v to 1v transition now - you have identified the connector as the issue.

Poor continuity from the ignition module to the coil (-) terminal

Think of the ignition module as the electronic equivalent of the points in a points based system.

If you have a poor ground at the points, or poor connectivity of the points to the (-) side of the coil, the (-) coil terminal will never drop off of the battery voltage reading. Unfortunately , there is no way to ohm out the (-) coil to ground as the contents of the ignition module are electronic, not mechanical like points.

I would make sure the ignition module is well grounded (ground the engine housing to the (-) battery terminal if you can ) and make sure the crimped connectors on the ignition module wiring are in good shape - maybe even refit new terminals to them if they look corroded.

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300zx

Thanks for the advice. The engine is not plugged into the molex connector. I wanted to isolate the engine from any problems with the tractor wiring for now. I did connect the battery cables. The positive to the starter and the ground to the mounting bolt on the engine. The starter works fine and the continuity from the engine block and heads back to the battery ground is good. The jumper from the positive battery post to the positive side of the coil is good. The connector to the control module measures 11 to 11.5 volts as I rotate the flywheel. If the control module itself needs to be grounded, there may be corrosion from the module to the engine. I am going to pull the flywheel this morning and see if there is any corrosion around the module. This engine ran fine 3 weeks ago in this same configuration on my shop bench. The only thing I can think of that may have affected it is that after I ran it the last time, I left the positive side of the coil powered until it ran the battery down over night. Since that time, the primary side of the coil has dropped from 3 ohms to less than 1 ohm, which is now out of acceptable range. I have changed to a good coil. I plan to change out ignition components from a good motor until I find the problem. Any other ideas?

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300zx

I pulled the ignition control and trigger ring from Rusty's engine. When I bench tested it, it would not trigger between 1 volt to battery voltage as specified in the manual. I then removed the ignition control and trigger ring from the engine that I knew would run. When I bench tested it the same way, it would trigger between 1 volt and battery voltage as the Onan diagnostics said. (I still don't know why it wouldn't do that by just rotating the fly wheel as the manual said) BTW, the trigger ring has two magnets, the first triggers the voltage to 1 volt and the other triggers it back to battery voltage.

So, my conclusion after all of this is: when I left the power on to Rusty's coil, it over heated and weakened the primay side until the current got so high it ruined my ignition controler. There is a caution in the manual to never put 12 volts on the negative side of the coil or it will damage the ignition control. This basically happened when the primary side resistance dropped from 3 ohms to less than 1. I know keys are accidently left on and ususally it just runs the battery down. In this case it took out my coil and then my ignition module. A $180 lesson!!!! Ouch!!!!

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300zx

Rusty runs great now with a new coil and ignition module. And I must have done something wrong last night, because tonight when I checked the new system, I did as the manual suggested and checked the voltage on negative side of the coil with the positive side powered. As I rotated the fly wheel, the voltage triggered between 1 volt and battery voltage, just as a good ignition control should do. I put the engine on the the tractor and it lit right up and ran well. The hydro seemed to be fine also. I will run it through its paces tomorrow and then begin to check out the electrical. I still haven't hooked up the molex connector. I plan to use dielectric grease on the connectors and fuses before I connect the tractor wiring to the battery.

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Save Old Iron

John, great news.

Always great when the test procedures you rely on prove to be valid.

:thumbs: :D

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