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shallowwatersailor

'Tis the Season for No Start

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I have been trying to get my 1989 520-H to start since yesterday. It last ran in January when I parked it. I have run through the usual checks and it seems to have settled on the ignition. It cranks and pumps fuel. I have changed plugs, swapped ignition wires with the other 520-H, and changed the fuel filter. I have not checked the condenser.  I have placed the inline spark checker between the ignition wire and plug, and I get a spark at the end but not while cranking. I have cleaned a number of terminals including the Molex connector. Now I have just put a lead between the + battery terminal and the + terminal on the coil. The 20 Amp fuse blew! That doesn't come up in any of the scenarios. Any ideas?

 

Also, has anyone ever changed the Molex connector to a Weatherpak connector?

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you absolutely sure you were on the battery (+) and not the (-) ??

 

you would totally bypass the fuse on the (+) terminal with no way of shorting out the fuse

 

you would short out the 20 amp fuse if you were on the (-) battery terminal .

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weatherpack connectors are indeed more weatherproof than Molex connectors

 

but the contacts are usually not rated for over 15 amps of current - so you would oxidizing the weatherpack connectors as quickly as the molex connectors - maybe even more so since the weatherpacks are more insulated and would retain more heat.

 

if you go the weatherpack route, separate the charge wire out into an additional 2 prong connector which divides the charge current into 2 pathways instead of one.

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Just duplicated it in case I did make the mistake. I had the same results.

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Try jumping the solenoid at the starter with the key in th on position and see is she starts. Sounds like the start side of the ign,. circuit isn't coming hot

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Just duplicated it in case I did make the mistake. I had the same results.

OK then,

 

here is how I understand this - with a jumper (red dotted below)in place between the + battery terminal and the + ignition coil, the 20 amp fuse blows. The reason I found this unlikely is the fact the current from the battery would go thru the jumper (bypassing the fuse entirely) and head toward the next best ground connection. This next best ground would be the purple pathway thru the coil and then thru the ignition module. The resistance of the ignition module should limit the current thru the jumper to about 3 - 4 amps.

 

Even if the condenser were shorted, the 20 amp fuse would blow when you turned the switch to the RUN position. The diagram doesn't show this properly but on the Onan engine, the condesor is on the POSITIVE coil terminal.

 

 

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Edited by Save Old Iron

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The only way I can see a no start condition and blowing the 20 amp fuse with the jumper is outlined in the schematic below

 

The red wire from the battery to the left side of the 20 amp fuse would need to be disconnected from the 20 amp fuse (this would give you no spark)

The left side of the fuse somehow is connected to ground (broken wire inthe fuseblock touching ground and fuse no longer connected to the red wire from the battery)

 

This odd condition would allow the best ground to be thru the fuse and not the ignition coil. Applying the jumper would cause current to flow toward the fuse - thru the fuse and then to ground. This would blow the fuse.

 

So if my understanding is correct and you are jumpering the points you mentioned earlier, then this is the scenario I could create to enable the fuse to blow only when the jumper is fitted to the tractor.

 

Check the wiring under the fuse block .

 

 

post-1689-0-04394600-1400474019_thumb.gi

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So I should check for continuity on the blue to see if it is shorted?

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I would leave the ignition switch in the off position. take a jumper and hook it directly between the battery and the coil positive side. Take jumper attached to the positive side of the battery and touch it to the other pole of the starter where the start lead from the ignition switch attaches. On my 416H it is a blue wire. You might have a bad switch.

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At this point, we can choose to troubleshoot the no spark issue or why the 20 amp fuse blows when troubleshooting. Either method will lead us to the problem.

 

Since my availability will be limited for the next few days, I would suggest downloading the Onan service guide and use the section for checking the ignition module.

 

Basically, the ignition module acts like electronic "points". The module is triggered by a magnet on the crankshaft instead of mechanical activation. The result is identical to the K series points. Power is applied and cutoff to the ignition coil, creating a spark when power thru the coil is interrupted.

 

Follow the checkout procedure in the Onan manual. Measure the voltage at the (-) side of the coil (use either a multimeter or a test light)

 

 

 

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. Rotate the flywheel by hand with the ignition switch ON. At some point in the rotation of the flywheel, there will be a transition on the voltmeter from 12 volts down to around 1 volt and then back again to 12 volts. This is the magnet triggering the ignition module. If you do not get this transition, the module or module wiring is suspect.

 

I'm not sure if we were going off into the gutter with the 20 amp fuse issue. It may be a contributor to your overall problem. I would check the integrity of the wiring in the fuse holder area. 520's and bad fuse holders .... gotta love it.

 

 

Please note the BLACK lead of the ignition model IS NOT GROUND. The BLACK lead is the "output" from the module. The METAL TAB on the module is GROUND.

Edited by Save Old Iron

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as a side note - why you see an initial spark on power up...

 

when you power up a 520 ignition module, the module contains some intelligence which sets the output of the module to the OFF mode. This cuts off the power to coil and keeps it off until the module sees the sensor ring rotating. The initial power applied to the coil and then shutoff by the module initializing causes the coil to spark just once.

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the ignition module uses the "latching" version of hall effect switching seen in the video below

 

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the trigger ring has 2 magnets embedded into the outer rim of the assy (magnets indicated by red dots)

 

1f225173.jpg

 

the fields produced by the magnets are seen on this magnetically sensitive plastic sheet.

 

0962ed48.jpg

Edited by Save Old Iron

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Sounds good. I'll try and get to it tomorrow. I really appreciate you time and effort.

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Well it is five months later and I decided to spend some more time on this tractor's problem. A few weeks ago I had gone to one of my dealers for parts. I happen to talk with one of his old time mechanics about the no-start issue that I was having. Believe me, I have come close to just selling it as-is because I was so frustrated. I mentioned that I had cleaned the majority of the contacts but still nothing. He suggested changing the ignition switch. I had thought that I had checked for continuity back in May but decided to get a switch to swap out. I became "a parts switcher" mechanic.

 

Today I changed the switch and also replaced the ignition wires with silicone 8mm solid core wire that I have had since the 70's. It was a bit of a search to find boots and terminals in today's market but I did. Connected a fresh battery that was on a battery tender, turned the switch and - voila, it started!!!

 

Reading through the thread Nick (km3h) suggested the switch. A BIG THANK YOU, Nick! Who would have known that it would have been that simple.

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I am very glad you finally found the problem. Most folks assume the switch is working properly because they turn it on and the lights work or the volt meter moves. The very simplest check when there is no spark is to check for voltage at the coil. If you don't have it you won't get spark. Now you know and the next guy who come up here with that kind of problem you will be able to tell him what to do because you learned by experience.

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