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Onan P-Series Ignition Module Replacement [Photos]

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 So if you've problem solved all other aspects regarding your non-starting Onan engine with electronic spark control and come to the conclusion that you need to replace the Ignition module, keep reading. This is a decent step by step and doesn't require the engine to be removed the from the tractor. Although it might be a little easier to work on while on a workbench.

 The part numbers for the Module and ring if needed are below:

Ignition Module: 166-0785
Spark Ring: 166-0767

Here is a list of the tools that I needed to complete the job from beginning to the end. They are named from left to right in the photo with the tools at the top of the photo last from top to bottom.


[From left in Photo]
1. T25 (for the stator screws but also may be three 5/16" head screws)

2. Phillips and Flat Head Screw Drivers
3. Needle Nose and standard slip joint pliers

4. Flywheel puller with 2 5/16" x 18 grade 8 bolts and washers (mine is technically a steering wheel puller)

5. 7mm and 5/16" deep well 1/4" sockets

6. 3" 3/8" drive extension, 3/8" socket 3/8" drive, 5/8" socket 1/2" drive and 5" 1/2" drive extension. 

7. 1/4" Socket Driver
8. 5/8" open end box wrench
9. 3/8" and 1/2" drive ratchets

[Top of photo- top to bottom]

10. Flash light (optional but very helpful at times)
11. Pry bar (used to lock flywheel from turning)

12 1/2" Torque wrench

13. Old Tooth Brush (for cleaning of dirt if needed)



- Obviously the first thing to do for safety sake is to shut of the fuel at the tank and remove the battery cables
- Next, remove the two screws that hold the fuel pump onto the shroud, move the pump off to the side and replace the screws into their holes for safe keeping.
- Carefully pull the fuel pump away from the shroud enough to work the hose clamp out and use the pliers to pinch the hose clamp and remove the vacuum line for pump. 


- Tie the fuel line, wiring harness, choke cable and battery cable all out of the way towards the rear of the tractor allow easy access to remove the shroud.


- Next remove the air cleaner box cap, then air cleaner cap and air cleaner tray by removing the three 5/16" screws from the carb and the two 3/8" screws from the tray and put all aside and out of the way. 

NOTE: Close the Choke all of the way and stuff a clean paper towel into the top of the carb to prevent debris contamination.

- Next, twist the air filter box bracket to the left (right side up and left side down) to clear space and use the 7mm, 5/16" and 1/4" socket driver to remove the black condenser wire, the red and black ignition module wires and the yellow ignition switch wire from the coil and then remove the coil using the 3/8" socket. (optional as it can be kept on the shroud but makes it a little more difficult.)
-  Next loosen the rear cylinders top shroud and engine hoist mount bolt with the 1/2" 3/8" sock and extension. (no need to remove the bolt as the shroud just needs to move freely. 
- Next, locate the wire retaining clip at the top of the shroud by the coil and open it up so the oil pressure sensor, stator and ignition module wires can all be removed when taking out the shroud.
- Last step before removing the shroud is to disconnect the throttle to gov arm spring and the throttle cable using  the 5/16" socket and driver.
-Next locate all of the shroud bolts and remove using the 3/8" socket.




- To remove the shroud, pull the bottom out first and then shift the entire piece upwards while working it from underneath the rear cylinder shroud, underneath the filter box bracket and over the flywheel on the left side. Work slow so you don't bend or brake anything and it will come off pretty easily. 
- Although everything was fairly clean in comparison to other engines at this step, my shroud was dirty and this can slightly decrease air flow which is important to maximize and keep optimal with an air cooled engine. 




So I took mine outside and pressure washed it to give it time to dry completely before reinstall. 



Shroud off... 




- Next, place the pry bar into the fins of the flywheel and lock the flywheel in place by placing the handle of the pry bar under the foot rest. Then use the 5/8" sock, 1/2" extension and loosen up the flywheel bolt about 3/8" -1/2" out of the hole. This needs to stay mostly in as a pressure point for the puller.

- Next, remove two of the flywheel grill center bolts and then use your two 5/16" grade 8 bolts and attach the puller to the flywheel while keeping the puller parallel to the surface of the flywheel to allow equal pulling pressure on both sides of the puller while cranking on the center pressure bolt on the puller. 


- Now it's time to remove the flywheel.  It may be a good idea to use the 5/8" open ended box wrench to stop the flywheel bolt from turning while cranking on the puller center bolt. 

- It should only take a 3-5 good cranks after finger tightening the center bolt and the flywheel will give a good pop and be ready to be pulled off. Don't forget about the flywheel bolt first! ;)



- Next, carefully pull the flywheel off of the crank being very careful not to get caught on the stator itself. 




My flywheel was pretty dirty on the inside and between the magnets so I felt it necessary to use the toothbrush and air gun to clean it out. 



- Now you'll need to remove the stator by using either 5/16" socket or T25 torque bit or driver. 



- Next, set the stator off to the side securely on a clean rag or similar. Underneath, you'll see the ignition module on the bottom side of crank. There are two 5/16" head machine screws. Remove those and open the wire retaining clip on the side of the gear cover and the module comes right off. Make sure not to lose the black rectangular insulator that goes between the module and gear cover. That needs to be there. 





I'm not 100% sure but I have a theory that these modules may potentially go bad from getting covered with grass and debris over time from not blowing out the air shrouds with a leaf blower or air gun after each use. This debris builds up enough to cause insulation around the module. The module being an IC (Integrated Circuit) is designed to handle only so much heat. Over time the extra heat and expansion with contraction when cooled back down causes a when point in the modules chip circuitry and eventually breaks. Thus causing the engine to shut off during operation for the first time. Once the engine cools, the chip materials contract, allowing the circuit to bridge again only until the engine heats up enough to break that signal and then becomes a cycle until replaced. 

- Moving on.. While everything is out, if needed (or your OCD kicks in), clean the cylinder heat syncs and general area before re-installation of the Module, Stator, Flywheel and Shroud. Mine wasn't to bad so I quickly used the toothbrush and air gun to break free any layered gunk and blew it off with air. 


-Next, once the insulator, module (and spark ring if chosen to) and stator are all reinstalled and screws all snugged up, carefully put the cleaned flywheel back on, install the flywheel bolt with a tiny tad of anti-seize on it and use the pry bar again in the flywheel fins but with the handle on top of the foot rest and use the 5/8" socket, extension and 1/2" torque wrench set to 55lbs and tighten up the bolt. 


At this point, go ahead and reinstall the Shroud making sure that the fuel pumps vacuum line is accessible through the front hole in the shroud, along with the shroud bolts, coil, coil wires (spark, condenser, module and switch) and tighten rear shroud bolt. 

NOTE: Yellow Switch wire, red module wire and black condenser wires all go on the Positive coil terminal (7mm nut) which should be on the right side of the coil. Only the black module wire goes on the left terminal. 



Connect the throttle to Gov arm spring, throttle cable, install the fuel pump vacuum line again and replace mount screws. 


- Lastly, re-install the filter tray, remove the paper towel from the carb, along with installing the air cleaner, pre-filter and clean cap and shroud to filter tray pipe. Then connect the battery and turn the fuel back on. 


 This should get you going again. 


 I think that I got everything but please feel free to comment on anything that I missed as far as documenting the steps or ask any questions about the process. 

 Hope this helps you see that it's not that bad of process to replace the ignition module (And spark ring if needed as some recommend replacing the module , ring and coil all at the same time).

I had to first remove this module from a P220G that was on my bench and awaiting repairs but the whole process took me a few hours. A good Saturday Morning task to be up and running by the afternoon for some work before the heat hits. I also ran the tractor for well over and hour after install to insure all was well. I went from a warm up process of idling and then a lap around my property. Did that a few times while repairing a cutting deck and then installed the cutting deck and mowed the lawn. No issues. Ran great! 

Take Care and God Bless!

Andrew C.


Edited by Mastiffman
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GREAT write up Andrew  and thanks for sharing! May be doing just this pending final  diagnosis. :handgestures-thumbupright:

Edited by WHX11
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Thanks for the excellent job.  :text-goodpost:


If anyone wonders why the Onans have a hotter rear cylinder, and valve seat problems, just look at how the oil filter mount and other parts block air flow.  If they wouldn't have mounted ignition and other parts to the cover we could easily pop it off and clean it at every oil change.

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True. but as long as it gets blow out with a leaf blower or compressed air after every mowing day, especially in the dry summer mowing days (more dust and lighter grass clippings get sucked into the air path easier) I think that should extend the duration in between having to worry about cleaning it out. 

 The issue I think mainly lies in owners that don't do this routinely. That debris build up, starts breaking down and then turns into insulation on that rear cylinder and causes failure. 

Still great engines! ;)


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:text-bravo:A picture(s) is worth a thousand words (and there might be a thousand words there too.)

Might want to get a MOD to pin this one to the top of the Engine section.

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I agree. I'm sure that this is a common interest with any Onan powered tractor owner... 

Here is a brief talk about it and the tractor running. 

Going to change the oil later today... 


Edited by Mastiffman

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I would say good job.  If I were to offer several ideas they would be to incorporate the troubleshooting of the ignition module, and coil prior to the disassembly process. You could probably sell it as a book.  The "Spark Ring" is nothing than a magnet. The simple test for that is will it repel another magnet. They very seldom if ever will go bad but may be damaged by junk like debris and such. They are what one would call a misunderstood part because most people simply like to throw parts at something thinking they are doing preventative maintenance. If it still will repel another magnet and is not damaged the part is still good.


You might touch on the importance of the thermal insulator under the module/sensor. Its job is to provide thermal protection. It is not to provide electrical isolation however.

Edited by 6bg6ga
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Great job with demo photos Andrew! As others have mentioned, a picture is worth a thousand words. I have a 1988 Model 518-H that suddenly would not start after turning it off and trying to restart within 1 minute. One backfire and never to start again. Long story short, I pulled the flywheel today and found the key that mounts on the crankshaft and extends into the ignition ring broken. The only piece remaining was the part that sits into the depression in the crankshaft. I was looking at your photos of your repair of ignition module, but I could not detect the key beyond the end of the shaft in your photo. As I do not have the missing part of the key, I "assume" (bad word) that this key actually does extend all the way into the larger key-way slot in the ignition ring?

The long key that sits in the flywheel and extends a bit beyond the inside of the flywheel was OK. The Ignition ring was free to move without it's locking key to the drive shaft, thus throwing the starting effort way off timing as it could free-wheel around the crank shaft.

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This is the key that is listed for the WH model 518-H, P218 Onan engine. My question is, which way does this mount on the crankshaft? If you insert the long end into the flywheel, there is not enough reach on the other end to reach the slot in the trigger ring. If you insert the long end into the trigger ring slot, the stub end is not long enough for the flywheel slot !! Are there TWO keys that must be used ? If there is only one key, does that mean that the trigger ring does not need a key in it's larger spaced slot, but uses just a friction fit on the crankshaft ?

flywheel key for Onan P218 engine.jpg

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I am currently working on a P220, this afternoon I'll pull the key if it isn't stuck, and let you know what I found.

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Posted (edited)

I looked at mine and found that the long tail goes back into the keyway of the ring.  There is plenty of key in the flywheel so I don't know what you mean about not being enough for the flywheel slot.  Get a new key and it can only go one way without sticking way out.  It might be possible that the ring is damaged in some way that it moved back, perhaps it should be removed and inspected.


just noticed that the illustration shown above shows the key opposite the way mine is installed.

Edited by lynnmor

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The key on my tractor was simply a long key with no tab at the end. The spark trigger ring should sit on the crank in a manner in which it will be locked to the correct timing by the flywheel key. There is only one key. 

If your ring sits too far back then pull it off and see if it is damaged on the inside bore in which maybe it's sitting back against the timing gear cover to much and not locking onto the key. If it looks like its centered properly over the spark trigger sensor module, then check to see if there is damage done to the key way on the inside of the ring. If not then maybe, just maybe the key was replaced from a previous owner because of shear and the replacement key was a bit too short and has moved. 

 I would think that it was one of those three things. 

Hope this helps.  

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