P220 Onan - running on one cylinder

[GH720 0]

I hope this is allowed.....I have a Grasshopper mower with a P220.  I've learned a lot from reading the posts on this website dealing with the same motor.  I'm hoping you might be able to help me. 

 

I just bought the mower.  I've hardly put any hours on it.  Was running fine last night.  Lots of power.  I noticed the prior owner had way too much engine oil, so I drained a bit off today and then went to mow   The engine was really doggy and it would hardly turn over the deck (and it runs rough).  I pulled the plugs and ground them to frame and cranked over.  The one side sparks OK (not great), but the other side doesn't spark much and is weaker.  The engine will run with the one plug in (the side with better spark), but it will not run with the one plug in on the side that has weaker spark.  I switched cables and plugs and regardless of plug or cable, when connected to one side of the coil, the spark is always weak.

I took all the wires off the ignition coil and measured the primary side resistance to be 3.4 ohms and the secondary to be 18,500 ohms (which I understand is normal).  Coil was still warm from running.  I put the wires back on.  I turned the key on (tractor off) and I checked for voltage on the positive side of the primary.  It measured about 11 volts.  I then connected my meter to the negative post, it measure 0.8 volts.  I turned the engine over by hand.  The voltage on the negative primary went to 12 volts.  I thought that was odd because I only had 11 volts on the positive side.  So I checked the positive side and I now had 12 volts.  I rotated over a couple times checking both primary negative and primary positive and the results were repeatable.  I had tried this test earlier with the engine cold, and I was seeing just over 1 volt on the negative side of the primary and 12 volts on the positive side of the primary. 

 

 

I suspect the coil needs to be replaced, but I'm not sure why all the readings are correct?  Having said that, maybe my ignition modules is doing something weird because I thought the primary positive was supposed to stay a constant 12 volts when turning over by hand?

 

Any help would be appreciated!  Thank you in advance.

[Martin 2,131]

I'm going to take a stab here and say the coil is weak when firing. i read your post through a couple times and all your readings seem reasonable. the only thing i can think it might be is that the coil doesn't have enough guts to fire both plugs at the same time. maybe check the plug gaps as well, could be that they are more than spec.

im interested in seeing what others have to say though and will follow along......

Edited May 11, 2014 by Martin

[Save Old Iron 1,566]

try swapping plugs and compare spark again

 

try swapping plug wires also and compare spark

 

measure the resistance of the spark plug wires and compare resistance values.

[Save Old Iron 1,566]

coils will fail from age, heat and vibration.

 

the heat and age weaken the insulation on the coil wiring and vibration will abrade away the weakened insulation.Heat will then expand the bared wires until they eventually make contact with one another.

 

shorted windings in a coil reduce the ability to produce adequate output to the plugs

 

shorted windings in a coil are similar to holes in a garden hose - the output is reduced as energy is diverted from its intended use.

[GH720 0]

I WILL report back when I get this working because I really appreciate the help. 

 

I will check the gap on the plugs later today and report back.  The guy I got it from put in new NGK plugs, so I hope they were done properly.  I read somewhere these engines don't like NGK?  But I also read another post saying they don't like Champion, so I think it might just be personal preference. 

 

How do I test the sparkplug cables?  Do you check resistance with the plug in or out?  I have unplugged them on both sides and tested, and the longer lead is quite a bit more resistance than the shorter.  I think it is 2 or 3 times more.  I can't remember the exact number.  I always have more spark when I plug either cable into one side of the coil. 

[Martin 2,131]

RS17YX is the best plug to use for these. others will have a different opinion, but I've read more positives about that plug than any other and i have no complaints running them myself. sounds like whatever sort of bridge or connection they used inside the coil to connect the two secondary terminals may have a weak or high resistance point. there would be some sort of connection there as the coil fires both at the same time and there is only one secondary coil......what does the 'socket' or hole that the wires sit into look like? are both of them the same, or is one corroded or different than the other?

 

its a ***** that the coil is maybe just weak on one side and the rest checks out ok.

 

just an oddball thought here. i wonder if you could put a "Y" in the good side and temporarily see if it outputs the same type of spark to both plugs? even rig something up so both of the leads are exactly the same connection and no advantage or disadvantage to either side. I'm not saying run it like that but you could turn it over slow and see if you get the same spark (however it looks) from each......

(maybe this is a stupid idea too)

[Save Old Iron 1,566]

  I always have more spark when I plug either cable into one side of the coil. 

sounds like a bad coil - sound troubleshooting has lead you to the fact the one coil winding is weak

 

not an uncommon problem at all

 

in regards to the spark wire resistance, carbon core wire is spec'd out in ohms per foot. if one wire is 2 to 3 times longer than the other, it will have a resistance 2 or 3 times the shorter wire.

[Save Old Iron 1,566]

regarding the change in positive coil terminal voltage from 11 to 12 volts, when a coil is drawing current, any unwanted resistance between the battery + terminal and the coil + terminal will drop battery voltage before it reaches the coil

 

when the coil is not conducting. no current flows to the coil and resistance in the supply wire to the coil will no longer drop any voltage - so coil voltage goes back toward battery voltage

 

if this issue persists, wiring will need to be checked between the coil and the battery. this means ignition switch connections, fuse block connections, etc.

 

if you want to measure the total voltage drop between the battery and the positive coil terminal, put the positive meter lead on the battery terminal, and the negative lead of the meter on the coil positive terminal

the meter will be set to read volts on a low voltage scale. the voltage on the meter will be the difference between the battery voltage and the coil voltage or "voltage drop" across all the wiring powering the coil. this drop should be very small, about 0.1 to 0.2 volts.

[GH720 0]

Thank you everyone.  I checked the gap on the plugs, and one plug was off quite a bit.  It was around 0.035" when the gap should be 0.025".  I regapped it, but it didn't make any difference.  Just to confirm again, I switched cables and plugs around and in all combinations, the one side of the coil was always weakest.  I will try to order an ignition coil on Monday.  When I get results, I will report back (which may be a couple weeks).  Thanks again.

[GH720 0]

One last question.  I read about a different way to test the ignition module.  In this method, they recommended removal of all wires from the coil and then jumping 12 volts to the red wire of the ignition module.  Then they recommended turning the motor over by hand while measuring the voltage between the black wire from the ignition coil and ground.  They said I should measure 1 volt for half a turn, and then 12 volts.  When I perform this test, I get zero volts for half a turn and 0.6 volts for the other half.  This is quite different from what I got when I had all wires hooked up.  Is this method of testing valid, and does it point to a problem with the ignition module?

[Martin 2,131]

i would think if you had spark when rotating the engine the ignition module is good, its only purpose is to trigger at a specific point the spark. 

 

i would be careful testing the ignition module any way other than what is recommended in the manual, if it tests ok leave it alone..... one mistake while testing and you will be replacing it. theres not much on these engines thats cheap to replace......

[GH720 0]

Thank you to everyone for your help.  I replaced the ignition coil, and my Grasshopper is running again!  Here's the details:

 

Original Part #:  166-0761  (this had plastic flanges with holes for mounting on engine; this is no longer available.  Had to buy the retrofit)

Retrofit Kit Part #:  541-0522 (this has the coil with no plastic flanges.  Instead, there is a steal clamp included that holds the coil and this clamp can be mounted to the motor)

 

The Retrofit kit came with instructions for 3 or 4 different types of installations.  None of them matched my engine exactly.  Many of the installations required drilling holes in the shrouds around the engine.  Fortunately, I gave up on the instructions and tried to figure out how I wanted to mount it and noticed that the holes in the new clamp lined up perfectly with some already existing holes (I re-used one of the original holes for mounting the ignition coil, and another was a bolt holding on a shroud that now serves double duty).  I was able to place the coil in the original hole, but I believe it mounted about an inch higher (but it still clears the air box fine).  The retrofit kit does not have any spare nuts for the primary, so don't lose those when taking the old one out (like I did).  The retrofit kit is also missing one bolt for "clamping" the ignition coil, but if you have any spare shop supplies at all, you will likely have a small bolt that will work.  So it took me a lot longer than it should have to install the coil - this is a case where throwing away the instructions would likely help

 

I got the part from Cummins in Calgary, Alberta.  They stock this part (which I'm told means they sell more than 4 per year).  My cost was $181 CND.  I see this part on line for around $99 US.   Yes, I know I paid a lot.  The grass is green, and I didn't have time to find a US dealer that would work with me and send it via USPS (so I don't pay high brokerage fees) and then hope it doesn't get tied up at the border. 

[Save Old Iron 1,566]

good news

[ericj 1,578]

i found this posting a little late but you said your engine oil was over full. i don't know why but every time , the first time you check the oil it will read over full wipe off and put back in and get a proper reading. hopefully you find this before any thing happens to your onan

 

 

 

eric j