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Posted

Looking for some guidance/tips on troubleshooting strategy. My Mooney has never been an easy starter. I actually recall the previous owner telling me it seems to start when you let off the key. Didn't think a ton of it, but with LOTS of starts in my back pocket, it's 100% the case - holding down the starter never results in a start. When I release the key and catch a compression stroke, it lights right up and runs perfectly. Until yesterday, I just managed.

Yesterday, away from home base, I went through the battery. Luckily, the FBO had a start cart, and with some help from a lifelong Mooney owner, we were able to FINALLY get it started. After about 20 attempts, he was pretty frustrated too. When it finally caught, it was that magic timing of key release and catching a compression stroke. The flight home was uneventful. I'm grounding myself until I get this fixed.

My mechanic is coming out Wednesday as soon as this snowpacalypse passes - but want to get a strategy so we don't just throw parts at it. I spent the whole day today reading through every thread here on MS - and it seems this symptom is not unique and can come from a number of sources.

A/C: 1970 M20F (IO-360), Mags are S4LSC-200 & S4LSC-204, serviced by Aircraft Magneto Service about 200 hours ago. Preflight mag checks are perfectly normal. Original SOS system (although I have a new in box SlickStart - but want to get this sussed first). 

Potential culprit 1: the switch - if anyone has the Bendix switch manual - I would greatly appreciate it. I'd like to test the resistance on all positions. I did find several threads where the switch was indeed the culprit. My logs are very complete and as far as I can tell, it's original. I *think* the current part number is Bendix 10-357210-1 (please confirm), I see several switches on eBay for $100 with this part number. Can anyone confirm that a "10-357210-1 is a 10-357210-1" - ie doesn't matter what a/c it came from? 

Potential culprit 2: something is messed up in the mag timing, retard gap, some kind of loose/frayed wire etc. My mechanic is going to bring a mag synchronizer and we'll check timing, all the leads, P-leads, etc.. Open to any troubleshooting or "gotchas" you may have discovered in the past.

Potential culprit 3: the mag might be failing or has something bum inside we wont see - I'm not opposed to sending it off to one of the quick turn inspection places, but would like to get after the free stuff first. But $500 is not a terrible price to know for sure the mag is ok.

Potential culprit 4: the SOS - I can 100% hear it if I turn the key to start (and not press in) - it buzzes loudly. I *assume* buzzing means working. When I push in, I can still hear it buzzing, so it's not like the switch is cutting out the SOS or anything. I mentioned earlier i have a new SlickStart in the box ready to install, but don't think my SOS is bad - but would like to install it later once this problem is behind me.

Am I missing any other options to investigate? I'd of course like to start with the free stuff and start buying parts as we eliminate free stuff. 

If anyone has a switch taking up space, I'd love to be a buyer (or a borrower) - to at least eliminate that possibility. 

And would love any testing/troubleshooting advice we can do on the switch itself.

Thanks all! 

Posted

It is almost certainly the mag or the switch.

I had this happen to my M20F. It was the wire in the retard mag on the retard points. The wire was touching the inside of the points housing and had worn through the insulation and was shorting out the retard breaker. I just flipped the spade log 180 degrees which moved the wire position and it worked fine.

  • Like 1
Posted

I’m putting my money on the left mag.  The retard circuit is not firing. When you are cranking the starter the right mag and the left mag run points are grounded and won’t fire. As soon as you release the starter the right mag and the left main points fire. That’s why you only get a start then. I’m surprised you haven’t had a kickback and broken your starter. 
I fought this issue intermittently after installing new mags. Replaced the starter switch to no avail. Turned out one of the plug wires was shorting out the retard breaker terminal on the left mag. My A&P/IA couldn’t find it but I did after intense scrutiny.  Rerouted the plug wires and installed insulating boots on EVERYTHING under the cowl. 

  • Like 1
Posted
1 hour ago, Buckeyechuck said:

I’m putting my money on the left mag.  The retard circuit is not firing. When you are cranking the starter the right mag and the left mag run points are grounded and won’t fire. As soon as you release the starter the right mag and the left main points fire. That’s why you only get a start then. I’m surprised you haven’t had a kickback and broken your starter. 
I fought this issue intermittently after installing new mags. Replaced the starter switch to no avail. Turned out one of the plug wires was shorting out the retard breaker terminal on the left mag. My A&P/IA couldn’t find it but I did after intense scrutiny.  Rerouted the plug wires and installed insulating boots on EVERYTHING under the cowl. 

Did you replace the ignition harness? Or did you just reroute everything and lots of shielding? 
I need to look how old my ignition harness is. I bet it hasn’t been replaced since the overhaul, which was 17 years ago. So that’s not a terrible option. I need to get a quote from Maggies. The off the shelf options look to be $600. 

Posted

The other thing to check is to make sure the left mag is wired to the left position on the switch and vice versa.

Most times, the start position grounds the non-starting (impulse or SoS) mag.   So if they are wired to the wrong position, when you are cranking your starting mag is grounded.  And when you release the key, it is ungrounded and fires.

  • Like 1
Posted

I'm going to be honest, I had a pretty similar problem the first year I owned my plane.  I decided I did not want to be left somewhere on a random field due to this silliness and the age-old SOS.  So, I replaced the left mag with a SureFly and I haven't looked back a second.  Do I get better fuel burn?  No idea... Did not really notice a difference.  What I did notice, though, was every single start has been easy.

Posted

Oh yikes…a $2k part. I assume I’d need to have a wiring hsrness built for that side (it’s Slick right)? This eliminates the SOS completely right?
We’re going to dive into the free stuff and make sure something isn’t backwards, wrong, frayed, etc.. 

It’s not a terrible investment as there’s no inspection costs / so over the long run they pay off. 

Posted

Mine had the same problem but it was me hanging on the key like an ape. I’ll need a switch repair soon but twisting the key without radial force is working for now.

Posted

Check the left magneto to see if the retard points are working properly. We recently had this problem during an annual and there was a broken part in the mag preventing the points from closing. The mag was 400 hours post IRAN.

Posted
5 hours ago, Rwsavory said:

Check the left magneto to see if the retard points are working properly. We recently had this problem during an annual and there was a broken part in the mag preventing the points from closing. The mag was 400 hours post IRAN.

The retard points are our #1 inspection. If I have to send the mag off, I might just bight the bullet and go SureFly and be done with this. If I go that route, I can sell my SlickStart (still new in box) and won't be out a ton of money. The e-mag a/c I have flown are STUPID easy to start. Literally like starting a car. How I dream of that...

  • Like 1
Posted
On 1/6/2025 at 4:36 PM, bigmo said:

Oh yikes…a $2k part. I assume I’d need to have a wiring hsrness built for that side (it’s Slick right)? This eliminates the SOS completely right?
We’re going to dive into the free stuff and make sure something isn’t backwards, wrong, frayed, etc.. 

It’s not a terrible investment as there’s no inspection costs / so over the long run they pay off. 

Yep! No more SOS.  One less part to mess with when we removed that.  You're correct, it was expensive... But I haven't looked back a second since upgrading to the SureFly.  I made a long post about my troubles and how we installed it, if you're interested, it's on my profile.

Posted

Mine did the same thing.  Your SOS is probably dead.  Do you hear it buzz when you are in the start position?  Go ahead and install your Slick Start and it will probably cure the problem.

Posted
3 hours ago, Jsno said:

Mine did the same thing.  Your SOS is probably dead.  Do you hear it buzz when you are in the start position?  Go ahead and install your Slick Start and it will probably cure the problem.

Yes, I can hear it buzz when in the start position (but not pushed in). When I push in, I think I still hear it, but it's hard to really know for sure...but I can still kind of hear the buzzing/rumble. 

But I am assuming buzzing means working - and I dont know if that's the case.

Posted
19 hours ago, bigmo said:

But I am assuming buzzing means working - and I dont know if that's the case.

As @Jsno says "buzzing is good".  No buzz=no good.  But you still need to confirm  that it's working.  Here is an article from DMAX that discusses some troubleshooting steps.  Read and understand the whole document.  Don't get whacked by the prop.

Shower of Sparks by Don Maxwell (mapa log 31.03).pdf

 

  • Like 1
Posted

I’d check the back of the Bendix Switch, there is an AD and replacement repair kit available for less than $100. You can try polishing the contacts and adding new grease, but I would probably just replace it if I went to the effort of taking it out. The repair kit part number i10-357515

  • Like 1
Posted
23 hours ago, BillyT0020 said:

I’d check the back of the Bendix Switch, there is an AD and replacement repair kit available for less than $100. You can try polishing the contacts and adding new grease, but I would probably just replace it if I went to the effort of taking it out. The repair kit part number i10-357515

Thanks! I had a hard time locating the repair kit and scored a "new old stock" - but in the box switch. I can;t recall what revision, but it was made in the early 2000's. It was -2F this morning, so as soon as the weather breaks, we're going to get after this. The switch will go in regardless. 

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