Help with ignition harness lead lengths

[sleeper-319]

Hey all, I’ve got a bad left ignition harness and need to replace it. Problem is I’m not exactly sure which one to buy. My plane has the ModWorks STC that replaces the IO-360-A3B6D with an A3B6 (two separate mags). 

The ignition harnesses that are installed M-4020 (RH) and M-2019 (LH) have fairly significant extra un-needed length on some plug leads (all the bottoms); as much as 7-10” of extra length. The top plugs have minimal extra length, if any at all. The slick application guide says an A3B6 should take M2506 (LH) and M-2507 (RH).

Given how expensive these are, I want to get the right size. Is there any way to know the lead lengths on the 2506 & 2507 so I can be sure it’ll fit before I buy them? Or does anyone else with this same STC know exactly which one I should buy? The STC paperwork doesn’t mention anything about the harness. 

Thanks. 

Plane is an 87 M20J; IO-360-A3B6

 

 

[Ragsf15e]

36 minutes ago, sleeper-319 said:

Hey all, I’ve got a bad left ignition harness and need to replace it. Problem is I’m not exactly sure which one to buy. My plane has the ModWorks STC that replaces the IO-360-A3B6D with an A3B6 (two separate mags). 

The ignition harnesses that are installed M-4020 (RH) and M-2019 (LH) have fairly significant extra un-needed length on some plug leads (all the bottoms); as much as 7-10” of extra length. The top plugs have minimal extra length, if any at all. The slick application guide says an A3B6 should take M2506 (LH) and M-2507 (RH).

Given how expensive these are, I want to get the right size. Is there any way to know the lead lengths on the 2506 & 2507 so I can be sure it’ll fit before I buy them? Or does anyone else with this same STC know exactly which one I should buy? The STC paperwork doesn’t mention anything about the harness. 

Thanks. 

Plane is an 87 M20J; IO-360-A3B6

 

 

I recommend New Horizons/Maggie harness.  They are good, sell one side if that’s what you want, and have good customer service.  If you call and talk to them, they’ll make you a harness.  Know which plugs exactly are coming from your left mag and tell them.

[sleeper-319]

I went to Maggie first. Unfortunately they are having fulfillment problems, and the ETA went from 3 weeks to “no idea.” I didn’t start with that because with that because they seem like wonderful people who are experiencing some unfortunate supply issues at the moment. :-)  Nevertheless, I’m AOG for a few weeks now and just want my plane to work, so if I can get a slick harness that works, that’s good enough. 

[N201MKTurbo]

The reason your bottom leads are too long, is because they are not routed according to the book. Six leads should go through the big hole on the baffles. Then four go to the right, through the mounting bracket in the center of the baffle. The leads for the right side bottom plugs go through the feedthroughs on the side of the baffles. The two left bottoms go straight to the plugs

Most I have seen route the leads straight to the bottom plugs and then the leads for the right top plugs are routed through the feedthroughs.

[sleeper-319]

I’ll take a look at the manual (but my service manuals probably are for the original A3B6D). When you say the six through the big hole in the baffle… do you mean the hole at the top rear near #4 cyl? So all the right side go through there, then the right bottoms go out the plastic clip on the right side of the baffle? Do you happen to have any photos of proper routing? (Edit: I’ve done some googling, and see some routed this way (but plenty routed the other wrong way too. Thanks for the tip. I’m still stuck on the “which harness do I need” question though. The M-4019 & M-4020 are not the right ones as far as I can tell).  

Also, the leads for the left side bottom are probably the longest. #2 bottom has the most excess, at maybe 10”.  Do you know what harness is supposed to be installed in this application? 

Edited March 30, 2024 by sleeper-319

[sleeper-319]

On 3/30/2024 at 2:24 PM, N201MKTurbo said:

The reason your bottom leads are too long, is because they are not routed according to the book. Six leads should go through the big hole on the baffles.

I was able to get the lead lengths from Spruce, and the M-2508 kit should work. Interestingly, both harnesses (installed and one the slick application guide calls for) are way too short to be routed this way. 1 & 3 bottoms are like 10+ inches too short. Anyhow, with luck, I should be back in business soon.

[N201MKTurbo]

On 3/30/2024 at 3:36 PM, sleeper-319 said:

I’ll take a look at the manual (but my service manuals probably are for the original A3B6D). When you say the six through the big hole in the baffle… do you mean the hole at the top rear near #4 cyl? So all the right side go through there, then the right bottoms go out the plastic clip on the right side of the baffle? Do you happen to have any photos of proper routing? (Edit: I’ve done some googling, and see some routed this way (but plenty routed the other wrong way too. Thanks for the tip. I’m still stuck on the “which harness do I need” question though. The M-4019 & M-4020 are not the right ones as far as I can tell).  

Also, the leads for the left side bottom are probably the longest. #2 bottom has the most excess, at maybe 10”.  Do you know what harness is supposed to be installed in this application? 

Are you sure you are putting them on the correct mag?

BTW, I'm not faulting you, it is a common problem. The lead sets should come with a routing diagram.

FWIW, The leads on my old Cessna are supposed to be routed in this crazy complicated way. I just did them the easy way. The engine in the Wikipedia article has the leads routed just like I did.

[sleeper-319]

Yep. I have everything labeled as it was when I took it off, so I just re-routed my current ones, and there's not way they'll work routed the way you describe. It's not even close for 1&3 bottom; they're like 10+ inches short routed that way. The ones coming from Spruce are little bit shorter on those particular leads, so if the ones I have don't work routed that way, the replacements probably won't either, but I'll check when they arrive to be sure.