Need help for TCI box compatibility

[Augshire]

Hey guys
Sorry about joining the forum just to get answers on one bike, I'm fixing a 1982 Yamaha xs400 DOHC Seca for a friend for pretty much next to nothing. He's a tightwad hipster that believes he can sell a running old ass bike without a title for close to 2 grand, you know the type

Anyway long story short I've gotten his electric system up to snuff after soldering in a new fuse box. However I noticed when I pulled each spark plug to check the spark I wasnt getting any on the right cylinder. Pulled the coils (god what a nightmare that was) and there was no reading from orange wire to spark output so I figured it was a bad coil

Replaced the coil (and all new plugs wires and boots too) and still same thing. Only spark it gets is when I depress the start button and it gets a single spark sometimes.

I've checked the wires from the tci box to the sparkplug boot and it's got perfect reading so let's just say the wiring harness is good. The battery is always charged up as well so I know it's getting enough juice. From what I've been reading I'm pretty much down to faulting the tci box itself. Would love to just buy one and plug it in but cant find an exact replacement on Ebay and theres only one new aftermarket model that's close to $300

My question is can I use another TCI box from something like a xj550? Cheap aftermarket replacement on ebay for around $35. Looks like the exact same plugs and the xj550 runs 2 coils with wasted spark so would it work on xs400 with 2 coils?

Sorry about the column of jabbering, I'm a vintage Honda guy and I would take dicking around with points over electronic ignition anyday of the week.

Here's a pic of the box

 

[CaptChrome]

Welcome to the forum. Vintage Hondas share the same troubles as vintage Yamahas, and Suzukis and Kawasakis and ....

A good used TCI might take a bit to find. You might consider posting a Wanted add in the Classifieds forum here as well as watch eBay like a hawk.

Regarding using the XJ TCI, I don't know for sure if it will work using the electrical connections on the box and the pickup coil arrangement, but I can tell you that the two bikes have a little bit different timing curve.

At 1200 rpm idle, the XS has a 10deg BTDC timing
At 1200 rpm idle, the XJ has a 5deg BTDC timing

The timing is advanced for both bikes as revs increase:
The XS advances to 39+/-2deg at 5500 rpm
The XJ advances to 38+/-2deg at 4000 rpm

So, the XJ box might work, but I have no idea of how the XS might run with this timing. You probably won't damage anything in trying it. For 35 bucks it seems worth a try. The bike might be a bit unrefined in how it responds (but tightwad hipsters might like that.) If you do end up trying the XJ box, at least give us one more post to let us know how it goes.

 

[gpounce32768]

Got the same bike and the same friend lol... .

My TCI is starting to look like a culprit also. Be sure to check resistance on the two pickup coil wires- these are the crankshaft ignition advance coils. They can be checked right at the TCI connector. My bike dies after about 10-15 mins riding at road speeds and needs 5 to 10 mins to cool off before it starts again. All the easy stuff is checked- my final test is to get it to stall again and re-measure the pickup & ignition coil resistances in the faint hope that its a hot engine fail in one of them. If not, then my TCI is the culprit. If/when I get to that point then I'll be into replacing capacitors in the unit- if THAT doesn't fix it then I'm dead in the water.

Sure not going to spend even more $$$ for an aftermarket TCI so if I don't fix it that will pretty much leave the bike dead pending finding a cheap replacement. Which would be a bummer because it only has 2500 or so miles, runs well and I've fixed every other damn thing on it

 

[Augshire]

Not gonna lie I just bought a used box on ebay from an '82 xs400 maxim. I managed to mess with the original box a little yesterday and got the bike to run on the right cylinder but the left had a weak spark.

Anyway I saw another on ebay for around $100, but it's from Australia and he isnt showing the faceplate, I'd ask the exact ID number to make sure

Best of luck man

https://www.ebay.com/itm/1852774181...rentrq:07122cb71890ad33d3ba1aa3ffff7040|iid:1

 

[CaptChrome]

If/when I get to that point then I'll be into replacing capacitors in the unit

I firmly believe that these can be fixed, and if you are going in for component replacement, also replace the two coil triggering transistors. They have been known issues. Those transistors probably aren't made any longer, but there will be replacements that have similar characteristics. The SOHC bikes originally had ETD41 transistors - the current replacement for those is the ST901T.

 

[gpounce32768]

I firmly believe that these can be fixed, and if you are going in for component replacement, also replace the two coil triggering transistors. They have been known issues. Those transistors probably aren't made any longer, but there will be replacements that have similar characteristics. The SOHC bikes originally had ETD41 transistors - the current replacement for those is the ST901T.

Ah, good idea- easy enough to crossreference them as needed; transistors certainly can go bad also. The ST901T is certainly a plausible transistor for the application. I'll document with photos, measurements etc if/when I go in.