81 XJ750

So attach the multimeter to both the red and orange/grey at the same coil correction? Sorry stupid question but better thab a stupid mistake

 

Check from the orange and gray to chassis ground either on the coils or on the TCI, whichever is easiest for you. If on the TCI it's the 4 pin connector, not the 6 pin which has an orange and gray pickup wire.

You will always have 12V on the R/W wire if the ignition switch is on and the kill switch is set to run. You are trying to determine if the TCI can control the negative side of the coil. If it has shorted output transistors those lines will be near ground and stay there

 

Ah okay, I'll give that a shot and reply. Thanks for the advice

 

So from the coils, I have 0V going from the grey/orange to ground. With or without cranking. Same with the connections on the TCI, the wires (orange and grey) running into the TCI have 12V, both drop to 10.5 when cranking, no fluxuation.
Would this indicate a blown transistor in the TCI?

 

Well it can't be both zero and 12V. With the key on and kill switch set to run you should have near 12V on the orange and gray wires on the coil, and the orange and grey wires on the TCI - note there may be a 3 second delay on one of the two as mentioned earlier.

 

It is value added to be patient and try several ignition on off cycles waiting 30 seconds or so between each to see if the orange or gray wire on the 4 pin TCI connector goes low for about 3 seconds.

The TCI will not become active without a signal from the pickup coil, and I don't see where you checked those.

https://www.xjbikes.com/forums/threads/the-information-overload-hour.27544/

"XJ750 air-cooled models:

Pick-up coils:
650 ohms +/- 20% = 520 ohms to 780 ohms acceptable range


Ignition Coils:

Primary side (input from main wiring harness):
2.5 ohms +/- 10% = 2.25 ohms - 2.75 ohms acceptable range

Secondary side (spark plug wires, without their end caps):
11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range


Spark plug caps:
1981-83 models: 5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range
1984 RL models: 10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range

Spark plugs:
0 ohms per plug"

 

Pick up coils are good
So are the coils, plug caps and plugs.
I've tried several attempts to get a reading to no avail.
I'm meant to be check on 20V DC with the meter correct?

 

Opened up my TCI just to see if anything looked off, top side it completely chared and as soon as the lid came off I was hit with a burnt smell. Gonna work on finding a new one.

Any Idea of what may have caused this?

Not sure if it happened while I had the bike or before the bike was parked. Worried that if I put a new one in I may just toast it agajn

 

If you have difficulty finding a replacement TCI unit you can buy a new aftermarket one.

https://xjbikes.com/forums/threads/aftermarket-ignition.45931/

 

I kind of ignored that because I don't know of any connector that connects to the TCI and rectifier? Which connector is it really? Does it plug into the TCI 4 pin connector?

Was the ignition fuse blown when you got the bike? That would have been a clue that something was wrong. Other than that maybe a battery reversal at one time. Or, if an ignition coil primary shorts out then the TCI will likely be damaged and the transistor would overheat and possibly short out. At that point it would just be the fuse protecting things, but the etch could be damaged in the process.

 

Fuse was not blown, when I got the bike though it had the original fuse box, the main fuse slot was a bit melted but the fuses were intact.

Also done goofed on those wires, 2 white wires from the rectifyer spliced into a single white wire running into the main harness. It had obviously been redone (very very poorly) so I redid that section of wiring completely.

Also possibly tracked down a TCI from a local partout. I'm going the check the coils again and make sure theyres nothinh that would blow this one. I've realized how hard they are to come across either locally or for cheap in known working condition.

 

Well found a part out with a good, tested tci. Put it in and I got spark! But only on cylinders 2 and 3 lol. Now I have good power to the coils for 2 and 3, but on the coils for 1 and 4 I get 4-6V cranking. Why would it be so much lower?

Also side note, battery wasnt fully charged but it wasnt low either. I'm charging and retesting it.

Also looking for that huge no spark checklist/diagnosis forum, if anyone has the link to that it would be greatly appreciated.

 

https://www.xjbikes.com/forums/threads/how-to-ignition-troubleshooting.21932/

Excessive load would cause a low reading, but since the R/W wires are all tied together with a splice within the harness then both coils should be low, as well as any other R/W wires.

The more likely problem is the splice in the harness where R/W wires are joined together has an issue. You might be able to diagnose that with an ohm check by measuring from the ignition fuse to the coil wires. The one reading low voltage may show up as a couple of ohms, which would cause excessive voltage drop

 

Checked the fuse to coil connection, had about 0.68 ohms of resistance on the 20k setting, presuming next step would be to cut open the harness wrap and trace the wire down from where it splits, looking for something wrong.
(Also flipped the coil connections and got good spark on the other set of coils)

And thank you very much for no spark checklist

 

So that would be 680 ohms, which would be way too high. So did the other coil R/W wire measure near zero?

Check the end you can get to really well before going into the harness - back probe on the coil connector, a good visual inspect of the crimp. It could be what ever burnt that TCI etch also did damage to that wire / connector along the way

 

Yea got 0 ohms to the other coil.
And thank you I'll check out the connector and crimp. Hopefully its that simple.

Also now with the new tci, power on the working coil is heavily fluxuating. Ive gotten readings like -0.02V to -5.35V but it is sparking so. When the key is turned, not cranking, its on it reads 0V. But on the bad one it reads 12V and drops to 5V when cranking. Just seems off to me.

 

So did some investigating and found that there were no flaws wity tge red/white wire. Which confused me. I tested from where the red/white split off from to each ground wire at each coil. And figured that since I was getting the same results as prior, that it had to be the ground wire. Traced the grey back and found it was taped off by a previous owner and burnt.

 

It can't be both

 

Sorry meant after the splice. I've redone the splice and replaced the wiring from the splice to the coil. Ohms are a bit lower at 480, still no spark. I checked the ohms from the fuse to the splice and got simular readings.
Im just confused bc if I have a high ohm reading at the splice how would I have power to one coil and not the other? Could it be an issue with the grey wire instead?

 

Alright, so got another TCI. Figured the other was bad bc when I fliped the grey and oranger connectors at the tci the spark flipped. But with another new tci I'm still facing the same problem. I know my wiring after the TCI (to the coils) is good bc flipping the connections at the tci causes spark on the other cylinders. I've checked my pick up coils and they were both in range (650ohms and 630ohms) any ideas of what it could be? Did I just happen to get another bad tci?