81 XJ750

Hey Noobie here. Bought a 1981 XJ750 that sat for 8 years. The bike cranks over, but I have no spark. Great conpression. Removed the safety relay and still nothing, I have power to the coils, but im not sure if its enough and im not sure how its supposed to change when I crank the bike. Any advice would be amazing. Im thinking of replacing the coils wires and caps. But im worried I missed something in my diagnosis.

 

Is your battery new? Check the fuses old Yamaha glass fuses clips get weak, common electrical problems because of this. Replace with a blade type fuse box. Make sure all terminals are clean. Don't replace parts, test them then replace might be a little corrosion on the end of the coil HT wires. Put a meter across the battery terminals and press the starter button, make sure the voltage does not drop below 10V under load. Carburettors will need attention too. You should have battery voltage to the coils no voltage drop at the low tension wires feeding the coils.

 

I put a brand new battery in the bike. And replaced the main and ignition fuses with inline fuses temporarily until I can find a proper blade style box to wire up. Testing the wires feeding the coils, there is a very minimal drop when I crank the bike over. Ill try cleaning the spark plug caps and double check the resitance through the coils, and through the wires. I know the resistance between the caps are about 5-6k ohms.

 

Clean all the connections on the starter circuit you can and the grounds to the frame. Check also your switch housing on the handlebar has not corrosion between the case and the handlebar, it earths to the handlebar. The spark is weaker on the TCI machines than on the CDI, and is harder to see depending on the light conditions. If you have no spark on all four it is a waste spark system so I think the coils are not the problem. Faulty kill switch, side stand switch needing cleaned, connections can get corroded. Have you tried for a spark with the bike on the centre stand and the side stand up?

 

https://xjbikes.com/forums/threads/no-spark-1982-xj750.119449/

 

Yea ive got no spark across all cylinders, Ive cleaned the grounds and most connections from the tci, but ill start cleaning around the start circuit. Alongside checking the connections around the handlebars. Ive been trying for spark with the side stand up, clutched pulled and in neutral. Thank youu

 

@SQLGuy read his write up here.

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

 

Will do thanks

 

Hey, just wondering. I have power to the pigtails that connect to coils, 20V stagnant and then 15V while cranking. Are they supposed to pulse while cranking or are they in proper spec? If not would that mean the tci is bad?

Also Ive cleaned and checked all the wiring in the starter circuit, and anything leading to the tci. Tci Recieves proper power. Ive check and tested all safety switches, fuse box is good and all the wiring around the handlebars are good including the killswitch. By process of elimination im down to the tci or the coils/wires. I think im going to go ahead and replace the coils/wires bc its cheap and easy to do and probably should be done for proper matenance after the bike sat for so long.

 

Usually if there is a problem with the TCI one transistor affects a pair of cylinders. Only way to test it is to put it into another XJ to see if the bike runs. I think the TCI is not the problem. How can you have 20 volts from a 12 volt battery? As for the pulse do you mean the power being cut same as it would with points in the old days so it generates high voltage in the coil secondary windings?

 

Sorry no clue where I got those results or why I didnt question them. I retested with a different ameter and got;
Primary coil: 9.7V drops to 7.5V cranking.
Secondary coil: 11.5V Drops to 9.5V cranking.
And yes sorry did some more research and realized I was completely off with that. Again still new to this but trying to learn.
Anyway I just ordered new coils + wires + caps and hopefully this fixes my issue. Checking the spark plug wires, wire to wire they were not reading within spec. The ones I ordered were meant for a 83 cb750, another thread mentions that they are compatable but I will double check the ohms before hand.
Just wanna say thank you for all the help and advise also.

 

That voltage is too low to create a spark. You need ten volts minimum when pressing the starter button for the transistors in the TCI to work.

 

I keep a bigger test battery around the shop just to check for spark. I hook it to the bike with thick jumper cables and crank the bike. Turning the lights off in the area also helps to see if you have spark. Looking for a good blue spark not a yellow color. Any spark is better than no spark thought.

 

Ah okay, so im probably losing power somewhere. Ill try a different battery just in case. And go over all connections.

Edit; Tried a larger battery, no change. Losing power somewhere along the way.

Replaced the ingnition fused, brought me up to 11V on either connection to the coils. But both are dropping to about 9V whilst cranking

 

Take the time to look at some of the connectors. I know it sounds like busy work, but it is not. Finding that one connection that is maybe green or worse and is not making good contact will really help out your troubleshooting. Do you have a manual yet? Do you have a multi meter? Those will be your next steps in tesing the coils and other parts of the bike to make sure it has good connections and the specs are correct for that parts that are making voltage and in the starting ckt.

 

I have both, checked all connections and cleaned. I get decent power to the coils, especially when boosting the battery. 14V and then 11V cranking on both. Checked the primary wiring on the coils is only slightly high in ohms. Second I cannot even get a reading on. Also realized I realized the mistake of running BPR7ES instead of BP7ES. I replaced thoses with some autolight equivelents for the time being. New coils/wires arrive tomorrow so hopefully that fixes the problem.

 

Think I might have missed this one, connects to the rectifyer and tci. Gonna replace the whole connector bc I dont think I can save that.

 

Good chance that is the issue.

 

Turns out it wasnt. I am reallyyyyy struggling with this bike.

I have new coils/wires/plugs. Proper spark plugs, all connections I can find have been cleaned/replaced. Tci gets enough power, coils get enough power, coils become warm when trying to starting bike?? All relay switches click, cleaned and checked clutch safety switch, and same for the side stand switch. Ive cleaned the starter button, ignition switch and kill switch, checked all wiring behind the head light (which is burned out). And I have a good battery.
I am at a total loss, last thing left would be the tci correct? Or am I forgetting something stupid?

 

That really shouldn't happen unless the TCI has shorted transistors. If you put a DMM on the Orange or Grey wire and turn the ignition key on one of them should go low for about 3 seconds. Knowing which one (orange or gray) is dependent on the position of the reluctor, so just as long as one of them does. Then after that 3 seconds or so both should remain high (near 12V) until the bike is cranked.

Note also when you see this happening a spark should be produced on the plugs for whichever coil went low and then high.

 

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?

 

So you can't have 480 ohms from the ignition fuse to the coil positive, it is not going to work. Perhaps it's a communication issue here and I don't understand what you are saying.

Be sure to check the pickup coils to ground also; the pickup coils should be open to ground

 

Ive fixed the issue with the resistance from the fise to coils. The pick up coils read 650 and 630 ohms of resistance to ground.
When I flip the grey and orange wire at the tci the spark flips.

 

OK, so now when cranking you have close to a solid 12V on both coils R/W wires?

The 650 to ground is OK if they are connected to the TCI and that's how you are measuring.

The troubleshooting guide suggests this, did you do this step?

5a. Switch the Orange and Grey wires from the pickups (6-position TCI plug) at the TCI. If the sparking and non-sparking pairs of plugs swap (i.e. if you had spark on 1/4 and not on 2/3 prior to swapping Orange/Grey and now have spark on 2/3 and not on 1/4) then you've got a bad pickup. If this didn't make a difference, swap the wires back and go to 5b.

If you don't get a swap suggesting a bad pickup and you have already done the coil wire swap then yes it would seem the TCI is bad. We often suggest the best test for a TCI is in another bike, or trying a known good one. Unfortunately, that is not always easy to accomplish. Could you have 3 bad TCI's, not likely but not impossible? Were they advertised as working units?

What is the voltage on the 4 pin orange and grey wires when the ignition key is turned on - remember the 3 second thing.

 

Yes I have good voltage on both red/white at the coils.
Ive done that step and rules out the pick up coils, when flipping the orange and grey on the 6 pin it makes no difference.
When I flip the orange to grey on the 4 pin the spark transfers to the other coil.

Im starting to think something on the bike might be frying that tci channel when I connect it? If so could it be a short somewhere? Leading to the tci, after towards the coils or both?

Because 2 in a row from a working bike seems a bit absurd to me. I got the tci's from know working parts bikes, off a friend who I know wouldnt try to screw me on something like that.

 

So the TCI is meant to drive a very low impedance primary coil, but that doesn't mean you couldn't damage one. However, to damage those output transistors you would pretty much have to short the ignition coil primary out, or apply 12V directly to the low side of the coil.

Odd that a working parts bike would have two TCI's, and that both were tested on this working parts bike?? If it is a working parts bike and a friend then just try them on that bike again.

I will ask again

And you could pop the cover on one of those new ones and look for damage like the original TCI where you found the burnt etch.

 

I will check the TCI's again on his bike, he's a motorcycle mechanic by trade and has a yard full of old bikes like that. TCI's didnt come from the same bike but he had one in a parts bin, another on a 81 maxim 650. From a quick look under the cover on the TCI's everything seems in order, no burnt smell like the first either.

I just dont know what would cause my no spark if not the tci. I know the wiring to the coils/plugs is good and I know my pick up coils are good