1984 XJ750 - Rear lights not working

Hey guys, I'm new to the forum, and this will be my first post. Please excuse me if this is the same post as maybe 100 others, i did try do a search (don't know how well), but came up empty handed.

I am from South Africa, and I have just bought my first Yamaha XJ750 - 1984 model. For now I don't want to do any mods, but going forward, would like to give her a semi cafe racer type look.

Issues that the bike had are:

Front brakes spongy
Rear light stays on
BRK warning light stays on

I have figured out what is causing the rear lights to stay on, and also i think the front brakes being spongy - they are both the same thing - the previous owner replaced the stock MC with another MC, and that MC i think is leaking, and also the part that is supposed to depress the switch when the brakes aren't in use isn't there. So i will need to replace the MC that he has fitted with a new one - was aiming at the FJ1200 MC as mentioned in another thread.
The BRK warning light was due to the replaced MC not having the connectors of the old MC for the brake fluid- so i just bridged those, and that warning is no longer there.

The problem I'm having currently is that when i swicth the bikes lights on the rear lights don't come one. Just want to confirm the only way to really diagnose would be to follow the cable from the rear lights and check that it isn't broken, and that it is plugged in wherever it needs to be plugged in? Unless there is a quicker easier way?

Thanks in advance.

 

this is a UK model 750 ?
first thing you need is a wiring diagram and a multi meter or at least a test probe light.

 

Thank XJ550H, yes, I think it is equivalent to the UK model. So what I have managed to get done, is print out the wiring diagram, and follow the rear light cable from the rear light through to the connector that goes to the Atari.

I did a continuity test of the cable from the connector under the seat to both rear light bulb holders. I also did a continuity test from the connector under the seat to the connector that plugs into the Atari. The cable makes connection all the way through.

I measured the voltage on the brake light when engaged and disengaged and it read just under 12v why the brake is pulled, and just over 3v when the brake is not pulled.
The rear light reads just over 3v whether the lights are on or off. Also, the light that are supposed to light up the Atari don't come on.

Does this mean the problem is with the Atari? And does it mean that I would need to replace the speedo/Atari?

All the other electrical system work, indicators, fog light, high beam and brake lights.

 

The MC is lilely the culprit. The stock MC has a brake fluid level sensor, most aftermarket units do not. If it was not sized appropriately you will have a soft feel to the brakes, and if the brake switch was wired incorrectly (likely) the brakee light could always be on.

The ATARI uses a polarized film to make the LCD display visible. That film can degrade with time. Take a pair of polarized sungalsses and look at the LCD. If you see the LCD cycle during startup then the polarizing film needs to be replaced.

 

On the 1982 model the tail light, the license plate light, and the Atari back lights power is routed through a 5 amp fuse located in the headlight bucket. I thought the 84 version was different (No Atari) but can't remember for sure. Anyway, since you have the Atari I would investigate the 5 amp fuse. There would also be a separate output on the ignition switch that is used for this circuit so that the parking feature is available. When the ignition switch is set to "P", all of the above mentioned lights are illuminated.

 

Thanks Rooster53, the license plate light isn't working either, I don't recall there being a fuse, but will have a look, so I hope it is that!

 

page 219 of haynes manual


does not tell where the inline fuse is but it is someplace it is depicted in the wiring diagram

check page 251 for 5 amp fuse location

 

Found the sneaky fuse, hiding behind the headlight, must be honest didn't notice it at all ... the fuse passed the continuity test, and visual inspection. I thought i should re-seat the fuse to test, and ended up braking the fuse ... so as a test, i just bridged the connection, and turned on the lights to no avail. So sadly it seems that the fuse wasn't the problem. Will try get a new fuse tomorrow.

But if anyone has any other ideas that i can check? It would be greatly appreciated. I hope this doesn't mean that I have to replace the Atari!?!?

 

Check the voltage at the fuse terminals. One side should be at 12V with the ignition switched to "ON" or "P" and is a unique circuit for the tail, license, and Atari back lighting. If you don't have 12V on one side, then look for a problem with the ignition switch or associated wiring/connectors from the ignition switch to the fuse. If the electrical diagram is correct, the back light and license circuits do not route through the Atari so the Atari would not cause all three to fail.

Just in case you find a fault with the ignition switch - no 12V on blue wire in "ON" or "P" position it may be salvageable with cleaning:

http://www.xj4ever.com/clean and lube the ignition switch.pdf

 

Thanks Rooster53, the help is greatly appreciated, i will follow those wires, and check out the ignition switch.

 

I can't seem to figure out the rear lights, ground is definitely working as the brake lights are working, and I have done a continuity test on both the ground and the light cable. The voltage measures just over 3V from the connector from the Atari, and the connector under the seat, and at the bulb, no matter if the lights are on or off.

Is there a way I can bypass the rear lights from the atari?

As for the lights that light up the speedo, and the park lights, I haven't had a chance to service the ignition yet, but I suspect it might be the ignition, as I don't think I'm getting the ignition into the Park setting.

 

To bypass the Atari - can I jump a connection from the headlight, to the rear light cable?

 

there is a thread on bypassing the atari

 

So do you have 12V at the 5 amp fuse terminal with the ignition set to "ON" or "P"? As stated before, the blue wire from the ignition switch supplies power to the 5 amp fuse. None of the lights (tail, license, or back light) are going to work without 12V being present at the 5 amp fuse.

Or try this:

Disconnect the positive battery terminal.

Ohm from the battery Positive terminal wire to the 5 amp fuse holder with the ignition switch set to "ON". You should get a reading of near zero ohms if the ignition switch and wiring are in working order.

The steering will need to be turned to the left to allow the fork lock to set and turn past that point to get to the "P" position on the ignition switch.

 

It is unlikely that the problem is in the ATARI.

 

Ok, so today i have put the bike into park, and measured the voltage at the fuse. When i put the bike into park, the rear lights come on, and so does the front park light. Also when the bike is in park, there is just under 12V measured at the fuse.

When the bike is turned to "On", there is 0V measured at the fuse.

So the good news is that the wiring that runs to the rear light is working as it should as per my continuity test. And the bulbs are in working order.

I disconnected the Positive terminal from the battery, and turned the ignition to on, and measured the resistance from the positive terminal to the fuse, and i got a reading of 4.9ohms.

 

The 4.9 ohms is a bit on the high side. At the very least it should go lower and not higher if you wiggle the switch a bit. If you see it open when wiggling the switch that would definitely be a bad sign. You might even get the lights to flicker by wiggling the ignition switch with everything connected.

However, with that said, the voltage test is the best way to troubleshoot this. If you have 12V in the "P" position, and 0V in the "ON" position at the fuse, then there has to be a problem internal to the ignition switch. Since the "P" uses a different set of contacts on the switch plate, it is likely the equivalent contacts used in the "ON" position are worn / dirty and not making a connection.


Borrowed pic from XJ4ever cleaning main switch PDF. Note this is a 650 and more contacts will be present in the switch for the 750.

 

his ignition switch has 6 wires coming from it being the UK style it also has a switch that turns on his lights unlike the north american 750

 

Yea I was confused on that, and you have a very good point. The trouble may be the lighting switch that supplies power to that circuit. Need to check the blue wire at the input of the ignition switch for 12V. I am assuming the headlight works and you don't have something simple like a blown "Head" fuse or damaged / broken terminals. Hope you get this before digging into the ignition switch.

Also, the lighting switch appears to have a "PO" position (park only?). With the lighting switch in either "PO" or "ON" the 12V should be present at the blue wire on the ignition switch.

 

@Rooster53 - Yeah, the headlight works, all the fuses are good. But i will run through them again to be sure.

And then i will venture down the ignition cleaning route.

 

As I have looked a bit closer with the correct diagram (UK version) the Atari back lights receive power from the lighting switch and not through the ignition switch like other versions. Therefore, if the Atari back lighting is not working with the ignition in the "ON" position, then the issue is most likely caused by the lighting switch or power to the lighting switch. That same feed (blue wire) to the back lighting also goes to the ignition switch and is used in the "ON" positon only for the tail and license light.

If the back lighting was working in the "ON" position, but the tail and license light were not, then the lighting switch could be assumed good and the ignition switch would be suspect.

 

With the ignition in the "ON" position, and the lights turned on, only the headlight, and "fog" light work. The tail light, license light and back lighting of the speedo don't work.

 

Finally got round to taking the ignition apart, once I managed to get the ignition out, and apart, I thought I had found the problem, I should've taken photos, but didn't think of it at the time. So inside the ignition everything looked pretty clean, however the wiring looked suspect, my ignition looks different to the one in the tutorial, I have 3 copper plates, instead of the 2 in the tutorial. They however looked nice and clean, and didn't mess with them. The blue wire, and blue and red wire looked a little dodgy, so I ended up resoldering the blue wire back on to the plastic base with the round copper contacts in them. I did a continuity test from each of the copper contacts to the connectors on the end and all checked out. I put the ignition back together, quite chuffed with myself at finding the issue, only to be disappointed again.

Turn the ignition to park, rear lights and front park light come on, happy that it is still working. Measure 12v at the fuse with the blue and red wire, which is great.

Turn the ignition to on, and switch the headlights on, and the headlight comes on, but not the dials backlighting, or the rear lights. Measure the voltage at the blue wire from the ignition, and there is no voltage.

So I'm guessing before blaming the ignition, and looking to get a new one, the next step, as you mentioned would be to check the light switch itself? Which I will hopefully be able to tackle tomorrow night.

 

It should be an easy check for the lighting switch tail circuit. The R/Y wire is the input power to the lighting switch, and the blue wire is the output power that is routed to the ignition switch and the Atari meter lights.

I would suggest locating the 4 pin connector from the light switch - it should have a R/Y, L, L/B, and Y wire and is located under the tank. Once located, unplug it and evaluate connections for corrosion or pushed pins. If all looks well then reconnect.

With the ignition switch set to "ON", the R/Y wire should be 12V, and with the lighting switch set to either "PO" or "ON" the L wire should be 12V. If you find 12V on the R/Y wire but nothing on the L wire with the ignition switch set to "ON" and the light switch set to "PO" or"ON", then it would be time to disassemble the control switch and troubleshoot for a broken wire or defective switch.

 

Thanks Rooster53, wish i had thought to check the forum before i started disassembling the light switch, and i could've gone straight to under the tank. Cause after disassembling the switch, and cleaning the light switch and putting it back together, and testing everything out, i came to the conclusion that i needed to check the connector, and following the cable - figured it was under the tank, and at that point was tired and ended up not going in under the tank.

Is there advise on how to take the tank off? And please excuse the next question, but does the tank need to be empty to take it off to get to the connector under the tank? I had a quick read through the manual, it looks like it should be easy to take the tank off, just remove the clip, remove the 2 pipes from the fuel petcock and disconnect the fuel level sensor.

Thanks in advance.

 

No need to empty the tank, although it could be a bit heavy with a full tank, but not normally an issue. And, you have the procedure down correctly. After removing the clip, fuel and vacuum line, and disconnecting the fuel level sensor just pick up on the back of the tank and while pulling it toward the rear and wiggling it side to side to release it from the rubber dampeners. As long as the petcock is working normally there won't be any issues with fuel leakage.

You could have done or could still do the voltage checks at the control switch. Cleaning was a good thought, but without testing you still can't really know what the problem is. With the control switch disassembled you could check input voltage at the R/Y wire and output voltage at the L wire. Depending on what you find there will dictate the next path.

No voltage at the R/Y wire and you are headed back through that 4 pin connector and into the main harness toward the fuse box No voltage at the L wire with the voltage at the R/Y wire and you are looking at a bad switch.

A switchable 12V at the L wire and once again you will be heading into the 4 wire connector and main harness to look for an open.

 

Thanks Rooster53, will def check the voltages on the switch before heading down the removal of the tank in search of the connector.
Although, i have a feeling that i should just head straight for the connector, check the voltages and look for any issues there.

 

Since you are already familiar with the switch I would suggest going there and test the voltages. If you find the switch is working by providing 12V to the L (blue) wire then it might be time to go under the tank to the main harness to find out what is causing the problem.

 

Ok, test done at the switch - measured voltage at the R/Y and there is voltage there. Checked voltage at the L in switch on, and there is voltage there. So I guess, under the tank we go.

Here goes ...

 

Seems like the best path to take. Probe at the back of the connectors. Check for 12V on the blue wire at the connector under the tank on both sides. I am really hoping this isolates it with no voltage showing up at the main side of the harness or both.

 

Rooster53, and everyone else who helped, thank you so much for your perseverance and help, at last I have managed to resolve the problem.

So, tank off found the connector, switched the lights on, and measured the voltage on the L, and low and behold 0V! Unplugged the connectors (x2), took the light switch off the handle bars, and began with my favorite ... continuity testing, and all cables checked out, except the blue cable. Turned out the blue cable was faulty, cut the old cable close to the connector, soldered on a new blue cable, did a continuity test, and the new cable checked out fine. Before routing everything back in place, plugged in the connectors, turned on the lights, and magic happened, the rear lights, and the speedo/tacho back lights came on.

Bike is now back together, and all lights are now working! Hooray!