Low voltage at headlight relay.

I apologize if this has been brought up before. I have a 1980 Yamaha Maxim XJ650 and I am only getting 6 volts at the white wire that connects to the headlight relay. I wanted to check if this is correct (I am assuming not) and where I should start looking if it is not.

I have been using this wiring diagram: http://www.polymetric.net/xj650_wiring_harness.pdf courtesy of a thread from
slackard. If I am reading the diagram correctly the wire in question goes to the diode block (located in the headlight housing) which I am unsure how/what to test. Then heads over to the Rectifier and alternator. If I am still reading it right that means that either there is a problem between the alternator and the relay or there is a problem with just that portion of the alternator.

Now for the stupidity backstory:
I may be the cause of my woes, I have had the bike for about a month and it has been running great. I have been building up all the parts I need to replace things(breaks, brake hoses, etc.) that have probably (are) worn out. Last week on a ride from work to school I filled up at a gas station and went to start the bike. There was no joy. I found out later that the AGM battery had an internal problem (shorted?) in that when load was applied the voltage would drop down to .4v and after a bit jump back to 12v a new battery solved that problem. One of the fuse holders also broke during troubleshooting and to fix that I just pulled the holders out of the plastic and kinda shoved them together. I think I may have damaged something because they came apart (the fuse holders) during testing and shorted against one of the other fuses (I replaced that fuse holder with a waterproof blade style fuse holder). The only thing is there are still 12 volts at the headlight fuse.

TLDR May have shorted something important and my relay is no longer getting enough voltage. Where should I look for the voltage drop?

PS. I did also replace the relay with a standard 12v 30amp automotive relay and it does not have a diode if that makes a difference.

Thanks!
--Jon

Edit: I do get ~14.5 volts at the battery when the bike is running at ~4,000 RPM.

 

The headlight relay cannot be replaced by any standard automotive relay. It is a latching relay.
Given that you created a short when you were working on things (disconnect the battery next time), you may have let the smoke out of the diode block.

 

That is what I was afraid of that smoke is always so hard to put back in. Is there a guide to testing the diode block, It is encased in something (hot glue, tar, rubber cement?) and I have no idea what diodes do what and where. Do I really need the latching of the relay? I thought that was just to keep the headlight going if the kill switch was tripped or if for some other reason the engine/alternator dies. Is the latching relay supposed to be a safety feature in that way? If that is the case I will order the correct latching relay.

Thanks for the quick reply!

 

There is a thread here somewhere about the diode block. Note that the internals vary depending on which model XJ you have ( I think the most detailed thread is about the 500 diode block).

The relay keeps the headlight off until the engine fires (or shuts it off when the stater is engaged, depending on model). It's only a safety device in that it allows the starter to get all of the amps that it needs to do the job, and leave enough voltage for the TCI to still provide spark (and yes it does keep the headlight on when the engine dies). With a different relay, you might not be starting the bike (particularly in cold weather).

 

I tested it and that still functions correctly. Key on and bike not started = no headlight, key on and bike started = headlight on. I thought the latching only kept it on if voltage was still applied to the fuse (red/yellow) wire so that the light would keep going even if voltage dropped to (correction from) the alternator (white) wire. Bear with me I am still figuring this stuff out, I am not very good at the electrics .

Thanks, Jon

PS. I will hunt down that diode block information.

 

Oh, well you're correct in your thinking. I was misunderstanding what you did.

 

So what happens if the original relay is installed? Here is a recent link on a headlight issue and SQLGuy gives a nice outline on testing procedures. It is for a 750, but I believe the only difference may be the White / Blue wire on the 750 is a solid white on the 650.

http://www.xjbikes.com/forums/threads/headlights-not-working.75793/

I would guess that 6V is normal if you are running with a non standard relay that does not have the internal latching diode. The reason is only one phase and one half that phase of the AC Generator output that is rectified by the diode block is used to energize the headlight relay via the white wire to the headlight relay. Normally, this only needs to be enough voltage to pull the relay in, and then the internal latching diode inside the headlight relay that goes from the output of the relay to the high side of the coil would hold this point at close to 12V keeping the relay latched until power is removed. When measuring this voltage with a DMM, this rectified DC voltage will be displayed as a RMS value, which is somewhat lower than what is actually present.

I would suggest replacing the entire fuse block with the blade style fuses as a good starting point, as this is something that most of these bikes need as you have noted how the originals are falling apart.

As for safety, it is true that one of the functions of the relay is to reduce battery load during starting, but to me there is a role it plays in safety in that if the charging system were to stop working while riding (say on a dark country road or such), and if you did not have the latching relay function, then the headlight is going to go out and you will be riding blind.

A working system as you have it, but not a reliable system as the added relay and the original relay were intended to have 12V applied to the coil. Relays have a spec on pull in voltage and holding voltage, and when running on just one half of one phase of the AC Generator there is a risk of the relay dropping out and shutting the headlight off.

 

I only tried the automotive relay after repairing the fuse and replacing the battery. The old relay did not work. I just tested the old relay and it clicks over with 12 volts going through it. It also works with 8.3 volts (two partially dead lion batteries) but it does not work with just over 6v.

I also checked the diode block that resides in the headlight (the one encased in rubber) and all the fuses test good. I can only assume that there is probably a problem somewhere else. I looked at the attached (and annotated) diagram.

My annotation is (in case it is hard to read):

Looking at the way this is wired it makes sense
that it is actually just 6v ac off of one of the
legs of the alternator. It kinda depends on what
the Rectifier/Regulator does with it.

I am wonderin if I fried someting in the
Rectifier/Regulator.

I plan on replacing the fuse block when the long dark hits and makes Ohio into the great frozen north I am just trying to get a few more rides in while the weather is decent.

On the non latching relay I read that on this form (http://www.xs11.com/forum/showthread.php?s=2f6366dafb90f8727600ade35a481252&p=172218#post172218) that I may be able to work around it but I do not entirely understand what is being said here. All I get is that a (latching) diode in the right place will work.

Let me know if there is anything else I should look at or test.

Thanks, Jon

 

Well, I measured this on my XJ750 by removing the connector to the headlight relay to simulate your condition of no latching diode. I got a reading of 9.8 VDC at idle, and up to approximately 12 VDC when the bike was revved up to near 2K. So, my previous speculation of 6V might be normal was incorrect.

Here's the thing though. If the replacement relay is working without the latching diode, then the AC Generator and rectified output of the diode block is sufficient to energize that relay, which leads me to believe there might be a bit more than the 6 volts you are measuring. Perhaps the order here is making a difference, and since you have worked on the fuse block maybe it is higher than previous noted.

As for the link you provided, it looks like they are saying to just add a diode or wire the relay to come on with the ignition switch. As was discussed earlier, the headlight on with the ignition switch would be a bad idea as it would deplete available starter current from the battery.

So, adding a diode to a non Yamaha relay is a viable option. Or better yet, if the OEM headlight relay is defective, just source a replacement OEM. They are not too difficult to find.

Here is a pic of a simplified diagram. The latching diode could be added internally or externally - obviously internal makes for a neater more reliable package. The simplest test for the headlight relay is as SQLGuy noted in the previously supplied link - turn on the key and apply 12V momentarily to the W/L wire (W in your case), the relay should energize and the headlight should come on and stay on until power is removed by turning the key off.