Vintage Dirt Bike Q & A

I have a 1976 250MR. the stock tail light works (brown and green wire), but I needed a brake light, so I bought a Maier universal with 3 wires, black tail light, red brake light, black and white ground. I used the brown to one side of a brake light switch I added and from the other side to the red at the tail light. I spliced on a jumper from the brown and ran that to black on the tail light. I hooked up the black and white ground to the green wire and ground them both to the frame. My problem is that the bright filament is always on and the dimmer one only comes on when the brake is applied. Just the opposite of what I would expect. I swapped the red and black wires, thinking it was wired backwards, and there was no change.
Do I need a relay, a different bulb ( I have been using a 6V dual filament, I think it is an 1154) or am I wire up wrong, HELP please.
PS : I tested the tail light and it lights the brighter filament when red and grounded, and lights the dimmer filament when black and grounded.

The 1154 is an automotive bulb, rated at 3A/18 watts on the brake filament and 1A/6W on the taillight filament. In my opinion this is about 4X too powerful a bulb. Perhaps Maier can suggest or supply another bulb that fits and is more like 4W brake / 2W tail or something like that (a 12V bulb may work OK). I don't know what kind of power the MR lighting coil puts out, but I wouldn't want to risk overheating it or burning it out.

If a bulb swap doesn't cure the problem 100%, it could be that the overall lighting voltage is dropping when both filaments are loading the lighting circuit, then popping back up when only one filament is connected. This can be confirmed with a meter.

If so, you may have to get rid of the jumper, then switch the 6V brown wire between the red and black wires on the Maier, leaving the ground in place. This will give you a tail/running light OR a brighter brake light, but not both at once. You'll need a SPDT normally-closed brake switch (3-wire) to do this. This will also allow you to run a brighter bulb, but I wouldn't exceed 6 watts max (double the stock bulb power) just to be safe.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,
Thank you for your response. First let me state that my understanding of things electrical is limited. If I am reading and understanding correctly, with the SPDT switch, I would have to manually switch between having either a running light or a brake light, but not both (the running light always on and the brake light operated by the brake pedal). If you could trace the wiring, starting with the brown wire, which I assume is the power wire, and each following step, perhaps I would be able to duplicate that on the bike. My goal is to get this thing inspected and on the road. For that, I need a running light and a brake light. Is the green wire just a ground wire. I remember that the wiring harness is grounded under the tank with a large green wire.
I will try changing bulbs. I have an 1157, which is 12V, and see what happens. No need for a relay ???
Thanks again, Bill

Bill,

I'd definitely try the bulb swap first - it's super easy and should get you through the inspection. If your state requires simultaneous brake and running lights, this is pretty much your only option. As I recall this is the way my MR175 brake/tail lights work, although it's been years since they've been powered up (I'm using a CR125 ignition now, without a lighting coil).

Re the SPDT switch - this is a 3-terminal unit that replaces your existing brake switch, and performs all needed switching functions automatically. When the brake is applied, the switch connects its center terminal (brown wire, power input) to its normally-open terminal (red wire, brake light filament) to light that up, and when the lever is released the switch connects its center terminal to the normally-closed contact (black wire, running-light filament) - basically a see-saw kind of deal. The downside is that it requires the SPDT switch and three wires from it to the taillight assembly.

You can also use a relay. This would be a 6V SPDT 5-terminal relay that could be mounted in the taillight assembly, or wherever. The switch section of the relay is identical to the switch above and is wired the same. The existing two wires from your brake switch are connected to the 6V relay coil terminals, instead of a bulb filament. When you apply the brake, the power that previously lit the bulb now activates the relay solenoid instead, which operates the relay's internal switch section to supply power to the bulb filaments.

It's really a lot simpler than it sounds. Most of the work involved is just running wire and physically mounting the relay if you use one. But, hopefully the bulb swap will do the trick and none of that will be needed.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,
Thank you so much for the detailed explanation. I get it now and will let you know how it came out.
This bike was a neglected wreck when I got it ($380), but all the major pieces were there, so I gave it a try, and it runs great, but every single phillips head bolt was either stripped at the head or frozen or both. Much time with the extractors, broken extractors, rescue bits, etc.
Thanks again for your patience, Bill

Bill I have thought of doing the same as you but never followed thru! When you get done can you post some pictures? Also curious what state your in,and do you have a title

Ray,
I have no title other than former Secretary for Life of the BMWMOV (BMW Motorcycle Owners of Vermont). My other bike is a 1977 R75/7, the last 750cc. bike BMW made, thus the last REAL BMW. I am a citizen of The Peoples Republic of Vermont (We're all a bunch of Socialists)
Since our last, I tried all the bulbs I have: 1154 (6V), 1157 (12V), 2057 (12V) and none made any difference. I did some more research and decided to order a 6V relay (SPDT). This is the way I hooked it up:
30-Brown (power)
Brown (jumper off Brown)-one side of brake light switch, other side of brake light switch to 85
86-Green (unswitched ground)
87A-Black (always on tail light)
87-Red (brake light)

My understanding is that the always on tail light would be powered by 30 to 87A, when the brake light switch is activated (brake applied), the coil in the relay is energized (85 to 86), and the relay switches from 87A to 87, which turns off the tail light and turns on the brake light. However, it doesn't do that. I have a light when no brake applied, no light when brake applied. I am going to try the 6V bulb and see if that helps. Maybe I should just buy an MT!!

Ray and anyone else who can figure this thing out,
I put in the 1154, 6V dual filament bulb. I got a very bright tail light, and when I put on the rear brake, I got a flashing rear light for maybe 6 flashes and then back to the bright tail light. While the light was flashing, the relay made a loud on and off buzzing sound, similar to the buzz made when one buzzes someone into an apartment building. I disconnected the ground/green/86 wire and the tail light was still bright, but when the brake was applied there was no change, which is what I would expect, as the coil would not be energized due to a lack of grounding to complete the circuit, and there would not be a change to 87 from 87A.
This just can't be this difficult.

Bill,

I figured the bulb swap would work - it really should have - so I left out a lot of essential info about the relay conversion, thinking it wouldn't be needed (sorry about that).

Long story short, the relay coil needs 6V regulated DC to work reliably, and the MR lighting circuit puts out unregulated AC that varies widely with rpm and lighting load. The buzzing noise you heard was the AC repeatedly slamming the relay contacts back and forth, after which the relay coil most likely burnt out.

You really shouldn't need the relay anyway. I think something's wrong with the brake switch or wiring.

At this point, I'd seriously consider picking up a 6V lantern battery, then unplug the wiring harness from the motor and connect the battery to the green (negative) and brown (positive) harness plug terminals. This will allow you to troubleshoot with the motor off and a steady DC voltage, a big improvement.

The fact that the bulb lit more brightly using the relay - and that the problems seem mainly related to the brake filament - tells me there's probably excessive resistance in the brake-switch circuit or related wiring. This will show up real quick using the battery, since you can easily connect the bulb direct to the battery terminals for comparison, bypass the switch and/or switch wiring using clip leads, etc.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for all your help. The explanation for the buzz makes perfect sense. I'll try the lantern battery. I have been using 4 - 1.5V AA batteries in a paper tube with wires taped on each end to get 6V. Our local shop (Pro Cycle) says the bulb swap should work, my buddy, Ted, says the bulb swap should work, so something must be wired wrong. I am suspecting the previous owner because everything else on this bike was totally messed up. I was going on the fact that the always on tail light worked fine, as well as the high and low headlight beams, which lead me to believe that all was in order. I keep hoping there is someone out there who has actually pulled this one off, and can give directions.
Thanks again, Bill

Bill,

AA batteries probably won't push enough current through that 1154 to brightly light both filaments at once. The lantern battery should be beefy enough, though, and four D cells should also work well. Even though it seemed to make no difference, I'd recommend going back to the 1157, since that will probably end up being the bulb you'll use unless it's not bright enough to pass inspection.

I've been wondering about the general condition of the wiring on the bike, I guess it's not so great? If so, I'd just try hooking up the taillight direct to the battery using clip leads, using one clip lead end as the brake switch (similar to the taillight test mentioned in your first post, but with both filaments energized during 'braking'). If that works OK, it's got to be a problem in the switch or wiring, not the taillight or bulb.

If you have a meter, try disconnecting both ends of the brake switch wiring and measuring the resistance with the switch on. You're looking for a very low resistance, less than a few ohms or so. It's also important to use at least the same gauge wire as the stock harness, and thicker / heavier-gauge wire wouldn't be a bad idea.

The MR175 and MR250 share the same wiring diagram, my MR175's tail / brake lights work fine using the parallel connection you originally tried, and the MR250 lighting system is surely at least as powerful as the 175's, so it really should work.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,
I took out the relay and wired it as I did initially, Brown to always on tail light, a jumper off of the Brown to one side of the brake switch, and the Red brake light wire to the other side of the brake switch. I switched bulbs to the 1157, and what I have now is: with no brake, I have a very bright tail light, when I apply the rear brake, there is a change/switch and the tail light is now a bit dimmer. Just the opposite of what I would expect. Basically right back where I started.
Then the thought occurred to me. The inspection guy and people behind me aren't going to know what is going on, all they will see is the tail light getting brighter and dimmer, which should, I hope pass inspection. So tomorrow I will get it registered and try for an inspection. Thanks again for all your help, and if I ever do something different with it, which works better, I will post it.

Ray,
The silicon rectifier (S3C-01) melted, all the silicon filler ran out the bottom. All bulbs burned out, as I assume the current is not being converted to DC anymore, so am ordering a new rectifier, buying new bulbs and seeing if that takes care of that problem. The bike crapped out on me, trucked it home, and it doesn't appear to be getting any spark. I wonder if the rectifier burning out could contribute to this second problem. The bike was running great, even after the rectifier and bulbs were shot.

Bill,

If the wiring diagram is correct - and your MR250 is wired like the diagram - the rectifier is part of the ignition circuit, not the lights. I disconnected the rectifier on my MR175 altogether (black/yellow to black/white wire, green wire disconnected) and the bike ran OK, but if the rectifier is completely fried but still connected it's likely you'll have no spark. It may have been only half fried earlier, when the bulbs burnt out.

I think it would be an excellent idea to check your wiring harness against the diagram - shouldn't take more than a half hour or so.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,
I had to do the walk of shame, when the bike died completely. Tested for spark by laying spark plug on head and got weak spark. Took ignition coil and rectifier to Honda dealer. Tested under load and for a length of time, both failed. The ignition coil was no problem to replace, but the rectifier is a rare item. I found a guy in Oregon, who will make me one. I am sending him my old one so he can take apart and use as a guide. I am his first MR250 customer, so mine will be free, even better. I'll let you know how it all works out.