I've been working on getting my DT400 back on the road after about 30 years of storage by the previous owner. I'm almost there, but I'm having issues blowing both headlight and tail light bulbs. Everything appears to be stock with the exception of the battery type.

So far, I've:
- Installed an aftermarket headlight (6v 35/35W) and taillight (6v 25/5.3W) as both were missing when I bought the bike.
- I cleaned all of the electrical connections on the harness with contact cleaner and a small wire brush. The harness appears to be in good shape with minimal resistance in the lighting circuit.
- I installed a new Yuasa NP4-6 (6v 4.5Ah) sealed lead acid battery after reading about people using them on here. The battery read 6.34v when I hooked the multimeter to it with the bike and lights off. I haven't checked it with the bike running yet.
- I also checked the (original) rectifier with my multimeter per the service manual. The diode appears be functional and correctly connected with continuity from the white to red wires and no continuity from red to white wires.

The tail light comes on with the key and handlebar switch on. When I start the bike, the headlight comes on (very bright compared to my other 6v bikes) and the taillight gets brighter.  As soon as I get over about 2500RPM, the headlight beam burns out. The tail light follows a few seconds later.

I've read every thread I could find about blowing headlights. Most seem to be due to various reasons such as bad batteries, poor connections, or a faulty regulator. It seems uncommon to blow the tail light as well as it should be protected to some extent by the battery?

I feel like I've checked the most common issues off my list. After a year of wrenching, this is the last item on my list before I can put it back on the road. What am I overlooking?

Edit: I wanted to add that I contemplated adding a voltage regulator. The service bulletin says to connect it to the blue wire in the headlight shell. If I'm correct, this would only prevent the headlight from blowing as it only regulates the AC voltage on the bike. It would do nothing about my taillight issue correct?

A voltage regulator is needed. The rectifier turns AC current into DC for the battery side. There is a bulletin from Yamaha in regard to this… I found a single wire regulator for my RT2, and since, no more blown bulbs.. there will be lots of info, and others on here that talk all about this topic.
Sneezles61

I have a voltage regulator on order, but I have a few questions. I'm not trying to be obtuse, but I'm the kind of person who learns through understanding the how and why of things.

1) Will this also fix my issue of blowing taillights somehow? It appears from the wiring diagram that the headlight runs off ac (which the regulator should keep at 6v), but the taillight runs off a separate DC circuit using the battery as a crude regulator. I have some electrical experience, but I don't see how regulating one circuit will regulate the other.

2) I'm assuming something else is wrong causing the lights to blow almost immediately after starting the bike. If it did that from the factory, Yamaha would never have been able to sell them, and the 75 didn't come with a regulator from the factory. The regulator may fix the issue, but it sounds like more of a fix to the symptom than a cure to the cause if that makes sense? Theoretically, if everything is in good shape, shouldn't my lights work for at least a while without a regulator before they burn out? I'm planning on going cross country on this bike eventually, so if I have another electrical gremlin, I want to find it and repair it now rather than it failing on me later when I don't have access to parts and tools.

I'm not thinking too straight right now, so without getting to deep--if the battery is 'in' the circuit it should soak up the extra & limit the lights from blowing [that's everything being as it should]. So is any charge going into the battery. I would have thought that model would come with a regulator--single yellow wire--possibly under the aircleaner box, so will need an earth being a plastic box.
I'd say you're going to be teaching us shortly--which is good.
If ya going cross country & are desperate you could pick a lower voltage output wire from the mag which would give less light at idle & pretty good at revs but no blow. Yellow is your highest output though.
Sorry if i'm just repeating what you've told us but i'll read the story closer later today--but higher wattage bulbs give less light on an unregulated mag system. Anyway, i think 35watt is original. Instrument bulbs soak up a little. Could run 12 volt headlight bulb. I did that on my RT1 360 ride to work bike using a huge headlight of an XJ650 with a 12 volt hallogen bulb from a wreck bike at work back in the "80's. No regulator & was great with a few revs on & never blew on full song. Blimey that was umm about 40 years ago haha. Sorry--woffling again!!.

ThebeardedOne,

Your absolutely correct in thinking something is wrong with your bike if blowing out bulbs as soon as you rev it up and you have to install a regulator to help it or stop it. Especially if you have a good known battery in the bike. Have you done a Charging output test on the bike? That would be the 1st thing to do. The specs are in the service manual. Make sure your electrical loads are correct and working. This means all light bulbs. Headlight, tail light, meter lights, indicator lights. If one or two are bad or not the correct wattage it can cause others to burn out quickly. 

You could have a defective charging coil with resistance way off. Could be a simple wire issue too. Go to the TECH LIBRARY link here and bring up the service manual for your bike. Its a DT250-400 manual. Go to pages 126-130. You seam to be pretty technical so all the info you need to find your problem is on these pages. Test,specs and proceedures. 

I personally don't use regulators on my bikes. Many members here swear by them though. I suppose it's good insurance to protect the bulbs so if one did burn out it would protect the others from getting overloaded. I guess ive been lucky and have not had a bike that burns out bulbs that quick? Knock on wood.   

Thank you! That's exactly the info I was looking for.

The battery in it is brand new. I actually have two because I'm in the process of installing lights on my 76 as well. I've tried both batteries with the same results. The batteries also work jumpered into my RD60, so it appears it's not a bad battery.

Tonight I installed a new rectifier just incase that was the issue. It doesn't appear to have made a difference. I did meter out the harness, but didn't have much luck everything seems to make sense based off the wiring diagram. Maybe I have a loose connection somewhere and the vibrations are wiggling it loose. It's hard to test continuity with the bike running.

I already downloaded the service manual, so I'll check out that section. I really should just print it off so I can flip through it while I'm out in the shop with the bike. No cell service out there makes for peaceful wrenching, but can be a pain when you're trying to look something up.

I have no problem installing a regulator, but it seems like I have a lot more power here than what the regulator is designed to handle. It blows bulbs at 2500 rpm or a bit less. I imagine it's cranking out some serious voltage at full tilt. That's on my list of things to check in addition to what's in the service manual. I guess I need to spend some more time getting aquatinted with my multimeter. 

I'll update with what I find out. Hopefully I have a chance to wrench again this week! Thank you again for your help!

You might also check to see if you have the original ignition/charging system. The 400 CDI is problematic and many have been replaced.

While there are DC and AC circuits...  the nature of a permanent magnet alternator with multiple coils under the same flywheel is that they are inter-connected.  That's why if a battery goes bad or gets disconnected the headlight will blow.

I don't disagree with Skydancer...  the systems are good and you normally don't "need" a regulator on an earlier model.  The thing is that today original bulbs are not as cheap or easy to find, especially OEM headlights.  Regulator provides insurance.

I have never used an SLA battery on a bike Yamaha without a regulator...  I am pretty darn sure an SLA battery has nowhere near the "voltage regulating" capacity of the original flooded wet cell battery.  SLA's allow voltage to climb high pretty easily with far less current than a wet cell.

So if system is original 1975 parts (flywheel and/or stator should have numbers) and you can't find anything wrong...  why not try using the correct battery the "balanced" electrical system was designed for?

 

Ok, read below at your peril lol. I'll leave it now i've done it but ignore if it's all B/s--t. & correct me.
While its blowing bulbs with ease it must be also charging hell out of the battery. & if its not then sound like the rectifier isn't in the battery circuit, battery & charge/lighting coil circuit.. T off the yellow which will be feeding ya headlight & tap the rectifier into it then direct to the battery red. It's got to charge then & use the battery as a sponge & soak up the excess [he says hopefully]. I'm only thinking out loud as to what i'd do in a similar situation. That's all without a regulator 'just yet'-- to see what happens. I test using an ammeter & have an old yamaha workshop one with a 5 amp scale & a 20 scale. "I connect my ammeter between the negative of the battery & earth/frame". I know the easy way is to put a volt meter across it & see what it rises too [& that's how i do my car but its regulated to safety] but on unregulated like yours you don't know the amps its going to or what negative draw it has with flasher operating, plus US ones might have the tail light off the battery too. Does it hold it's own & keep on the plus side of charging when brake is applied or flashers operated. The lower wattage bulbs you use the more charge you'll get to the battery plus brighter lights. But there's a cut off point where the bulbs will blow if too small wattage is used. I'm mostly talking smaller scooters from the "70's, like if 25/25 was std then 20/20 would give more charge plus brighter lights, but 15/15 would blow at full revs. My ammeter zero's out in the middle so i can see when something draws below zero like brake light or flasher etc & how many revs you need to get it above the line. After all my woffling about nothing much i'm still thinking yours isn't charging--battery not in the right circuit. Ignore me, i'm typing not thinking. I keep adding bits in so i'll edit again probably. I'll go back n read your story. Fitting a Regulator is good idea of course, but why are you having this problem that requires one. They were partly fitted as original equipment from about your model because no-one ever checks their battery & they run dry of water then bulbs quickly blow.

I'll start again as below is just woffling about something only slightly related.

On my honda trx300 quad with blown reculator/rectifier [just about set me seat alight lol] i just use an old yamy rectifier & testing like i said up top with amps, so i could then pick an alternator wire with minimal output so as not to boil the battery & been like it for couple or 3 years, Is 12 volt of course but the rectifier doesn't care, 6 or 12 [or might if i had it charging off the 20amp scale--& battery would explode. I just ended up with about 3 amps max at high revs but i don't rev it super hard heading down the farm anyway.

Sorry Mark, typing at same time, i'll read yours haha.