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'78 DT400E Headlamp Function & Wiring Questions

15 Mar 2020 11:07 15 Mar 2020 11:32 by Tom S..
Tom S.
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'78 DT400E Headlamp Function & Wiring Questions #1
The headlamp on my 1978 DT400E (US model) will only function while the engine is running. With the ignition key in the on position but with the engine not running, the headlamp will not light up. Is this normal?

No answer to my question was found by using the search function on this site. I did find multiple posts about the oil light/neutral light connection. :)

The motorcycle starts easily and runs fine. The tail light and brake light function when the key is on, regardless of whether the engine is running. The headlamp high and low beams function, but, again, they only will operate while the engine is running.

I have a 1982 Honda CB125 with 6v electrics that works in this manner and in that case I accept it as normal. The Honda's headlamp only will function while the engine is running.

This machine came to me with a dead battery and without turn signals, but with the other electrical components and wiring largely intact. After five years of ownership I started it for the first time just three weeks ago. It sat idle (indoors) for more more years than that before I purchased it and I have never spoken to the prior owner.

I installed a new 6v battery and the recommended replica turn signals are on order, but I'd like to know for sure how the lamps are supposed to operate before I dig deeper. There is no plan to convert to 12v or use lamps of a different style from OEM. I am being very careful to avoid damaging the sealed beam unit. It cost a lot of money to replace that part on my RD350 and Honda MB5.

I have spotted one wire that has been patched with a crimp-on connector. I have not yet determined the function of that wire or whether the repair was effective. I have not yet tested the turn signal leads.

All six of the 6v3w lamps in the speedo and tacho are burned out and the horn does not work. Common problems?

I appreciate this site's library of manuals. The service manual (and supplement) for my DT are silent on the question of headlamp function. There is no owners manual for the '78 DT400E available on his site. There is an owners manual for the 1977 DT400D, but that bike's switchgear is different from the 1978 model. In 1977 there was an on/off switch for the headlamp. The '78 model lacks that feature (thanks, US DOT!).

Knowing how the lamps are supposed to function will guide my further exploration and investigation. I will then check the ground points and other connections, voltage regulator function, the one patched wire, and anything else that you might recommend in order to bring it up to snuff.


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15 Mar 2020 11:44
Gr8uncleal
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'78 DT400E Headlamp Function & Wiring Questions #2
Certainly with the older models (eg my RT2), the headlight only works with the engine running.

For some markets (I am in the UK), you need to go a second click round on the ignition to get the lights working. I think you're probably ok.

Regarding blown bulbs, they obviously do go eventually and often people just don't bother replacing them.

Most pre 1976 models needed to have a voltage regulator retro fitted to prevent bulbs blowing regularly. However, your bike should have had one fitted from new - just check that it is still present and functioning correctly. Maybe just double check that there aren't any obvious shorts that might cause those bulbs to blow.

Good luck!
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15 Mar 2020 11:48
MarkT
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'78 DT400E Headlamp Function & Wiring Questions #3
Yes, on all enduros from 68 to 81 the engine must be running for the headlight to work.

Split system... DC battery side runs signals, horn, brakelight and after about 1970 in the US, taillight. (Law that said taillight has to stay on with engine off came into effect about then in the US)

System is also "balanced"... stator output is matched to load of bulbs and battery charging which is how the voltage was "regulated" before semiconductor regulators were available.

AC power from stator runs headlight and gauge lights...

Smart move replacing battery though your 1978 should have an AC regulator on the headlight circuit. While systems are "split", they are interconnected... a bad or missing battery allows more power to go to the AC side and blows the headlight... more of a problem on the earlier models.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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16 Mar 2020 04:13
Makotosun
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'78 DT400E Headlamp Function & Wiring Questions #4
Correct on the headlight. Only on when running.

The 78 did come with a regulator so you won’t have headlights blowing if you run with a bad or low battery. The horn generally needs a good battery to sound off.

I have the same bike - uours looks to be in good condition. They are great woods bikes. Not eye opening power, but nice and torquey.

IF it isn't one thing, it's another.
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17 Mar 2020 12:29
Tom S.
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'78 DT400E Headlamp Function & Wiring Questions #5
Thanks for the tips.

I figured that was how the headlamp is supposed to work, but I wanted to hear from others to be certain. Bulbs for the instrument lamps have been ordered. I visited three local auto parts stores with huge racks of bulbs on display. Not one of them is 6v. :lol:

While waiting for parts to arrive I have set about to tune up the wheels. I enjoy building and adjusting wire wheels.

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17 Mar 2020 12:51
KennyV
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'78 DT400E Headlamp Function & Wiring Questions #6
No 6v bulbs? I've been able to find the 1129 turn signal and 1154 tail / brake light 6v bulbs at Autozone and O'REILLY stores around here in VA.

71 Z50 Minitrail
72 RT2
74 250A
78 XT500E
06 F650GS Dakar

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17 Mar 2020 12:56
KennyV
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'78 DT400E Headlamp Function & Wiring Questions #7
Or I should say I have been able to find them in the past. It's been awhile since I've looked for them. :Ugh

71 Z50 Minitrail
72 RT2
74 250A
78 XT500E
06 F650GS Dakar
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17 Mar 2020 13:23
Gr8uncleal
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'78 DT400E Headlamp Function & Wiring Questions #8

Tom S. wrote: Thanks for the tips.

I figured that was how the headlamp is supposed to work, but I wanted to hear from others to be certain. Bulbs for the instrument lamps have been ordered. I visited three local auto parts stores with huge racks of bulbs on display. Not one of them is 6v. :lol:

While waiting for parts to arrive I have set about to tune up the wheels. I enjoy building and adjusting wire wheels.

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That looks like a very tidy Portuguese Orange RD350B in the background.:likey

I've got a US import one myself to restore (I'm in the UK), but it's miles off being finished.
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17 Mar 2020 13:24
Gr8uncleal
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'78 DT400E Headlamp Function & Wiring Questions #9

KennyV wrote: Or I should say I have been able to find them in the past. It's been awhile since I've looked for them. :Ugh


I think I read on the forum a while back that 6v bulbs are becoming harder to source in the US.

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17 Mar 2020 13:57
FrankC
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'78 DT400E Headlamp Function & Wiring Questions #10
I like your style that 78 400 is pristine.

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