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Makotosun

1978 dt400 Source Coil Trouble Shooting

  • Marty’sS
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1978 dt400 Source Coil Trouble Shooting was created by Marty’sS

I lost ignition spark and am trying to narrow down what failed. My ignition coil reads reads 1Ω and 5.77Ω for the primary and secondary coils. The voltage coming into the coil reads 41.6 volts ac.

The charge/source coil brown wire reads:  208Ω and 91 volts ac, with the ignition key on.
The pick up coil, white and red wire reads: 46.5Ω and 5.5 volts ac, ignition on.
The pick up coil, black wire reads: -.1Ω and 0 volts ac, ignition on.

The service manual I have has the ignition coil in spec. and the charge/source coil brown wire should read .33Ω.

Does anyone have any additional information on component testing and trouble shooting?
 






 
18 Feb 2023 16:02 #1

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  • Ht1kid
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Replied by Ht1kid on topic 1978 dt400 Source Coil Trouble Shooting

Greetings from Tennessee.   

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18 Feb 2023 16:37 #2

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Replied by msavitt on topic 1978 dt400 Source Coil Trouble Shooting

have you tried to get spark with ignition and kill switches disconnected?

On mine, it didn't spark when I got it but after I disconnected the block connectors that connect the CDI unit to magneto and other related connections up with a blast of contact cleaner I got spark.  And I had funky resistance readings too...

Also, I do not think this bike needs a battery to run, hence the source coil which energizes the primary...which is why it's shorted to ground I think (I am NOT an expert) to stop the  bike via key or R/H switch

this is such a killer bike, good luck!
 
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18 Feb 2023 17:36 #3

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Replied by RT325 on topic 1978 dt400 Source Coil Trouble Shooting

Without researching i'd think the .33 should be 330 ohms. & on a "peak meter" i'd be looking for 160volts kicking. But 91 is more than half & a std meter reads half so in theory should be good. So i dunno--still sounds sus. Think--[think i think] i'd now hook ya drill up to the mag nut, plug out & spin it up & see if you get spark at faster than kick speed spinning the drill in reverse of course [anti clock]. Not sure if that helps. I'm guessing the pickup is white/red so the black will just be earth or the earth end of pickup windings but didn't think it had two wires unless the black is just a stator plate earth just to be sure to be sure of it all earthing. I'll get back in the corner & do a bit of research. I was in the shop when those were born but of course they don't play up till 45 years later & by that time i've forgotten what i knew lol.
Sorry, keep editing. think i've sorted it.
Last edit: 18 Feb 2023 22:12 by RT325.
18 Feb 2023 21:31 #4

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Replied by Marty’sS on topic 1978 dt400 Source Coil Trouble Shooting

Greeting’s Tennessee,

I’ve visited there a few times, including a trip to Clarksville in 1978 from Louisville, on the bike I’m working on now. What a great place and state.

I tried the switch connector disconnect from harness and connector engagements, early on my quest for spark and still no spark
19 Feb 2023 08:39 #5

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Replied by Marty’sS on topic 1978 dt400 Source Coil Trouble Shooting

Attached is my most current test readings.1) It looks like my ignition coil is good and there is not enough voltage leading to it.2) The 208 ohm reading on the brown wire seems out of range.3) I don’t know what the line voltages should be, I’m just a tea leaf reader at this point, describing the tea leaf’s.The . 33ohm spec does seem a bit low for resistance on a good sized coil, could be a misprint. Feel free to tell me what you think. If I can get some good operating values for resistances and voltages posted, it would help me and anyone who experiences the same.
19 Feb 2023 09:03 #6

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Replied by Marty’sS on topic 1978 dt400 Source Coil Trouble Shooting

 
19 Feb 2023 09:06 #7

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Replied by Marty’sS on topic 1978 dt400 Source Coil Trouble Shooting

Looks like attachments are disabled, no spark and no attachments too…
19 Feb 2023 10:01 #8

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Replied by msavitt on topic 1978 dt400 Source Coil Trouble Shooting

don't mean to press my idea, but I had no spark and when through same taffy pull...and got spark once I blasted connectors with cleaner
19 Feb 2023 10:13 #9

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Replied by RT325 on topic 1978 dt400 Source Coil Trouble Shooting

I wrote a spiel up there in #4 & now after sleeping on it--maybe their .33 just means 33% of 1000 or 330 ohms. Bit of a confusing way of writing it. Spin it up on the drill & see if ya get spark, & if so it sort of proves a weak sourcy thing.!!
19 Feb 2023 13:27 #10

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