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Repair XS400 DOHC Pulsar Pickup Coil

Discussion in 'The Garage' started by XSPen, Feb 7, 2020.

  1. XSPen

    XSPen XS400 New Member

    Hey guys, new to the forum!

    Just wanted to thank everyone, this has been a wealth of knowledge this past week while trying to get my XS up and running.

    My left side pickup coil blew out and I was getting massive voltage spikes in the circuit (while cranking) but for the life of me could not figure out why. i rewired all grounds and primary connections, put in a new fuse box, checked and resoldered every joint and resistor on the TCI, did all the basic peripheral checks, and traced shorts to no end, until I found this little DIODE buried in the harness beneath the seat.

    It had given out and was Backfeeding current from the headlight circuit, into the ignition (via the handlebar start/stop controls) and down into the coil through the TCI.

    Anyway, I needed my bike running and didn't want to buy a pickup coil, so I made one.

    I used a pair of vice grips and a small hammer to separate the magnet from the coil, and dremelled the plastic cover in half to retrieve the bobbin.

    It took me half an hour to get the (lightly) epoxied wire off of it with a metal pic. I didn't have any 42AWG wire sitting around, but I did have an old forklift voltage regulator with similar guage windings.

    After carefully unwinding the regulator's coil onto a screwdriver, I used the wire to rewrap my bobbin, which took about an hour altogether.

    Once I finished, I epoxied the bobbin directly to the magnet, wrapped with electrical tape, more epoxy, soldered on a couple of leads and presto! It's functioning beautifully.

    So, if you're in a bind and have some spare parts around, it is doable lol

    Attached Files:

    xschris likes this.
  2. pcream

    pcream XS400 Enthusiast

    Very cool! Would you be able to describe your initial problem in more detail? You replaced the left pickup coil but couldn't get it started and you found this faulty diode? Where was it wired into and what was its purpose?

    The reason I ask is I'm having a problem right now where I can't get the plugs to fire while cranking, but whenever I turn the handlebar kill switch to off, I get a really big spark. Perhaps I'm having a similar backfeeding problem. I still need to check the pickup coils themselves, but very strangely my turn signals won't blink anymore either, just get a constant light in them.
  3. XSPen

    XSPen XS400 New Member

    The diode is in the wiring harness under the seat. It ties into the headlight circuit and keeps power from Backfeeding into the system.

    First I would run down the check list.

    40 year old wiring connections and components can create too much resistance in the circuit, often times requiring more voltage than normal to get a spark.

    If you have the original fuse box, go to your local auto parts store and get a new one asap. ($3-$15 depending on quality)

    Grab some electrical contact cleaner as well. Open and spray all connections under the seat and in the headlight. There are more connectors in the frame under the tank, but I'd just go for the accessible ones first.

    Verify you have 12.6 volts at the battery at the very least.

    Take care of these things first and I'll do my best to go through diagnostics with you

    Also are you grounding the spark plug to the frame when you test for spark?
  4. pcream

    pcream XS400 Enthusiast

    I'll get some contact cleaner and get to working on it, gonna tow it tomorrow. Pretty sure the fuses are fine based on continuity, but the box is a mess, PO had soldered in one of the fuses. I hear the relays under the tank clicking when I switch the key to on and the handlebar switch to on, but I haven't tested all of the different relays as detailed in the DOHC manual on this site. I'm almost positive it is a short somewhere, based on it being in the rain the day before.
  5. XSPen

    XSPen XS400 New Member

    It was running the day before it rained? How about the signals, same?

    All of my rear connectors have been moved under the seat, sorry I forgot it didn't come like that. You'll probable have to remove panels to get to the TCI and such
  6. pcream

    pcream XS400 Enthusiast

    It got rained on over the weekend (forgot my cover before I left town) and I road to work monday morning with no issues at all, signals worked fine. Went to start it when I was ready to go home and it would crank over fine but not catch; eventually found out there was no spark on either side except when the handlebar switch was moved to off.

    Yeah the TCI and voltage regulator/rectifier is behind the left side panel for me, I had to replace my TCI a few months ago because I couldn't get a spark on the left side and that fixed the problem. Can't imagine another one has gone bad this quickly, but it's a possibility.

    After a little research, I've also seen mentioned in the forums here that the turn signals won't blink unless the bike is running, the voltage is only high enough to trip the relay when the alternator is going I guess. So the turn signals not working might just be a red herring.
  7. pcream

    pcream XS400 Enthusiast

    I apologize for the delay, but I finally got around to starting to troubleshoot over the weekend with the good weather that we had. Ran through the ignition checklist n the manual: check connectors -> battery voltage -> fuze continuity -> coil resistance -> pick up coil resistance and everything was reading correctly. Decided to try to start it and bam it started up immediately. Whatever was wrong had fixed itself. I'm now almost certain there was a short that finally dried up with the 60F weather we had over the weekend. I'm going to go over everything with some contact cleaner, but based on the symptoms I think there was something shorting between TCI and the coils, so I'm going to give those a good clean.

    If I find something during the cleaning, I'll post it here so these threads can have a final resolution to the problem.
  8. pcream

    pcream XS400 Enthusiast

    Ok, update for my problem. Looking at the circuit diagram I saw that the neutral cutoff relay was in the ignition circuit and decided to test it by bridging the red/white wires that are connected when the neutral gear switch is engaged. Fired right up! I've attached pictures of it, it's a black little cube with connectors under the tank on the left side of the bike. So I just bypassed it and twisted the red/white wires together and now it has no issue starting anymore. Wanted to post this so if anyone in the future with an '82 DOHC bike has this problem to try bypassing this relay. I apologise for slightly hijacking this thread!!

    Attached Files:

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