Topic: mUnit - Destroyed

[jakew]

Hello out there! I’m curious if anyone has had an mUnit burn up?

I have a 1972 CB500 that I’m in the final stretch of rebuilding. Working through wiring now and went with an mUnit Basic, Rick’s RR, 8 Cell AntiGravity battery.

I had the bike running 10 days ago, everything seemed great. Went out one night to fire it up and the mUnit arched(?). The lock input burned up! I ordered a new one, went through all wiring and confirmed things were fine. My sensing wire from the RR connects to the key wire and then goes into the lock input. I replaced these connections and confirmed all grounds were good on the bike.

Connected the new mUnit, testing each input/output as I added more. Was able to turn everything on (head/brake lights, turn signals, aux) and called it a night. Unhooked the battery. Came back the next night, connected the battery, tested battery at 13.3V, lights in the mUnit signaled I had connection. I turned the key and it arched again! Burned up the same lock input.

I pulled the mUnit to see if I could test it without everything on the bike. Connected the battery/ground, key, and a light with a switch. Turned the key on and nothing happened for about 5-7 seconds…then the same lock terminal started smoking again.

Any ideas? Could it be an issue with the key? I’m using what I had on the original bike.

I used this wiring diagram from Revival: https://technicalarticles.revivalcycles.com/hc/en-us/articles/360009106131-m-Unit-Blue-Basic-Universal-Wiring-Diagram

Thanks for any advice!

[RAFster122s]

Shorting to ground somewhere in that circuit is my bet...

[jakew]

Thanks, RAF122S. I appreciate the reply.

I talked to a guy from Revival and he thinks it might be the key switch. I spent a little time testing last night and was able to recreate the problem/fire. All I had connected to the unit was the key switch so it looks to be the culprit. Images of the setup attached including a video of how it burns.

I was also told that in 8 years of working with these he's never had this issue. Has anyone else run into it? Is there a way to confirm it's the switch and not the battery or my ground setup?

Thanks for the help!

[albzn]

Hi, quick question. Do you still have the burned out m.units at home? I'm currently working on my own, open-source, type of m-unit and I'm wondering what kind of electronics that are inside!
Thanks in advance

[albzn]

Hi, quick question. Do you still have the burned out m.units at home? I'm currently working on my own, open-source, type of m-unit and I'm wondering what kind of electronics that are inside!
Thanks in advance