2002 Suzuki VZ800 (Marauder) - Mystery Module

[monkeyfarm]

Dumping my notes so far and will reorganize as I go, hopefully someone finds this useful or derives something from it. A good laugh at least?

Problem: The starter relay control on older VZ series seem to die due to unknown reasons. The available material on the issue doesn't go into the reason behind the failure of the module and the available DIY fixes don't sit well with me, more on that later. Replacement modules (Mitsuba RK-0313) are rare or expensive, IMHO, and it seems that unless they're new old-stock, they will likely fail in short order as well.

What it ain't: Lots of sources refer to it as the MMM, Mitsuba Mystery Module, or just the Mystery Module.

What it is: Part of the ignition system, it allows the starter relay to energize only when the clutch lever is pulled in, as indicated by the wiring diagram. I gutted it and soaked it in acetone to remove the potting compound. It seems to be based around a A1568 transistor, and the other board components seem to indicate it was configured to act as a relay itself with a snubber network as well. Considering the starter relay is in line with this module, it could have been fried by it.

Found Fix: Replace it with a relay, or just bypass it by splicing some wires together. Both solutions could end with the electrical contacts within the clutch lever welding themselves together. There is some merit to replacing it with a relay that has a built in diode, which I may consider if I give up.

Proposed Fix: Reverse engineer and repair, or reproduce the entire module. If it's worth doing, it's worth overdoing (famous last words)...

 

[monkeyfarm]

Going in for a tire change and safety check tomorrow so I had to hide my shame and improve on my wiring hack. I'll continue on reverse engineering the module at some point this month, hopefully, but for the time being I just want to get out and ride.

I have a bunch of relays from a buddy's long gone '99 Corolla (tear and beers, it will be missed) and I cracked a few open tonight out of curiosity. I initially thought I could add a diode in parallel with the coil, as mentioned in post #1, and stuff it all back together. After taking the thing apart, I noticed there's already a resistor wired in parallel, which is great. It takes longer to dissipate the ~200 volt spike with a resistor versus a diode, but its still only about 10-20 milliseconds. Between starting the bike and letting go of the clutch just needs to be longer than that amount of time and the clutch contacts are safe.

The sacrificial relay:


The innards:


The hack job:


The fix (temporary):

 

[TwistedKestrel]

I don't understand what problem Suzuki originally solved with this. It seems like an unnecessarily complicated way to implement a clutch interlock

 

[monkeyfarm]

I think the logic was to run as little current through the clutch interlock switch as possible, to extend its life (and maybe something else?). If you used a relay, or nothing, because the interlock circuit is not disabled when the motorcycle is on, every time you pull in the clutch lever, the interlock switch would close and pass current to indicate a safe starting condition. With a high current passing through the thin components of the switch, arcing could cause the contacts to erode or weld closed. Using a transistor based circuit, rather than a relay, would mean that significantly less current would be passed through that interlock switch. The support components within that circuit could also absorb the inductive spike coming from the starter relay coil downstream (~200V) further protecting the rest of the motorcycles electrical system. You could sort of replicate this with a relay and a diode or resistor, but it would be crude less effective. I am by no means an expert, and am open to being corrected.

Since the season is upon us, I went with "crude and less effective" for the time being.