DMC failure?

L

laverda180

Junior member
Location
Amberley, New Zealand
Today I started my series 2 Jota after a rebuild that included the rebuild of the engine and carburettors; thanks OCT, new Nology coils and a home made wiring loom integrating a number of solenoids and the DMC ignition that I originally fitted in 1988. Bike started fairly easily and ran for 20 seconds, but clearly needed some setting up so I shut it off to fetch tools. I hadn’t balanced the chokes was the likely reason, centre fully open, right just starting to lift, left half open! Set them, but it wouldn’t restart. I think the DMC has died. DMC internal resistance to each pick up is 137 Ohm, the pick ups 230 Ohm. The coils are both 2,7 Ohm. The DMC internal resistance for lines to coils is however strange. L/R output is 4,3 MOhm with ignition and or run off and open circuit when both on. This is consistent over many tests. The centre is initially 4,2 MOhm and 8 MOhm, but when either switch is turned off and then back on is 590 kOhm with either / both off and 0 Ohm when both on. Resets if I turn both switches off, but only to reappear. Power to coils and DMC is only on when both ignition and run are on, so does not appear to be a wiring issue. Are these resistance values correct? Battery and charge are healthy. Sparks; plugs out, are weak and appear unreliable with new Champion DC3, even worse with new NGK B8ES. Pick ups are both gapped at 0,3mm. Curve 1 selected for standard 32 degree advance (7mm BTDC), BTDC determined with vernier caliper.
 
I'm not able to provide DMC resistance values to compare with what you have measured - it's generally not a good idea to apply voltage (for measuring resistance) to electronic circuits without knowing details of the internal circuitry. Hopefully you haven't damaged the unit. The pickup input transistors are particularly vulnerable.

The pickup resistance value for the Bosch pickup fitted to series 2 180s and all 120s is approx 210 ohm.

The DMC instructions provide troubleshooting guidance - assuming you have the early DMC unit, screengrabs attached. Complete document is in the library along with info for later DMC2.

The DMC unit is intended to work with 3 ohm coils - so your Nology coils will be drawing more current from the DMC than it is designed for. The original Nippon Denso coils are very reliable, as well as having the required higher resistance. Many of us have successfully substituted with TEK KP03/MP08.

You may have a problem with the original pickup wiring which goes brittle and flakes off with age, shorting out the very low voltage. Or the pickup wiring connections in the DMC plug. Again, the Witt troubleshooting guidance will rule this in or out.

Apparently the early DMC is susceptible to failing if running voltage goes high - I assume you haven't got your bike hooked up to a battery charger?
 

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Battery is usually charged off the bike, but for sure has been charged at times whilst installed! Pick up wires and connectors are all new. The inputs to DMC are at least consistent so without specs my guess is that side is good. Original centre coil was not great, can’t find data hence change to Nology which were used for approx 1000km before this major overhaul inc new loom and reg/rec as original was faulty. A couple of months ago, during the rebuild, the centre Nology failed through over heating, L/R was fine. I had left ignition and run switches on whilst doing something or other. I read now that DMC is supposed to protect ignition coils in this case, but seems it did not. Thanks for manual extract, will try the suggested test
 
Failed reg/rect will likely have seen high volts which may have damaged the DMC box.

As you note, fried HT coil should not have occurred since the DMC should turn off if engine isn't running ... maybe another indicator of failed box.

Using Nology coils pulling 10% more current than designed may have shortened output transistor life.

Final thought - it's worth checking your new loom earth (or ground) connections are all good. And the DMC prefers to be connected direct to battery negative.
 
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Thanks Rob. DMC test 4 showed it has failed. Probably combination of factors you suggest. New loom earths could be better by design, however the design fault shows all systems are either dead or live depending on earth case. Seeing as I now have the Nology coils I am thinking to install a Sachse unit as rated for coils from 2-5 Ohm. Are these units also sensitive to charging battery when installed?
 
Can't find a tech spec for the Sasche but would expect it to be capable of handling normal working voltages. DMC and Ignitech quote up to 18v. Ignitech will happily work with low impedance coils and I believe DMC can now be similarly specified.
 
Sachse ignition will work with coils of 1Ω – 3Ω impedance.
Ignitech will work with coils of 0.5Ω – 5Ω impedance.

Ignitech is a pretty impressive ignition and is the better choice in my opinion. It has way more features than either Sachse or DMC (or any other aftermarket ignition that I'm aware of). It's fully programmable with two switchable advance maps that respond to both revs and engine load (manifold pressure) and it has a heap of other useful features. And it's cheaper that pretty much anything else on the market.

I have no financial interest in plugging Ignitech. I just think they make a brilliant product. I've installed them on Ducati, Laverda, Moto Guzzi, BMW and Honda bikes. They've always worked flawlessly.

Ignitech will work with your existing inductive pickups, or with Hall effect. If you're using hall effect, you just have to specify that at the time of ordering. But rather than re-invent the wheel, talk to Red (Redax5 on this forum) about his bolt-on alternator upgrade with Ignitech/Hall ignition system.
 
I've fitted Ignitech adjacent to a reg/rect with no issue. Be aware that the reg/rect needs cooling air... I wouldn't recommend it being in contact with the ignition black box.
 
On my 3C, the reg/rect is on the rear mudguard and the Ignitech box in the tool box.
On my wife's 500, I put the reg/rect in the tail on a piece of alloy plate that acts as a heat sink.
Paul
 
Paul Marx said:
On my 3C, the reg/rect is on the rear mudguard and the Ignitech box in the tool box.
On my wife's 500, I put the reg/rect in the tail on a piece of alloy plate that acts as a heat sink.
Paul
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Interesting. The tool box and tail piece are two places I wouldn't put such things. :)
Storage spaces are premium real estate on a bike. I like to use the tool space and tail unit for useful stuff like tools (no such thing as too many tools). The tail is an enclosed space with no ventilation. A heat sink won't sink heat forever if it can't cool itself.

When I bought my Jota, the PO had wrapped the DMC unit in rags and stuffed it into the tail piece. He said the rags were to protect it from vibration, but it got very hot in there. Too hot to touch. So I mounted it somewhere to get some air circulation but it died not long after that. I guess its years of heat stress in the tail unit took a toll.
 
Years ago I was building up a four-stroke dirt bike and wondered where would be the most benign location to fit the reg/rec. I settled on fitting it in the air box as a steady flow of cooling air is guaranteed and its out of the way of knocks and general grime.
 
Dellortoman said:
Interesting. The tool box and tail piece are two places I wouldn't put such things. :)
Storage spaces are premium real estate on a bike. I like to use the tool space and tail unit for useful stuff like tools (no such thing as too many tools). The tail is an enclosed space with no ventilation. A heat sink won't sink heat forever if it can't cool itself.

When I bought my Jota, the PO had wrapped the DMC unit in rags and stuffed it into the tail piece. He said the rags were to protect it from vibration, but it got very hot in there. Too hot to touch. So I mounted it somewhere to get some air circulation but it died not long after that. I guess its years of heat stress in the tail unit took a toll.
Click to expand...
The original reg/rect on the 500 is behind a side panel. Tail piece is likely cooler.
I only take tools on longer runs, in which case, it's tank or saddle bags. Touch wood, I've never had to regret it.
Paul
 
Paul Marx said:
I only take tools on longer runs, in which case, it's tank or saddle bags. Touch wood, I've never had to regret it.
Click to expand...

A few years ago some friends and I were on a ride and they were having a bit of a laugh about the vast quantity of tools secreted in every nook and cranny on my bike. Jibes about me being prepared for Armageddon and stuff like that.

Later that day one of them suffered an electrical failure and his bike stopped dead. Out came the multi-meter and the fault was soon found. It was a cable that had broken inside the insulation at a spade terminal. It wasn't obvious to the eye so would have been very difficult to diagnose without a meter. I didn't have spare terminals and a crimper among my tools, but I did have a gas soldering iron, so I soldered the terminal back on. We were on our way again in no time. There were no more jokes about my tool inventory after that.
 
Dellortoman said:
A few years ago some friends and I were on a ride and they were having a bit of a laugh about the vast quantity of tools secreted in every nook and cranny on my bike. Jibes about me being prepared for Armageddon and stuff like that.

Later that day one of them suffered an electrical failure and his bike stopped dead. Out came the multi-meter and the fault was soon found. It was a cable that had broken inside the insulation at a spade terminal. It wasn't obvious to the eye so would have been very difficult to diagnose without a meter. I didn't have spare terminals and a crimper among my tools, but I did have a gas soldering iron, so I soldered the terminal back on. We were on our way again in no time. There were no more jokes about my tool inventory after that.
Click to expand...
Indeed, I have similar stories. I’ll never understand people who ride old bikes and don’t have at least basic tools.
Mind you, soldering iron…? 😉
 
My Pantah started slowing on an expressway on a very hot day, cracking 40c. I had adjusted the lever height on the rear brake and taken too much free play out causing the brake to drag get hot and lock the brake on. All I had was a lone 13mm ring openender and what I needed to loosen and back off the high adjuster was a 10mm openender.I did have a 20c coin though, thick enough to fit into the 13mm spanner and allow it to undo that 10mm nut. Guess if I now carry more tools plus I always carry water. You learn about things like that. BTW that was the same 40c day I learned that Gel inserts sown into motorcycle seats to improve comfort don't do well being parked in full sun for hours, my arse was well cooked from the heat that gel socked up and held for hours after leaving skin burning heat. It now has a sheepskin cover to stop that nightmare from happening again.
 
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