Makita 6401

[heyduke]

UPDATE ON SAW,
Took the saw back to the dealer this morning. After talking with him, he assured me he checked the fuel line and impulse line before returning to me. I put a spark tester on it after I couldn't get it to start, it did have spark, BUT as stated earlier it still may not be enough or strong enough. The dealer believes it may be a bad coil. I will update when the saw is returned from the dealer.

i think coil (module) failures might be a problem with this model. if you read the contributions from poleman above, when he mounts a husky 272 module he has to advance the ignition 30 degrees. that tells me there is a bunch of circuitry in the dolmar/makita modules that not only limits rpms but alters timing too. those are components that could overheat and fail. also, the fact that you see a spark doesn't assure me that it's happening at the correct time. another thing, the fact that so many guys are curious about replacing the dolmar/makita modules could indicate a high failure rate.

 

[fordf150]

i think coil (module) failures might be a problem with this model. if you read the contributions from poleman above, when he mounts a husky 272 module he has to advance the ignition 30 degrees. that tells me there is a bunch of circuitry in the dolmar/makita modules that not only limits rpms but alters timing too. those are components that could overheat and fail. also, the fact that you see a spark doesn't assure me that it's happening at the correct time. another thing, the fact that so many guys are curious about replacing the dolmar/makita modules could indicate a high failure rate.

Replacing with the husky module is to get rid of the limited coil mainly. That's my understanding anyway.

 

[fordf150]

chris-

i tried everythin i could but there just wasn't enough wire. i tried different routing but no luck. i need an inch an a half but only can find 1/4". good news is, after taking of the ignition module and unscrewing th spark plug wire, cleaning and putting it all back together again, its running good.

Plug wire is cheap. I keep a bulk roll of it around for replacement jobs. It is tough to get a new wire through the sleeve on the 7910. Be careful taking those clips out that hold the wire under the flywheel. Dolmar recommends replacement every time they are removed. I just spread them back out so that they "dig" in when reinstalled.

 

[heyduke]

Plug wire is cheap. I keep a bulk roll of it around for replacement jobs. It is tough to get a new wire through the sleeve on the 7910. Be careful taking those clips out that hold the wire under the flywheel. Dolmar recommends replacement every time they are removed. I just spread them back out so that they "dig" in when reinstalled.

you may be right about guys wanting a non-limited coil, but my perception has been that there has been a higher than expected failure rate and one is faced with a decision, pay over $130 for a dolmar module or $15 for a more robust husky module.

thanks for the heads up about the clips. i can see how they might cause problems if they come loose behind the flywheel. i was careful to get the clips back in securely but will pay even more attention next time. i'm curious about where you found a roll of 5mm plug wire. stihl sells one foot lengths with a stihl part number. weedeaterman seems to sell it by the foot but it's unclear whether they have two foot lengths. i don't think one foot would be enough.

 

[fordf150]

you may be right about guys wanting a non-limited coil, but my perception has been that there has been a higher than expected failure rate and one is faced with a decision, pay over $130 for a dolmar module or $15 for a more robust husky module.

thanks for the heads up about the clips. i can see how they might cause problems if they come loose behind the flywheel. i was careful to get the clips back in securely but will pay even more attention next time. i'm curious about where you found a roll of 5mm plug wire. stihl sells one foot lengths with a stihl part number. weedeaterman seems to sell it by the foot but it's unclear whether they have two foot lengths. i don't think one foot would be enough.

I will PM you later today or tomorrow with a price on plug wire. Didn't realize it was that hard to find or it would already be on my website. I buy it in 25 foot rolls.

I have only ever sold 1 7900/7910 coil so failures aren't to common but the 12800 limiter is really annoying to tune and with a ported saw ......guys may be against the limiter even with a 20" bar buried

 

[heyduke]

I will PM you later today or tomorrow with a price on plug wire. Didn't realize it was that hard to find or it would already be on my website. I buy it in 25 foot rolls.

I have only ever sold 1 7900/7910 coil so failures aren't to common but the 12800 limiter is really annoying to tune and with a ported saw ......guys may be against the limiter even with a 20" bar buried

thanks-

i suspect the limiter is there for a reason. dolmar uses the same case and crank for saws from 64cc to 79cc (and we bump that to 84cc). maybe their engineers/designers don't feel comfortable running the 79cc saw above 13k rpm. my experience has been that any limited coil is more likely to fail. there's just more parts to screw up. bear in mind too that the module is switching around 170 times per second and has a .5 msec delay circuit that operates at the same freq (30 degree retard) and there is no heat sink, just epoxy. and the whole mess is positioned where oily chips shoot out the front. shouldn't work but it does, for a while. i'm not whining about dolmars. i have two 84cc makitas and i think they're great saws. all the manufacturers have the same problem.

 

[Paragon Builder]

thanks-

i suspect the limiter is there for a reason. dolmar uses the same case and crank for saws from 64cc to 79cc (and we bump that to 84cc). maybe their engineers/designers don't feel comfortable running the 79cc saw above 13k rpm. my experience has been that any limited coil is more likely to fail. there's just more parts to screw up. bear in mind too that the module is switching around 170 times per second and has a .5 msec delay circuit that operates at the same freq (30 degree retard) and there is no heat sink, just epoxy. and the whole mess is positioned where oily chips shoot out the front. shouldn't work but it does, for a while. i'm not whining about dolmars. i have two 84cc makitas and i think they're great saws. all the manufacturers have the same problem.

Well thanks Heyduke! I was trying to figure out why the unlimited mod required a 30 deg advance with the coil in the same spot.
Now I know!!![emoji106]

 

[heyduke]

Well thanks Heyduke! I was trying to figure out why the unlimited mod required a 30 deg advance with the coil in the same spot.
Now I know!!![emoji106]

glad that helped clarify things. the numbers, other than the 170hz switching freq, may not be correct but i think they're in the ball park. i suspect that the reason they throw in a .5 msec delay is to allow switching noise to die down before the limiter circuit decides whether to turn off the spark plug or to let it fire. to have that kind of circuitry hanging off the side of a hot, dirty, oily, hyper-kinetic chainsaw is crazy, but like i said, it works for a while.

 

[fordf150]

shameless plug for my site. http://performanceoutdoorequipment.com/search?q=plug+wire&x=0&y=0

 

[Chris-PA]

glad that helped clarify things. the numbers, other than the 170hz switching freq, may not be correct but i think they're in the ball park. i suspect that the reason they throw in a .5 msec delay is to allow switching noise to die down before the limiter circuit decides whether to turn off the spark plug or to let it fire. to have that kind of circuitry hanging off the side of a hot, dirty, oily, hyper-kinetic chainsaw is crazy, but like i said, it works for a while.

Some misconceptions - the 0.5ms is about 30deg at 10,200rpm. This is how they control the timing "advance". Clearly a circuit cannot fire a spark before it gets the trigger, but since it is a cyclical process you simply rotate the magnet 30deg before you want the spark (more or less TDC). Then you can delay the spark to TDC, or any point in between. So the 0.5ms "delay" is actually part of the spark advance curve.

The time delay should not cause any additional heat, and the kind of parts needed for the ignition can be quite tolerant of heat. Also, the ignition module is sitting right outside of a big spinning fan, and so it should be well cooled during operation. Heat soak after shut off would be the big worry. But of course some designs are better than others, and often the people doing the mechanical design don't consider the electronic design and vice versa.

 

[heyduke]

Some misconceptions - the 0.5ms is about 30deg at 10,200rpm. This is how they control the timing "advance". Clearly a circuit cannot fire a spark before it gets the trigger, but since it is a cyclical process you simply rotate the magnet 30deg before you want the spark (more or less TDC). Then you can delay the spark to TDC, or any point in between. So the 0.5ms "delay" is actually part of the spark advance curve.

The time delay should not cause any additional heat, and the kind of parts needed for the ignition can be quite tolerant of heat. Also, the ignition module is sitting right outside of a big spinning fan, and so it should be well cooled during operation. Heat soak after shut off would be the big worry. But of course some designs are better than others, and often the people doing the mechanical design don't consider the electronic design and vice versa.

chris-

thanks for the feed back.

i did the calculations in my head, just to one significant figure, and as i stated it was "ballpark." two points regarding the 30 degree retard, i don't see how that could be part of the advance curve. that would be way too much. i'm no engineer and i don't have the bosch handbook at hand but i would guess that going from starting to 12.8k rpm would require something like ten degrees of advance. also, i would assume that the unlimited husqvarna coils employ a spark advance circuit that doesn't need a 30 degree delay. and as far as thermal issues go, why are we having more frequent ignition module failures? husky used to guarantee their modules for life (maybe that was an epa requirement) but that's something no one even remembers. by the way, i only had to use that guarantee once, a 272 in daily use by an arborist's crew. and regarding the fan, 6400/6401's accumulate bar oil and chips in the area around the ignition module, in front of the flywheel/fan. they can become completely impacted with schmutz that encases the module, which i think can lead to thermal failure. and no matter how hard you blow on it, epoxy is a lousy heat sink.

the one-shots (time delay) would indeed add to the thermal load but i don't know if that would be significant. however, in an overheated module they would be subject to failure or malfunction just like the switching circuitry.

 

[Paragon Builder]

So if it is a straight up .5ms delay, wouldn't it have the most advance at idle, and retard as the rpms rise?

 

[Chris-PA]

two points regarding the 30 degree retard, i don't see how that could be part of the advance curve. that would be way too much

Yes, but it likely would be using some of that delay just for the supervisory circuitry. My assumption is that circuitry has been added to the standard CD design that varies the delay based on rpm, and this circuit may have a additional delay of its own. So if you want to vary the advance from say, 0-20deg, you might instead vary it from 10-30deg, moving the magnet ahead another 10deg and giving you a little more time to work with. I'm only guessing there though.

as far as thermal issues go, why are we having more frequent ignition module failures?

Well, not all things are designed equally well, or made with parts of the same quality.

So if it is a straight up .5ms delay, wouldn't it have the most advance at idle, and retard as the rpms rise?

Yes it would, but it probably has a delay that is fixed in degrees, not in seconds. And more modern ignitions will have a delay that varies with rpm.

 

[Paragon Builder]

Yes it would, but it probably has a delay that is fixed in degrees, not in seconds. And more modern ignitions will have a delay that varies with rpm.

Oh ok. I thought someone said it was a fixed number. I'm understanding better now!

 

[heyduke]

So if it is a straight up .5ms delay, wouldn't it have the most advance at idle, and retard as the rpms rise?

actually, i think you have that backwards. timing is most retarded for starting and advances as the rpms rise so that combustion can be completed at tdc.

from the wikipedia article on ignition timing:

"Spark timing, relative to piston position, is based on static (initial or base) timing without mechanical advance. The distributor's centrifugal timing advance mechanism makes the spark occur sooner as engine speed increases.

Yes it would, but it probably has a delay that is fixed in degrees, not in seconds. And more modern ignitions will have a delay that varies with rpm.

doesn't really matter whether you think in degrees or time. the figure of merit is the amount of time it takes to burn a particular grade of fuel vapor. at a higher rpm you have to start earlier because because the crank rotates in a shorter period of time. any part of combustion that occurs after tdc is wasted.

Yes, but it likely would be using some of that delay just for the supervisory circuitry. My assumption is that circuitry has been added to the standard CD design that varies the delay based on rpm, and this circuit may have a additional delay of its own. So if you want to vary the advance from say, 0-20deg, you might instead vary it from 10-30deg, moving the magnet ahead another 10deg and giving you a little more time to work with. I'm only guessing there though.

i was "only guessing" when i hypothesized that some of the delay was to disable detecting of rpm until switching noise settled down, but i still think that it's a good possibility. i've seen it before in analogue to digital circuits. one thing has come of this discussion, though. it's time to order a new copy of the bosch automotive handbook from abe.com.

and i'll stick to my guns. the increased failure rate of ignition modules is likely due to their more complex designs, not component quality. it's the electronic version of the stihl flippy cap.

 

[fordf150]

it's the electronic version of the stihl flippy cap.

 

[Paragon Builder]

actually, i think you have that backwards. timing is most retarded for starting and advances as the rpms rise so that combustion can be completed at tdc.

from the wikipedia article on ignition timing:

"Spark timing, relative to piston position, is based on static (initial or base) timing without mechanical advance. The distributor's centrifugal timing advance mechanism makes the spark occur sooner as engine speed increases.


.

Yes this is how it should work which is why I posed the question. With a set time delay the opposite would occur.

 

[Chris-PA]

the increased failure rate of ignition modules is likely due to their more complex designs, not component quality. it's the electronic version of the stihl flippy cap.

It is inherently true that increased complexity provides increased opportunity for failures.

Still, these things are not that complex, nor are the rpm limiting or spark curve functions new - there are plenty of ignition modules out there doing both for years now. So I think that if a particular saw has a high failure rate of ignition modules (and I'm not sure that's been proven), then I'd say they blew it!

 

[heyduke]

It is inherently true that increased complexity provides increased opportunity for failures.

Still, these things are not that complex, nor are the rpm limiting or spark curve functions new - there are plenty of ignition modules out there doing both for years now. So I think that if a particular saw has a high failure rate of ignition modules (and I'm not sure that's been proven), then I'd say they blew it!

i haven't seen any hard data supporting a high failure rate but it is my perception. these saws are rare compared to stihls or huskies but there seems to be a lot of interest in their ignition modules. someone said earlier that it was only because guys were looking for an unlimited coil but i think most makita/dolmar owners "don't mess with it if it ain't broke." that certainly describes me. that's what got me curious. i have two 6401's one had symptoms of a failing module, wouldn't run after it got hot, wouldn't restart until half an hour after it cooled down. i balked at the cost of a new module, $130. when checking to see if it was red or black i encountered the oil/chips impacted module. i cleaned it up as well as i could without removing it and it started running when hot, though it felt like the rpm limiter wasn't working. it would rev to 13.5k, maybe more. so i took the module off and cleaned the back side and the cavity where it rests. now it runs correctly. so my conversion to 272 mode has been put on hold.

 

[Andyshine77]

and i'll stick to my guns. the increased failure rate of ignition modules is likely due to their more complex designs, not component quality. it's the electronic version of the stihl flippy cap.

The 7900 does not have increased coil failure rate that I'm aware of. I also don't see a problem with a solid state coil being a problem, or overly complex.