Cordless Mulching Mower woes (Black and Decker CMM 875)

I have a B&D cordless mulching mower that suddenly stopped running. I've switched out the battery, but no joy. The motor turns freely, so does the blade. The unit quit in tall, wet grass so I am thinking some sort of overload kicked in. Unfortunately, I wasn't mowing. The kid doing the mowing for me said it just stopped, but I suspect he took too big a "bite" of the wet, tall grass and burned up some protective element. The unit's simple: A motor, a controller board with a relay, a handle switch and a wheelchair sized 12V SLA battery. The motor shaft will, on occasion, turn a few degrees with the pulse of the switch on the handle, but mostly it just strains and whines slightly.

Any suggestions or clues welcomed. FWIW, the next step is to run the motor directly off the battery without the intervening controller board (not sure what it does, actually - it's got a black plastic cover so solidly attached I can't remove it to trace the circuit without seriously damaging it). What I can see of the controller board is a heat sink about 1" by 5" by 1" that I assume is a voltage regulator and a small black relay. Also, there used to be a clicking sound when the unit engaged (the relay, I'm guessing) that I no longer hear which leads me to believe that the relay has failed or is not getting sufficient power.

I've been drying the unit out (it got very wet trying to mow tall, wet grass) for a few days, but that doesn't seem to have made a difference. )-:

TIA for any help. Bad comedians need not apply. (-:

-- Bobby G.

Reply to
Robert Green
Loading thread data ...

Suggesting that you see about downloading the manual first

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Reply to
Michael B

I see that one good first step would be to call 1 800 54 HOW TO

Reply to
Michael B

Yes, Bob, the blade spins very freely.

They happen to be easy to locate and inspect and they look intact and in good contact with the armature, but I'm not a motorman, so I am not exactly sure what to look for other than good contact and not even sure how to determine that precisely. Based solely on the small amount of use the mower has seen over the years, I tend to believe the problem lies elsewhere.

I can hook a multimeter up to the brush contacts and look for high resistance reading - I'll Google "motor brush inspection" to see if I can find out more about the brushes. My gut feeling, from the way it performed (well) up until the moment of its death, that the brushes are not the culprit. I would expect that mode of failure to be gradual and noticeable in performance/function degradation. This was five seconds of spin and then nada, zip, zilch, nothing.

First thing. Even swapped in a known-good battery of similar voltage. I would never have expected a sealed lead-acid battery to last 16 years, but apparently it did. The original (looks to be a 40Ah job - can't see the label without dismounting it, though) is roughly 12" on all three sides. That's why the next step is to bypass the controller board (containing a relay and a voltage regulator) and to power the motor directly from a known good battery. Since one of my theories is that something got wet, I am waiting until the unit has been indoors under humidity control to dry out. Also thinking of squirting it with WD40 to displace any moisture still trapped.

If I could get to the solder side on the controller board, I would trace the power to the relay. The whining sound it makes when trying to engage doesn't come from the motor, it comes from the controller board, and when I find my stethescope, I will try to determine precisely what is whining. I suspect it's the relay. Removing it isn't going to be easy but may be the only option if no replacement parts are available.

Thanks for your input!

-- Bobby G.

Reply to
Robert Green

difference. )-:

I see that one good first step would be to call 1 800 54 HOW TO ===============================================

That's the manual for the CORDED mulching mower - I haven't been able to find similiar information for the Cmm 875 - the "C" being for Cordless.

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goes nowhere.

B&D's helpfulness when contacted by email was to simply give me the name of a local repair shop - for B&D hand power tools!!! We'll see what they have to say on Monday but from what I've seen of parts lists from two different parts vendors, the key parts for these mowers (control boards and motor armatures) are no longer made. This was a 1995 model mower that's spent most of its life in my basement. It's almost brand new but apparently B&D makes parts that can't survive the simple test of time. )-:

BTW, when I tried B&D's own "Servicenet" website gave me an error message that said "Under Construction - please revisit us after Nov. 15, 2010. !!!!!!!????

Thanks for your input!

-- Bobby G.

Reply to
Robert Green

difference. )-:

Suggesting that you see about downloading the manual first

formatting link
============================================ Turns out the damn typeface B&D uses for their nameplate makes a 6 virtually indistinguishable from an 8 so the real number is CMM675, which shows a

*lot* more hits than the non-existent CMM875. Judging by the amount of hits for the non-existent parts number, I was not the only one fooled by B&D's idiotic choice of type faces.

I found the exploded parts diagram at

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but alas, the parts I need are not available from B&D either, which likely means it was defective from the get-go and all the spares in their inventory sold out long ago. When I get the damn controller board apart, I'll post pictures of that along with the nameplate and the eight that's really a six.

The only thing more stupid than a lousy typeface is including zero's and oh's and one's and l's in the serial number. Is it 10100 or lOlOO? Hard to tell if you're using a fancy typeface instead of just trying to communicate basic information in a non-ambiguous way. My next mower WON'T be a Black and Dickhead. )-:

-- Bobby G.

Reply to
Robert Green

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If you have an ohmmeter, measure the two leads going to the motor and see if you read something in the range of 50 ohms or so, I have no idea what the correct number should be, but under 100 ohms and more than 1 ohm seems reasonable. Then, assuming there are only 2 leads to the motor, clip lead the motor directly to the battery bypassing the control, something should happen, like the motor runnning full speed. You haven't said if there are more than one speed for the mower, if it is push or self-propelled, etc. A lot of us electrical types could be more helpfiul with more info.

Reply to
hrhofmann

"hr(bob) snipped-for-privacy@att.net" wrote in message news:2b010cfe-1311-45ba-ab82-On May 22, 8:31 am, "Robert Green" wrote:

If you have an ohmmeter, measure the two leads going to the motor and see if you read something in the range of 50 ohms or so, I have no idea what the correct number should be, but under 100 ohms and more than 1 ohm seems reasonable. Then, assuming there are only 2 leads to the motor, clip lead the motor directly to the battery bypassing the control, something should happen, like the motor runnning full speed. You haven't said if there are more than one speed for the mower, if it is push or self-propelled, etc. A lot of us electrical types could be more helpfiul with more info. =========================================================== It's a push mower, not self-propelled. Also, as I recall, but can't confirm, it has only one speed but ironically, I can't really bet my life on it. The handle is designed to have only two positions, ON and OFF but the diagram I found shows the handle switch has three wires coming out of it, which would seem to indicate something more than a normally open single pole, single throw switch. I am hoping that it disconnects rather easily so I'll be able to measure continuity and resistance in the switch circuit and perhaps determine the function. Regrettably it seems to be part of the sealed, lacquered control board and access might only be via a vampire tap. That might not give me good readings since it won't be isolated from the main circuit if I have to pierce the existing cable. I suppose I could always cut and resolder the switch wires - they are pretty small gauge wires.

You may think it's odd I don't remember whether it's variable speed control, but I always ran it "flat out" because it's pretty anemic compared to a gas or corded mower. Thanks for the instructions about reading the resistance (I have about 20 ohmmeters scattered around - I collect them like I do flashlights - you can never have enough). I'll try that next.

Running the motor directly off of 12VDC isn't going to be easy because the wires to the motor are thick and their termination connections on the controller board are both nutted and soldered and then covered over by a thick plastic plate and tons of lacquer. I may have to cut them to get access to them and I don't think I have any good replacement wire - it looks like #10 or thicker stranded wire but I'm only basing it on diameter - it could have an incredibly thick jacket. Oddly, the cables to the motor are thicker than the cables going to the battery. I thought it would be the other way 'round.

The current plan is to consider modding the mower as follows: if the motor runs directly off a 12VDC battery then I'll bypass the controller board with one of my own design that incorporates a relay and perhaps a voltage regulator. I'll also modify the slow 2A two day charger with one of the much bigger 15A wheelchair battery chargers I have and make the battery

*easily* removable - something it took B&D 15 years to incorporate into their design.

The original battery could mow the lawn if it was just a few inches tall and not wet. This way I can either take a brief break and restore enough charge via the high amp charger to finish the lawn or swap in a fresh battery (B&D part costs 132 bucks, similar batteries cost $60 on Ebay).

Biggest concern about the modification is that there may be some function of the controller card that I am missing that's critical to operation. Considering that spare parts are no longer available, even if the sucker burns up it won't be a great loss. It's a neat, nearly noiseless device that cost me $159 in 1995. Currently they want over $300 for the simplest model and close to $500 for a three (removable) battery top of the line model. Apparently I wasn't the only one who noticed the early models didn't really have enough power to do more than a very small lawn. (-: If the mission's a total failure, there's another company that makes much cheaper cordless mowers - Earthworx, IIRC. I think I've had enough of B&D. I've got a whole big box of B&D cordless gear that died prematurely and need battery rebuilds that would cost as much as the original tool. )-:

Any ideas how to determine whether the relay has failed? Since it's a mechanical part, I'd suspect it was the problem long before I'd "indict" a passive component for the problem. I found my old stethescope but the membrane is cracked. I'm going to convert it to a solid bar head so that I can place the metal against the various board parts to hear whether they're vibrating (or buy a solid head 'scope from HF for $5 on my next visit).

Thanks for your input, Bob. Much appreciated.

-- Bobby G.

Reply to
Robert Green

Pretty sure that's the one I had, bought in early '97. One speed.

Think I paid $189 - Sears. It was cheaper than a decent gas push mower. I liked it, and it did my lawn on a charge for maybe the first 5 years if the grass wasn't too high, then it tailed off the next 2 years so at the end it would take 3-4 charges to do the lawn, so I left it at the curb for the trash pickers. Don't know what's small or large, but I'm on a corner and it went all around and in back before slowing down at the end - first 5 years. Good luck fixing it. Think I would have bought a second one, but like you I saw the price of these really escalated. Can't figure that one out.

--Vic

Reply to
Vic Smith

Yes, I believe it would take far more electronics than a cigarette-sized board to enable variable speed - some sort of pulse modulator/chopper. IIRC, it ran at one speed, too, but I couldn't be sure.

At the CURB?!!!! Probably with the controller board (the piece I believe I need) intact!!!!! Oh, the humanity!!!!! (-"

I hope I can restore it. It's had very little use - still on the original blade although I bought a spare and some other accessories for 20 cents on the dollar when Hechinger's went belly-up.

It did its job and did it well for as long as it lasted. It sounded like all you needed was a new battery. For a small lawn the cordless is much better than screwing around with gas engines or cords. Maybe I'll get lucky and find someone who's still got one stored away somewhere with an intact controller board. Oh well, it's going to be a fun project to restore it if I can successfully bypass the original controller board.

If that doesn't restore it, I'll buy one of the cheaper cordless "Earthworx" mowers. $500 is too much to pay for a frikkin' push mower.

Thanks for your input, Vic.

-- Bobby G.

Reply to
Robert Green

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You could skin off the insulation on one side of the wires going to the motor to clip onto them, and then wrap them with several layers of duct tape after you are done experimenting. One thought if it turns out to be the battery, is to convert the mower to a plug-in-the-wall mower, feeding AC power to your battery charger and then feeding the

12V output to the mower via a regular extension cord. OR you could mount the battery charger in place of the battery, and then run the entire mower directly off of 120V extension cords. Let us all know what finally happens.
Reply to
hrhofmann

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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

My bet would be a totally failed battery.

Reply to
bob haller

firsthttp://www.black

You could skin off the insulation on one side of the wires going to the motor to clip onto them, and then wrap them with several layers of duct tape after you are done experimenting. One thought if it turns out to be the battery, is to convert the mower to a plug-in-the-wall mower, feeding AC power to your battery charger and then feeding the

12V output to the mower via a regular extension cord. OR you could mount the battery charger in place of the battery, and then run the entire mower directly off of 120V extension cords. Let us all know what finally happens.

==================================================== No corded mowers for me, thank you. I grew up using corded electric lawnmowers and snow-blowers. Never again! I think the motor is too powerful to run off A/C through a DC power supply. Those wheelchair batteries have a lot of oomph in them but it might be possible.

As for skinning the wires, it might come to that. I found some self-fusing black rubber stuff that's like heat shrink that you pull and wrap. On the few tests I've done, it seems quite a bit better than black electrical tape. Much thicker, too. Thanks, I think I might end up going that route for the test if I can't get access to the wires any other way. I also might just cut them and resolder using adhesive-filled heat shrink tubing. I buy it in assortments and always end up with lots of the large size stuff left. I'd like to keep it fairly waterproof.

Thanks for your input!

-- Bobby G.

Reply to
Robert Green

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there may be a thermal fuse somewhere near the motor or controls, to protect things.

I hate stuff with sealed boxes and no tech info available. Its a excellent reason to never buy another product made by whatever company did it........

Reply to
bob haller

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I'd suspect the controller. That it has some sort of controller board instead of a simpler system would tell me that you need to proceed carefully. It might be does more than simply switching dc on and off to the motor. It's challenging to do that with straight dc, a lot of current, and electronic components. And motors have a pretty high surge to deal with as well.

Can you tell if the motor really is a dc motor? Are the only two wires going to the brushes? If you hold a metal wrench near the side does it have permanent magnets?

Did you measure the voltage of the original battery? Hook a small load like a car tail light bulb to it for a while.

Reply to
jamesgangnc

The name B&D goes along with cheap power hand tools, thats a name I avoid.

Reply to
ransley

They also own DEWALT, I guess B&D is low endof market Dewalt the pricier one

Reply to
bob haller

firsthttp://www.black

My bet would be a totally failed battery. ===========================================

You'd be wrong just like my buddy. (-: It was his first guess but the mode was not like any battery failure I've ever seen. As Vic described, when these mower batteries die, they die very, very gradually and it's quite noticeable in the amount of lawn they'll mow. This was still mowing the whole yard with one charge when it died, almost the same as when it was new. Besides, I've got a number of known good wheelchair batteries of the wrong form factor (too tall) but equivalent amperage. When I swapped them out they got the same whining noise out of the controller board. The battery cables come off easily unlike the motor cables, so that's the first thing we tried!

Battery has seen little use and was working fine for the previous two outings. This is something else. Either a failed, non-resettable thermal overload protector or something else on the controller board. The things that were different this mowing were that the lawn was too tall and was very wet. That's why I am thinking controller component failure (the relay is Prime Suspect) or some sort of motor overload protector. The problem with that approach is that the motor spins slightly when you pull the switch. I would agree wholeheartedly that it looks just like catastrophic battery failure except that a test with a known-good battery produced the same controller board whining and no spin on the motor.

Part of the problem is that the kid who was operating it at the time has not reported much useful information about how it failed and on previous outings has worked the unit too hard in grass that was too tall. He was rushing through it instead of taking smaller slices, the preferred way to deal with high grass. That makes me think some sort of overload protector has failed. Finding it's going to be difficult because of the way the controller board is sealed up.

Thanks for your input!

-- Bobby G.

Reply to
Robert Green

The controller shouldn't whine, I know that much, so I tend to agree with you. It's a pretty simply controller compared to the ones I've seen on power wheelchairs but they are far more complex. The have to be able to vary speed to each side's motor (to be able to turn) and depending on the turn, can even move the left drive wheel forward and the right drive wheel backward to almost spin in place. The wheelchair controller has large capacitors and far more components than the mower does. I believe you are right about high surge currents and wonder if that's what fried the relay (if it is indeed fried).

Yes to all of that. I've read a little about repairing these and have learned that the motor magnets are glued in place and that removing them almost certainly means never getting them back in alignment without scraping away all the old glue and regluing them at precised the same height as the old ones. Parts diagram also indicates "Magnet: $47.95" that sits around the motor armature. So I am pretty sure it's DC (red and black wires go to brushes that are mounted 180 degrees apart).

Yes, when it finished the charge cycle the red light went out and the green light came on, indicating full charge. A voltmeter pegged a freshly charged battery at slightly over 13VDC. I have several "test loads" I will try today including a truck's cab fan and an old tungsten headlight. First I've got to locate some thick enough wire to make jumper cables for the testing I need to do. I already tried swapping a known-good wheelchair battery and got the same whining noise from the controller, leading me to look elsewhere for the problem's source. Also, the two times I've used it previously this season it mowed the front and back on a single charge each time, and the failure mode of these batteries is usually very slow, and noticeable only in the declining run time of the mower each session.

I agree that the controller/motor combo might have important features which could imperil the motor during straight battery to motor testing. However, with no spare controllers available, I have little left to lose by mucking around with it myself. Gonna print out these messages and run the tests people have suggested in the next few days after I get someone to help me get the sucker up on the bench to work with. With a bum knee, it's very hard to work on the floor and my bench isn't clear enough to raise the sucker up onto it. The controller can't be removed from the motor and battery wires very easily. To their credit, they have really connected them VERY TIGHT.

Thanks for your input, James.

-- Bobby G.

Reply to
Robert Green

This is one of their earliest cordless models. It took them a while to get things right. It's only recently that they've come up with easily removable batteries that don't require disassembly to swap. From what I can tell from the exploded diagrams, they've been making a lot of improvements in design over the years. Look at how long it took car makers to make oil filters accessible. (-:

-- Bobby G.

Reply to
Robert Green

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