Cordless Mulching Mower woes (Black and Decker CMM 875)

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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.
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Suggesting that you see about downloading the manual first http://www.blackanddecker.com/outdoor/MM875.aspx
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I see that one good first step would be to call 1 800 54 HOW TO
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wrote:

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.
http://www.blackanddecker.com/outdoor/CMM875.aspx 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.
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wrote:

Suggesting that you see about downloading the manual first http://www.blackanddecker.com/outdoor/MM875.aspx
===========================================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
http://servicenet.blackanddecker.com/Products/Detail?productNumber=CMM625#pp-parts
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.
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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.
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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.
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On Sun, 22 May 2011 16:50:25 -0400, "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
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wrote:

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.
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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.
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wrote:

My bet would be a totally failed battery.
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wrote:

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.
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Thanks to all who contributed to the thread. Here's how it wound up. B&D had virtually no parts for the mower or schematics for it so I decided fix it myself or scrap it. Nothing to lose. The grass was getting so high I went back to my old lawn guy because he did a much better job then my neighbor's kid, whose heart just wasn't into it.
I decided to localize the buzzing sound with my stethoscope. I hooked up the original B&D battery that, according to the charge light AND a voltmeter was fully charged, and rigged a bypass for the deadman switch (a trashbag twist tie defeats it nicely!). When I turned it on, searching for the part that was buzzing, it turned out conclusively to be the relay.
During the test, however, the whining noise from the relay dropping in pitch. I took that to mean the battery voltage was dropping. The voltmeter kept dipping downward when I connected it to the battery terminals. Not by much, but definitely a downward trend.
I took out my test headlight from my '71 LTD that had a burned out low-beam filament and hooked the hi-beam filament up to the mower battery. It glowed brightly for a moment, and then quickly dimmed down to a dull red glow. I did the same test with the supposedly good wheelchair battery which had recently come out of a working chair. It, too had tested above 13VDC when fully charged with a standalone SLA charger. And it too glowed for a second and faded to red.
The B&F original battery was a 24AH 12VDC unit. The wheelchair battery I tried as a test had 34AH 12VDC. But neither appeared able to supply the current needed to spin the blade. So I whipped out a battery from my spare wheelchair that's 55AH and was working well the day before and, well, you know the rest. Mower hummed away like it was new.
What lead me to not suspect the battery was that it worked fine for the last two outings, both within two weeks and then it failed catastrophically in heavy, wet grass. Just stopped. The unit would squeak and the rotor would spin a degree or two every few starts, but otherwise dead. From what we could see of the controller board, there's a voltage regulator with a hefty heat sink. I believe that's why the unit just stopped cold. I wasn't seeing a gradual slowdown of the motor as you might expect with a deteriorating battery. The overload, the dead drop off, the moisture and a good voltage reading all pointed in the wrong direction, at least for me. Kudos to those of you who stuck with the simplest explanation: "It's the damn battery!" (-"
I've put the mower back in storage. No sense in buying a spare until I'll be doing the yard work again since they do die over time, used or not. This was a good lesson in not over-thinking the problem. Now my friend that I give gas to for hooking up a hard drive's Molex power connector the wrong way and frying its circuit board can rib me about insisting this was something other than a simple, garden variety dead battery.
Sheesh.
-- Bobby G.
"I have yet to see any problem, however complicated, which, when looked at in the right way did not become still more complicated." Poul Anderson
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wrote:

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.
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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........
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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.
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wrote:

<<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.>>
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).
<<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?>>
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).
<<Did you measure the voltage of the original battery? Hook a small load like a car tail light bulb to it for a while.>>
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.
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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.
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I have not tried that yet. Got sidetracked by an ant invasion and an X-10 "jamming" problem that de-automated half my house (the RF controlled part) for the last 5 days. As you might imagine, getting to the bare wires is not terribly simple because they're (thankfully) well-sealed against moisture (as well as servicing!).

I did try this as the relay measured only 5.5 volts powering the coil. That's when it started to rain so I had to put it back in the shed. Without knowing anything about the controller card (so far no joy in finding a schematic) and without even being able to see the component side without cracking it open, I can't determine whether it should have more voltage than that. IIRC, the lettering on the relay faced in. Tomorrow's the new day set aside for lawn mower surgery and I'll be trying to document the controller board as best I can, even if it means cutting it free from the well-attached cables and cracking the cover. It could be worse - they could have potted the whole thing.

IIRC, this has a black metal case and is about 1" by 2" by 3/4" tall. My guess is a relay failure and it looks like it will be a bitch to desolder it or even remove the casing. I haven't played with it since the original post hoping that it was simply wet in the wrong spot and would dry out. What I call the "minimum effort" fix.

That's an interesting thought. None of the people who've used it can remember whether the blade stopped instantly or not. Sounds like it would be a good safety feature but hard on the parts. Would it short out the motor or use the relay to briefly apply reverse polarity to the motor? This relay looked like it had too many pins to be a simple On/Off control, but then again, the might have already had thousands of that model around. Hard to tell without cobbling together a schematic.

Not so curious if it's spitting an arc like that!

Visual inspection by my bud who grew up rebuilding motors in his dad's shop said the motor looked remarkably unused (which it is for a 16 year old mower). Spins freely and from what he could see of the brushes, they looked fine. But he didn't have his reading glasses and he could have been looking at a beached whale. (-: The motor does spin very freely along with the blade. Tomorrow we'll try to isolate the source of the noise, which IMHO *has* to be the relay. What else could buzz like that? If the contacts were welded together, wouldn't the device be in an "always on" state? All I get is the buzz and an occasional movement of a few degrees of the armature.
Thanks for your input, Bob!
-- Bobby G.
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On Thursday, May 26, 2011 3:07:57 AM UTC-4, Robert Green wrote:

Sorry to Join this game so late, but I have a B&D cmm625 (working nicely with a 5 year old battery I just desulfated) and a CMM630 that I am fixing for a friend.
The CMM630 was shutting down after 5 min or so in tall grass (not even wet!) and we suspected a battery (only 1 year old, 35 AH). However, when it looks to be something along the lines of your struggle (above). We found the battery to be good (better than mine, when moved to my unit) and not dropping badly in voltage either during operation or immediately after the mower abruptly stops. We then suspected the motor or the control unit (there's not much else!)
It's not easy to swap motors/control units on these mowers, so I carefully swapped out the control unit and the brushes (crimped at to the controller). In this way I was able to test that the motor ex brushes was fine with my good mower's controller.
Both motors with the known good controller were drawing about 20 Amps. Brushes on each looked identical as did the rest of the parts of the . The 'bad mower' was drawing 33 amps on start up from its controller, though. While we had both mowers open we lubed the top bearing (a felt hole at the top) and the bottom bearing (after SAFELY removing the blade etc from the bottom of the mower) with 3 or so drops of good oil (we used Mobil-1). After reassembly the 'bad mower' was down to about 26 amps, so not sure if we've helped much. The 'good mower' battery is only 26AH, so it's possible that the electronics don't like a battery with the ability to put out more current, or simply that they are a bit fried. We hope that lubrication buys us a little more headroom before the silly thing shuts down due to 'overload'.
If anyone has the schematics please direct us to them! (we have 2 'version 5' controller boards...and I have a version 3 I got from a junked unit).
By the way, we run our mowers off 100% solar (easy to do with one smallish solar panel...see my site:http://solarpowermower.wikia.com/wiki/Solar_Power_Mower_Wiki
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