Welll, the 20 year old Ryobi BMM-2401 beast stopped in its tracks today.
Any ideas on where the short might be? Here's my story....
I didn't hit anything unusual - just a thick patch of grass and it just
stopped. I could see that the thermal fuse had tripped. I reset it,
started up again and it tripped again. This time I saw smoke coming
from the relay board, and the red wire connecting the hot power from the
power relay to the motor hot terminal was hot.
So I tore into it, took the relay board out and sketched the wiring
First thing I saw was a large burned spot on the relay board at the
connection of a 12awg wire that connects to the motor positive terminal.
Sorry, camera is not working right now....
I disconnected all the wires connected to the board and checked
continuity. I found that the load contacts remain closed whether the
coil is energized or off. So that pretty much tells me I need a new
relay. Luckily they are still making an exact replacement and not that
That wouldn't explain why the board was burned, so I'm guessing that
there's another short somewhere. Probably on that same board.
But just in case it's not, I started checking the motor for short. I
disconnected the pos & neg motor terminals and measured the resistance
at 2 ohms. That sounds pretty low to me, but motor TS info online says
that's in the proper range. I=E/R means 24/2 = 12A wouldn't it?
That's pretty high. Maybe that's not important -- the battery spec is
Motor part number - 971051-003. Obsolete and no specs I could find
Shaft length end to end 8"
Motor diameter 3 7/8"
Shaft diameter .675" (just under 11/16th)
Iron stator housing length 4.5"
Thanks in advance!
Misc Info for future reference...
Similar to BMM2401:
Review in Popular Mechanics, March 1994, page 76:
Ryobi Outdoor Products
550 N. 54th Street
Chandler, AZ 85226
(Mine was $20 at garage sale with bad onboard charger. Paid $25 for a
charger at the auto parts store)
New battery in 1994: $125
(Actuall it's two 12VDC in series) $20-35 each now for new ones on ebay
<But just in case it's not, I started checking the motor for short. I
disconnected the pos & neg motor terminals and measured the resistance at 2
ohms. That sounds pretty low to me, but motor TS info online says that's in
the proper range. I=E/R means 24/2 = 12A wouldn't it? That's pretty
high. Maybe that's not important -- the >battery spec is 5A max.
YOu can not use the normal i=e/r on motors. That is true for only a
fraction of a second. Once the motor starts up, it also acts like a
generator and creats a back EMF. The current will be a lot less as the
motor reaches its running speed. This is true for any common motor , AC or
It might also be know as the LRA or locked rotator amps. It will often be 3
to 5 or more times the running current.
For a motor you have to know what it is rated at at full load and measure
the current when the motor is running.
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Thanks a lot, Ralph. That's what I was hoping to hear.
I was expecting to see some kind of voltage regulation or filter
circuit, but don't see anything. Is it normal for this type of motor
to run connected straight to the battery like this? With this units,
there's a chance that someone else has fiddled with it before I got it.
Before the relay went back, never had any trouble in 15+ years.
Thanks, JB. That's exactly what it was. I ended up bypassing the board
and installing a relay & base elsewhere and rewiring the other
connections around the board.
This was an interesting project for me. I was surprised at how little
there is to an electric mower. It's basically a motor and battery. Add
a thermal circuit breaker or fuse, relay, switch and charger and that's
it. Mower chassis is 2 pieces of molded plastic. Only and some brackets
are metal/ New ones made in China sell for $300 and up. Hmmmm......
I took way too many photos and will upload a few soon.
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