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 diagram.

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 much $$...

http://www.digikey.com/product-detai...B710-ND/813777

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 5A max.

Motor part number - 971051-003. Obsolete and no specs I could find online. Measurements:
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!

Bob

Misc Info for future reference...

Similar to BMM2401:

https://www.youtube.com/watch?v=mFpP-4tv0tw

Review in Popular Mechanics, March 1994, page 76:

http://books.google.com/books?id=heM...0motor&f=false

Ryobi Outdoor Products
550 N. 54th Street
Chandler, AZ 85226

Original costs:
Mower $379
(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






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