TS DC Motor on Electric Lawnmower

[Guv Bob]

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 $$...

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:

Review in Popular Mechanics, March 1994, page 76:

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

319

[Ralph Mowery]

"Guv Bob"

[Guv Bob]

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.

[J Burns]

Maybe the relay is shorted from the contact input to the coil ground. I'd check those two terminals on the relay. I'd check the board without the relay.

[Guv Bob]

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.