I have a Ryobi RY481111 electric riding lawnmower wherein the blades refuse to spin most of the time, they will work on occasion. The major parts, outside of the batteries, are the drive controller and two blade motor controllers. I've tested the controllers and they appear to be fine but it's the relay that is causing some angst.

The relay is as pictured below. The low current is 12V, high current is 48V. According to the diagnostic document, the testing instructions are as follows;

1. Power Mower On
2. Set DMM to "DC Volts"
3. Check for 12V on the small terminals (under the black caps)
4. Check for approximately 39+V on the large terminals (under the red caps)

Now. Powered on the 12V measures properly, but I get no voltage testing the large terminals. The relay does actuate (confirmed via sound, and visually with the top cap removed). However, if I test with the mower off, I do get a measurement from the high side (just over 39, if I remember)

My relay knowledge is rather rudimentary, so it has me confused. Thus, questions;

- Does this indicate a bad relay or something outside of the relay?
- Can I bypass the relay by connecting the high current terminals to see if that powers the blades of this mower? (Hopefully indicating that replacing the relay may work).
- While getting a new relay is relatively inexpensive ($100 or so), I don't want to waste money/time if it is unlikely to be the issue
- The red-black between the two low terminals contains a diode.

Any advice would be greatly appreciated. Unfortunately, Ryobi hasn't returned my questions.

 

Yes, an electromechanical relay can be bypassed. But we'd need to know how said relay is connected and its function to give you a more detailed answer.

As for the diode, most likely it's there as a snubber to protect the circuit that's activating the relay's coil, and/or to minimize EMI.

 

If you are getting no voltage across the switching terminals of the relay check for a bad connection somewhere.

 

Trying to think of what additional information would be helpful.

I've attached the testing and diagnostic pdf I'm using. It does have a wiring diagram though it doesn't seem as helpful as it could be because it doesn't break out the wiring harness.

Everything else works on the mower, it moves fine, lights work, USB etc. (I'm not convinced a lawn mower really needs a USB port)

I have had it working at times. If I disconnect one of the blade motors, the other will start and when reconnecting both, then both will work. Although, this method doesn't seem to work any longer. When it is working I can turn it off and on and the blades will continue to work. After the last time, I left it for a few hours, came back and the blades would not spin.

If one of the leads of the high current side is disconnected then it will measure 48V (wire-end to wire-end) whether the machine is turned on or off. It's when both are connected to the relay that the voltage measures 0 when it's on. (39 when it's off)

 

It seems from the description that there is a failing electrical connection in the portion that powers the blade motors.
If that block diagram is correct then the relay also switches on power for the drive motor, and so if the mower can be driven with the relay engaged then the relay is not the problem. Or there might be a second relay controlling the blade motors that is not mentioned.
My guess is that the problem is in the safety circuit associated with the blade controls. Usually safety circuits are the cause of functional issues with consumer electrical devices.